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Butyrate Way back when we talked about SCFAs positive effects on inflammation and permeability of the gut, we said we would get back to its actions outside of the gut, and here we are. In addition to butyrate’s peripheral anti-inflammatory effects via keeping LPS contained inside the gut, it also directly inhibits TNF-alpha and the inflammatory response to LPS (460, 461). Elimination of the gut microbiome (thus SCFA production) with antibiotics decreases IGF-1, which is restored with SCFA administration (462). Dietary administration of fellow SCFA, propionate, up-regulated the expression of GH, IGF1 and down-regulated myostatin (463). Butyrate improves insulin-resistance in skeletal muscle, along with its induction of Akt (464, 465). And, it increased muscle fiber cross-sectional area along with improving glucose metabolism in aged subjects (466). Let’s take a closer look at some of the several mechanisms through which it works. GLP-1 Activation of Free Fatty Acid Receptors (FFAR2 & FFAR3) in the gut by butyrate stimulates the release glucagon-like peptide-1 (GLP-1) which then enters the blood stream (467, 468). Much like insulin, GLP-1 activates the anabolic Akt/mTOR pathway (469, 470, 471, 472). It also promotes dilation of muscle microvascular (473, 474). This enhances nutrient uptake in the muscle cell and is dependent on Akt/mTOR upregulation of nitric oxide production (475, 476). Its effects in this regard were both independent of, and additive to, insulin (477, 478). Recall also the positive effects of NO on satellite cell activation, muscle regeneration, and repair discussed earlier. In addition to skeletal muscle microvasculature, GLP-1 also significantly increases vasodilation and blood flow in large vessels like the brachial and radial arteries and femoral vein (479, 480). Treatment with GLP-1 improves exercise capacity and mitochondrial function, as well as skeletal muscle mass and strength (481, 482). HDAC While GLP-1’s positive effects in muscle begin with butyrate activity in the gut, butyrate, itself, is also taken up from the gut and enters the systemic circulation producing direct actions that support muscle growth. One of the primary mechanisms is its function as a Histone De-Acetylase (HDAC) inhibitor. This is epigenetic stuff. To put it simply, epigenetics involves (heritable) changes in gene expression without change to the DNA sequence, itself. It basically changes how the DNA is interpreted, similar to translating a foreign language book. The original book (DNA) stays the same, but a different author (epigenetics) is going to translate it differently. Negative epigenetic changes are a huge part of the build-up of dysfunction with aging in everything from metabolism, to muscle mass, to the brain, with inflammation being a particular culprit (483, 484, 485). Propionate and acetate also augment histone acetylation, but the bulk of the data is on butyrate (486, 487). HDAC inhibition amplifies Akt/mTOR signaling, as well as preventing induction of atrophy genes (488). Increased histone acetylation blocks downstream activity of glucocorticoids, including FoxO (489, 490). Inhibition of HDAC during muscle disuse significantly attenuated both disuse muscle fiber atrophy and contractile dysfunction via FoxO (491). The effects of acetylation on FoxO, and other targets such as mTOR, appear to be quite similar to phosphorylation with Akt, though data is still new and scarce (492, 493). But, that will definitely be something to keep an eye on. Finally, inhibition of HDAC activity significantly enhanced androgen receptor mediated protein synthesis (494). You have likely heard the term “muscle memory”, but you may not know that skeletal muscle stem cells do indeed have a memory that is created epigenetically. Stem cells from muscles of young, aged, physically active, and diabetic subjects carry on their altered metabolic characteristics when isolated and cultured (495). In other words, the bad (or good) epigenetic build-up semi-permanently alters them to such an extent that it is maintained when they are taken out of subjects and grown in a lab. HDAC inhibitors promote muscle regeneration through epigenetic regulation of both satellite stem cells and differentiated muscle cells (496). Via upregulation of follistatin (basically the anti-myostatin), HDAC inhibition also blocks the adipogenic potential of stem cells, pushing them toward the formation of muscle cells rather than adipocytes (497, 498). The importance of HDAC inhibition reversing long-term damage from inflammation and aging basically cannot be understated. Heat Shock Proteins Butyrate also induces Heat Shock Proteins (499, 500). Heat Shock Proteins (HSPs) are called such because they were initially discovered in cells subjected to hyperthermia, but they function as a protective and subsequent regenerative and repair mechanism against all kinds of cellular stressors (501). Other HDAC inhibitors induce HSPs as well, suggesting this as butyrate’s mechanism in this regard (502, 503). Induction of HSPs protects intestinal epithelial tight junction barriers, decreasing LPS leakage, and reducing the inflammatory response (504). Increased HSP levels also reduce TLR-4, and the subsequent production of TNF-alpha and NF-κB (505). HSPs strongly blunt increases of cortisol to stressors (506, 507). The synthetic glucocorticoid dexamethasone decreased myotube diameter and protein content, and heat stress prevented this along with recovering Akt signaling (508). HSPs directly bind to and protect Akt, and HSP induction defends against glucocorticoid induction of catabolic FoxO via Akt (509, 510). Silencing of HSP genes decreases Akt and myotube diameter while increasing FoxO, and treatment with an HSP inducer reverses this (511). Exercise also increases HSPs, along with Akt and downstream anabolic signaling (512). Aged subjects have a blunted HSP response to exercise, along with decreased muscle repair, which is reversed with HSP overexpression (513). HSPs’ positive effects on muscle repair and regeneration seems to be to some extent from protection of satellite cells (514). Androgens and Clenbuterol also strongly upregulate HSP expression, with this likely being particularly important for Clen’s anabolic effects (515, 516, 517, 518). Angiotensin II Last but not least, butyrate protects against the negative effects of Angiotensin II (Ang II). Like butyrate, itself, Ang II produces effects both inside and outside the gut. But, its effects are negative. It is kind of a wingman of LPS in that regard. It also displays bi-directional communication between the gut and brain in hypertension, much like cortisol with stress (519). In addition to its effects on blood pressure, it is strongly induced by LPS and mediates some of the inflammatory response to it (520, 521, 522). LPS induction of Ang II may be through TNF-alpha, but it is also a direct ligand for TLR-4, just as LPS is (533, 534, 535). It is a really interesting molecule, and Renin-Angiotensin a really interesting system, as it ties high blood pressure in with inflammation, insulin resistance, and the cardiovascular system in Metabolic Syndrome. You will likely hear a lot more about it over the next 5-10 years, but parts of the understanding are still relatively in their infancy, so we are going to keep it fairly brief. There is a decrease in microbial richness and diversity in hypertension and with Ang II infusion, as well as decreases in acetate and butyrate producing bacteria (536). This is accompanied by increased intestinal permeability and decreased tight junction proteins (537). Butyrate administration elevated Akkermansia levels, with significant positive effects on inflammation and ROS, and led to improvement of hypertension (538). Butyrate significant reduces blood pressure, as well as TNF-alpha, in response to Ang II infusion (539, 540). Data on other HDAC inhibitors indicate that this may be a primary mechanism for butyrate’s antagonism of Ang II’s actions. HDAC inhibitors prevented inflammation and ROS from Angiotensin II (541). They also protected against Ang II induced hypertension and vasoconstriction (542). And, again, the semi-permanent nature of epigenetics makes this especially important. Outside of the gut, Ang II basically does all of the same bad stuff as LPS because, as we mentioned, it mediates some of LPS signaling, plus shares signaling downstream from TLR-4. It shares the same link between inflammation and insulin resistance, and ACE inhibitors or Ang II receptor blockade reverses these (543, 544, 545). It reduces protein synthesis and increases catabolism, leading to muscle atrophy (546). Ang II inhibits the insulin and IGF-1 signaling pathways via Akt inhibition (547, 548). It impairs insulin stimulated nitric oxide and vasodilation (549). This is, once again, via the Akt/mTOR pathway (550, 551). As a result, Ang II also reduces muscle regeneration and satellite cell differentiation into muscle fibers (552). Finally, it increases the glucorticoid/myostatin catabolic pathways (553, 554). Will it work for me and what to expect With the science out of the way, the obvious question is “How much do you need and/or should you want pro- and prebiotics to fix your gut and your body?” Because the gut and systemic inflammation affect every system, and basically every cell, in your body, a good probiotic/prebiotic combo kind of stands apart from any other category of supplement. It is most analogous to going from a shitty diet to a good diet or from an okay diet to a perfect one. We briefly mentioned hardgainers and the “skinny fat” phenotype, earlier. You definitely want to fix your gut. I would expect around an extra 10 lbs of muscle in a year, as your body starts living up to its genetic potential. For significantly fat people (and, even moreso, if showing signs of glucose intolerance), you absolutely need to fix your gut. I would expect very noticeable body composition changes in 1-3 months, and borderline miraculous ones in a year. This article has been about muscle mass, but as we alluded to with the mention of Metabolic Syndrome, the gut and inflammation play a huge role in health, as well. Other general parameters pointing toward its usefulness for you are being over 30, the fatter you are, the worse your diet is, being in a calorie surplus, and having a (personal or family) history of inflammatory related conditions (heart disease, blood pressure, auto-immune, IBS/IBD, etc.). On other hand, if you are 19, quite lean, on great diet with low-moderate carbs including fruits and veggies, with an iron stomach, and in a calorie deficit, it is not going to noticeably do a lot for you. It would be much more of a preventative measure to keep your cells young as you get older, to keep you still being awesome 5-10+ years from now. A big exception would be during bulking phases – and, the dirtier the bulk, the more it would help. Likewise, if you tend to go off the rails during holidays or vacations, it is damage control.
Outside of the Gut If you are not already convinced of the benefits of pro- and prebiotics on muscle mass, it gets even more interesting outside of the gut. Getting back to Lipopolysaccharide (LPS) produced by a dysfunctional microbiota, once it has escaped the leaky gut, it sets off an anti-anabolic, pro-catabolic cascade of inflammation and reactive oxygen species (ROS), systemically (298). As we have mentioned, your body views LPS as an outside, pathogenic invasion via TLR-4. And, it literally is, as these bacteria are living, foreign invaders, thus immune defenses are activated. In such an attack, metabolically expensive skeletal muscle is not prioritized – quite the opposite, in fact. Amino acids and protein synthesis are prioritized for the immune response, at the expense of muscle tissue (299). The activity of insulin and other growth factors like IGF-1 are reduced, as are the levels and signaling of testosterone (300). At the same time, catabolic signals such as glucocorticoids, myostatin, NF-kB, and FoxO are upregulated, activating atrophy producing genes, which initiate the physical breakdown of proteins in muscle (301, 302). Obviously, that is all very bad for muscle size and body composition, so let’s take a look at these pathways in more detail. These will be kind of complicated, so some may better visualize and understand by also viewing the attached GRAPHIC LPS and Anabolic Pathways This is simplified, but the anti-anabolic signaling pathways of LPS basically proceed as follows: LPS/TLR-4/TNF-a/ROS DECREASES Insulin&IGF-1 signaling/Akt/mTOR/Protein Synthesis LPS/TLR-4/TNF-a/ROS DECREASES Amino Acid signaling/mTOR/Protein Synthesis LPS/Cortisol/ROS/Myostatin DECREASES Akt/mTOR/Protein Synthesis To reiterate at risk of being repetitive, the body views lipopolysaccharide (aka endotoxin) as an attack. LPS is a ligand of TLR-4, which literally exists to recognize molecular patterns of pathogens and toxins, then subsequently activate the inflammatory immune response in self-defense. This is great, when your body needs to occasionally protect itself. It is very bad when it is constant and chronic because of diet and lifestyle. LPS injections result in a 50% fall in protein synthesis in skeletal muscle, along with a 60-100% increase in muscle protein degradation (303). Decreases in muscle protein content from LPS are equivalent to those with starvation (304). However, the rate of protein synthesis in the liver and plasma proteins, especially albumin and immunoglobulins, is greatly increased to match (305, 306). Basically, amino acids from diet and muscles are being commandeered for the battle (307). You are not going to get that high of LPS levels from dysbiosis of the microbiota, unless you have sepsis inducing food poisoning or the like, but it goes to show how powerful LPS is as an anti-anabolic, pro-catabolic trigger. Obesity and type-2 diabetes are known to be associated with a chronic, low-grade inflammatory state, and this is accompanied by impairment of protein metabolism such as a lack of stimulation of protein synthesis by insulin and amino acids, as well as lower inhibition of proteolysis by the same (308, 309). The protein synthetic response to exercise is blunted in obesity compared to the lean as well (310). Disruption of the mTOR pathway, and its stimulation of protein synthesis, is also seen in these subjects (311) (312). However, chronic excessive energy intake and increased adiposity, without the metabolic disturbances, do not induce any changes in tissue protein synthesis rates, indicating the primacy of inflammatory pathways in these effects (313). LPS diminishes the anabolic sensitivity to BCAAs and EAAs (314, 315). It also reduces IGF-I levels (316). Repeated LPS administration decreases muscle weight and muscle fiber cross sectional area (317). In addition, LPS treatment reduces blood flow in muscle by as much as 70% (318). So, not only are your muscles less sensitive to various anabolic signals, less of those are even getting there. Exercise reduces the LPS receptor, TLR-4, along with LPS induced inflammation (319). Via TLR-4, LPS massively increases TNF-alpha, the next step in the inflammatory equation – and, inhibition of TLR-4 reverses this (320, 321). TNF-alpha is highly involved in muscle wasting and kicks off the ROS cascade that invokes myostatin, NF-kB, and ceramides, which we will get into in a bit (322, 323, 324). LPS induced TNF-alpha increases result in decreased body and skeletal muscle weight, and TNF-alpha shares with LPS an elevated rate of BCAA oxidation (325, 326). LPS promotes TNF-alpha mRNA transcription, with subsequent declines in IGF-1 (327, 328). Direct TNF-alpha administration also lowers IGF-1, along with gastrocnemius weight (329, 330). TNF-alpha completely prevents insulin-mediated augmentation of capillary recruitment and blood-flow as well, inhibiting skeletal muscle glucose uptake by more than half (331). Reducing TLR-4 and TNF-alpha increases the anabolic signal transducer “Akt”, which we will talk a bit about, now (332). Akt You possibly have never even heard of it, but Akt (also known as Protein Kinase is one of the most important molecular signals controlling skeletal muscle mass. It affects both anabolism, through mTOR regulation of protein synthesis, and catabolism, through FoxO regulation of protein degradation (333, 334). Anabolic growth factors such as insulin, IGF-1, and testosterone, as well as factors inhibiting anabolism such as TNF-alpha and myostatin, transmit their cellular signals on hypertrophy and atrophy by altering the activity of Akt and its phosphorylation of its numerous downstream substrates (335). Basically, Akt turns pro-muscle growth targets on, and anti-muscle growth targets off. Genetic activation of the Akt/mTOR pathway causes hypertrophy and prevents atrophy, whereas genetic silencing blocks hypertrophy in vivo (336). Testosterone administration activates Akt signaling (337, 338). It upregulates the insulin-dependent Akt/mTOR signal transduction pathways in an androgen receptor dependent manner (339). Another androgen, Nandrolone, increases IGF-1 expression and its activation of Akt/mTor signaling while decreasing catabolic FoxO transcription (340). Resistance training induced muscle hypertrophy increases Akt and phosphorylation of mTOR, with a parallel drop in FoxO, and detraining does the opposite on all parameters (341). Mechanical overload also induces muscle hypertrophy via activation of Akt and its downstream anabolic pathways (342). LPS decreases Akt, along with its phosphorylation (thus, activation) of mTOR, as well as upregulating catabolic NF-kB and FoxO (343, 344). Increases in TLR-4 and TNF-alpha also downregulate Akt (345). Akt downregulation by TNF-alpha reduces skeletal muscle protein synthesis and increases protein degradation (346). Inflammatory cytokines like TNF-alpha decrease IGF-1’s activation of Akt, subsequently increasing expression of muscle atrophy-related genes (347). Elevated reactive oxygen species (ROS) production by TNF-alpha inhibits Akt/mTOR pathways and upregulates atrophy promoting genes (348, 349). And, as would be expected, ROS directly promote resistance to insulin signaling in skeletal muscle (350, 351). Amino acids and insulin fail to stimulate activation of Akt/mTOR mediated muscle protein synthesis in aged rats, a model of chronic, low-grade inflammation (352). Likewise, insulin resistant subject have reduced muscle Akt phosphorylation and negligible Akt mediated anabolic response to physiological insulin levels (353). mTOR You may be at least somewhat familiar with mTOR, as it is known to mediate increased protein synthesis from BCAAs such as leucine (354). LPS administration reduces phosphorylation of mTOR by Akt in skeletal muscle (355). Activation of TLR-4 by LPS inhibited the Akt/mTOR pathway, decreasing protein synthesis (356). Unlike insulin, amino acid action on mTOR induced protein synthesis is not modulated by Akt (357, 358). LPS also blocks leucine stimulated muscle protein synthesis, independently of Akt (359). Nitric Oxide and Satellite Cells The Akt/mTOR pathway also mediates the upregulation of nitric oxide (NO) by insulin (360, 361). Prolonged exposure to insulin (i.e. insulin resistance) desensitizes this pathway and blunts NO production (362). You probably are familiar with NO, as it is one of the more popular supplement categories, but we will still take a quick look at a bit of data. NO is a key messenger in myogenesis, particular in response to repairing muscle damage, such as from working out (363). It promotes muscle satellite cell activation and proliferation, as well as induction of myogenic genes such as myogenin and follistatin (364, 365). This satellite cell activation is dependent on the Akt/mTOR upregulation of NO production (366). Aging (which is associated with inflammation, hindered insulin signaling, and muscle loss) results in reduced activation and speed of satellite cell migration to half of that of young cells, which is reversed by NO (367, 368). Ceramides Ceramides are reactive lipid species that, for all intents and purposes, behave like reactive oxygen species (ROS) within muscle tissue (369). It is a 2nd messenger in TNF-alpha inflammatory signaling cascades (370). Ceramide accumulation in muscle is higher in obese and aged subjects, concomitant with decreased sensitivity to insulin and its anabolic effects (371, 372). Ceramide also decreases sensitivity of mTOR to amino acid induction of protein synthesis (373). In addition to insulin signaling, it diminishes anabolic responsiveness to IGF-1 by elevating Akt degradation (374, 375, 376). As would be expected, ceramide also reduces glucose uptake and glycogen synthesis, via inhibition of Akt/mTOR (377, 378). Of note, even though we are focusing on the anabolic and anti-catabolic actions when discussing insulin signaling pathways, uptake up glucose and amino acids, as well as glycogen synthesis, are generally increased when insulin sensitivity is increased. These are not divergent pathways. It simply is not our main focus, here. In addition to decreased anabolic activity, the decreased activity of Akt by TNF-alpha and ceramides also takes the breaks off of catabolic signaling via FoxO and NF-kB, promoting muscle atrophy (379, 380). Exercise (surprise, surprise) reduces muscle ceramide content, restoring insulin sensitivity (381). Testosterone The testis barrier basically works exactly the same as the gut barrier, including 1) being susceptible to increased permeability to LPS and inflammatory damage, 2) the enhanced expression of tight junction proteins and improvement of barrier function by bacterial fermentation products such as butyrate, and 3) modulation of all of this by the gut microbiome (382, 383). LPS administration in healthy subjects inhibits testosterone production directly in the Leydig cells of the testes (384, 385). Germ free mice (which do not have bacteria to produce SCFAs) show increased blood-testis-barrier permeability and lower testosterone production, which is fixed with probiotic administration (386), 387, 388). Obesity and metabolic syndrome are associated with lower testosterone levels, along with the low-grade inflammatory state (389, 390). A close relationship exists between the development of a pro-inflammatory state and the decline in testosterone levels, and these are thought to be very much causally linked (391, 392). Finally, heavy endurance exercise training (like marathons and such) is consistently associated with persistent low-grade systemic inflammation together with reduced free and total testosterone levels (393, 394). You are no doubt familiar with the positive effects of testosterone on muscle, so we won’t get into that, but I will just note that testosterone inhibits the LPS/TLR-4/TNF-alpha inflammatory, anti-anabolic/pro-catabolic pathways (395). LPS and Catabolic Pathways This is simplified a bit, once again, but LPS induced increases in catabolism basically proceed as follows: LPS/TLR-4/TNF-a/ROS INCREASES NF-kB/Atrogenes/Muscle Breakdown LPS/Cortisol/Myostatin DECREASES Akt INHIBITION OF FoxO/Atrogenes/Muscle Breakdown LPS/TLR-4/TNF-a/ROS/NF-kB<->Myostatin<->ROS<->NFKb and FoxO/Atrogenes/Muscle Breakdown Just like with inhibition of anabolic signaling, LPS escapes the gut, increasing inflammatory cytokines like TNF-alpha and reactive oxygen species throughout the body. This elevates NF-kB, which then triggers atrophy promoting genes (atrogenes) that subsequently induce the physical breaking down of proteins in muscle tissue. LPS also amplifies cortisol release, which increases myostatin, which inhibits Akt. This takes the breaks off of FoxO activated atrogene expression which, again, subsequently induces the physical breaking down of proteins in muscle tissue (396). There is also a feed forward myostatin/TNF-alpha/ROS/NF-kB loop, where they all increase each other, which results in activation of both NF-kB and FoxO (397, 398). NF-kB and FoxO, together, account for 95% of muscle fiber atrophy, so let’s take a look at some data on those two (399). NF-kappaB Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-kB), along with FoxO, is one of the two primary atrogene activating pathways at the end of the LPS induced catabolic chain (400). It is induced by reactive oxygen species generated by TNF-alpha, and blocking ROS production prevents its activation by LPS administration (401, 402). Blocking TLR-4, upstream of TNF-alpha, also inhibits NF-kB (403). NF-kB is implicated in various models of atrophy (404, 405). It is significantly elevated, along with TNF-alpha, in myocytes from obese type-2 diabetics (406). Insulin resistant subjects display higher elevations in NF-kB in response to LPS than normal glucose tolerant ones (407). The resistance to the anabolic effects of exercise seen in aging is also associated a rise in NF-κB activity (408). Interestingly, the increases seen with aging, and the associated inflammation, is normalized with lifetime calorie restriction (known to be anti-inflammatory) in animal models (409, 410). Though, it increases catabolism in both slow-twitch and fast-twitch muscle fibers, NF-kB is more highly expressed in slow-twitch fibers (411, 412). It is particularly noted in disuse atrophy, so hopefully you already have that part of it covered with exercise (413, 414). It also inhibits Akt, perhaps via myostatin, which would ultimately also induce catabolic FoxO, which we will talk about next (415). Thus, its indirect negative effects of NF-kB on muscle may be even more important than the direct ones. FoxO The Forkhead Box O (FoxO) family is the second major atrogene activating pathway at the end of the LPS induced catabolic chain. It is induced by cortisol and myostatin, and it results primarily in atrophy of fast-twitch fibers (416). It is stimulated by ROS, likely through myostatin (417). FoxO is negatively regulated by Akt, as we mentioned previously (418, 419). When Akt’s inhibition of FoxO is removed, atrogenes are induced and loss of muscle mass follows (420, 421). It is inhibited by both insulin and IGF-1 signaling via Akt (422, 423). FoxO is likely the more important catabolic pathway vs. NF-kB, both because of the preferential fast-twitch degradation and because activation occurs in more common atrophy promoting situations due to the nexus with the ubiquitous Akt. Blockade of FoxO also prevents muscle atrophy from glucocorticoids, which leads us to our next segment (424). Cortisol Cortisol strongly activates both anti-anabolic (via inhibition of insulin signaling) and catabolic (via myostatin and FoxO) pathways, but we are putting it in the catabolic section. Being a stress hormone, it is triggered by not just perceived emotional stress, but energetic stress, such as with starvation, or for our purposes, the metabolic stress of the LPS induced inflammatory response (425). Before we get into the negative effects of gut dysbiosis on cortisol in relation to muscle, it should be mentioned that there exists a bidirectional communication system between the gastrointestinal tract and the brain (426, 427). A full treatment of the Gut-Microbiota-Brain axis is well beyond our scope, but in addition to dysbiosis and inflammation messing up the stress axis, stress and anxiety are signaled from the brain to the gut and mess it up, as well (428, 429). In other words, it is a feed-forward vicious cycle. Getting back to LPS and cortisol… As a systemic immunological stressor, LPS invokes a prolonged activation of the hypothalamic-pituitary-adrenal (HPA) axis via cytokines such as TNF-alpha, ultimately increasing cortisol release from the adrenals (430, 431). Cortisol is anti-inflammatory, and synthetic glucocorticoids are well known as being useful for this purpose (432). This anti-inflammatory activity, from an evolutionary perspective, likely serves as a mitigating factor in fever/ROS induced cellular toxicity (433, 434). Indeed, impairment of the HPA axis results in greatly increased lethality from LPS/endotoxic shock (435). Getting to cortisol and muscle, the purpose of cortisol with cellular stress such as from the LPS inflammatory cascade is to rapidly mobilize carbohydrate, fat, and protein stores to provide energy for the fight (436). With inflammation, protein is particularly in demand, as it is necessary for synthesis of acute phase reactants, which are a group of proteins that modulate the immune response (437). Thus, muscle protein synthesis is suppressed while breakdown is activated to provide said protein (438). This response also increases gluconeogenesis from amino acids preferentially over fatty acid oxidation as fuel to quickly provide ATP for increased energetic demands (439). We’ve basically mentioned all of that before, but it is worth repeating, as it is the “Why” on something meant to protect you going so terribly wrong. Cortisol quickly initiates muscle fiber atrophy, as IGF-1 and insulin signaling are blunted while myostatin is increased (440, 441, 442). This is mediated, downstream, through the Akt/mTOR and Akt/FoxO pathways, respectively (443, 444). Even worse, this atrophy is preferential toward high energy demanding fast-twitch fibers (445, 446). Glucocorticoids also initiate NF-κB activation, likely through the myostatin-ROS pathway mentioned earlier (447). In addition to insulin resistance through Akt/mTOR inhibition, glucocorticoids also reduce the activity of GLP-1, which we will talk about in a bit (448) Lactobacillus has been found to block restraint stress induced increases in LPS and cortisol (449). Interestingly, though cortisol inhibits mTOR’s anabolic activity, mTOR modulates glucocorticoid receptor function, counteracting its catabolic effects (450). So, more good news on BCAAs. Myostatin Myostatin is the major mediator in cortisol’s negative effects on muscle size, so we will give it its own little mini-section. It is upregulated by glucocorticoid administration and stress (451, 452). This stimulates muscle atrophy through a cascade of signals that includes activation of FoxO and NF-kB (453, 454, 455). Myostatin overexpression results in an atrophic phenotype with fast-twitch fibers being most sensitive (456). Myostatin also inhibits Akt, thus insulin and IGF-1 anabolic signaling through mTOR (457, 458, 459). Part 5 finale on Thursday, June 28th
Inside the Gut Short Chain Fatty Acids (SCFAs) Along with outcompeting the LPS producing bacteria that trigger inflammation, one of the primary and most basic ways by which probiotic bacteria work their magic is by fermenting prebiotics to produce SCFAs (primarily acetate, butyrate, and propionate). So, we are going to talk a bit about those, now. SCFAs primarily work through three mechanisms: 1) Decreasing inflammation and permeability in the gut 2) Activation of free fatty acid receptors, FFAR2 and FFAR3, in the gut 3) Inhibition of Histone De-Acetylase (HDAC) in skeletal muscle We will talk about the first one, now, as it occurs wholly inside the gut (though, it ultimately prevents bad things outside of it). The second one begins in the gut, but mostly does its work outside, so we will cover it a bit here, then get deeper into it and number 3, later, in the section on all of the stuff outside of the gut. I should probably further emphasize that the Gut-Microbiota-Muscle axis is not straight-forward linear and compartmentalized. There is inside and out, as well as back and forth, communication. There are also overlapping functions and pathways. LPS/inflammation increases gut barrier breakdown, thus its own leakage into the body, and SCFAs/butyrate directly reduce inflammation in addition to reducing LPS/inflammatory leakage by strengthening barrier function… in addition to directly attacking problems caused by inflammation related pathways in skeletal muscle. Anyway, back to SCFAs. Both acetate and propionate reduce inflammatory pathways of lipopolysaccharide like TNF-alpha and NF-kB (238, 239). However, butyrate is significantly more potent (240, 241). Butyrate also plays the most critical role in maintaining colonic health via modulation of intestinal cell growth and differentiation (242). It is the primary fuel source for enterocytes, being responsible for up to about 70% of their energy use (243, 244). Butyrate also dose-dependently reduces LPS impairment of tight junction permeability and intestinal barrier integrity. We’ll get into this more in muscle, but one mechanism by which it increases tight junction proteins is by preventing LPS induced inhibition of the anabolic Akt/mTOR mediated protein synthetic pathway (245). Butyrate also dose-dependently increases mucin protein contents of the mucosal layer of the intestine (246). The mucosal layer is the first line of defense against noxious substances and pathogens (247, 248). In addition to being food for some of the best bacteria, mucin improves adherence of probiotics to the mucosal layer of the intestine, thus mucins are perhaps the most important aspects of their viability and colonization (249, 250). Butyrate also improves intestinal barrier function via activation of AMPK (251). Sodium butyrate has been specifically found to be an AMPK agonist (252). And, butyrate increase tight junction assembly, thus improving barrier function, specifically through AMPK (253, 254). This seems like as good of a place as any to add a bit more about AMPK, really quickly, as it is one of the major targets in all of this inside the gut. AMPK AMPK is a primary signaler in the maintenance of tight junction integrity and intestinal barrier function. It is one of the most important pathways in preventing the “leaky gut” we have spoken of earlier in regard to LPS and other inflammatory and infectious molecules escaping into the body to wreak havoc (255, 256). As we’ve mentioned, modern food processing and the Western diet is a particularly egregious malefactor in all of this (257). In addition to its involvement in barrier function, AMPK activation is extremely positive for the great bacteria that we can’t get commercially. Metformin increased Akkermansia 18-fold through AMPK activation. Also, against a high-fat diet, it restored Bacteroides levels and normalized microbiota constituent ratios to that of lean subjects (258, 259, 260). It inhibited LPS induced inflammation and gut permeability increases, while improving glucose uptake and insulin sensitivity (259). Akkermansia increases are likely at least partially due to greatly elevated production of its favorite food, mucin, which is stimulated by AMPK. Its activation also reduces insulin resistance and adipose tissue inflammation in a high-fat diet (260). Free Fatty Acid Receptors Activation of FFAR2 by SCFAs suppresses insulin signaling in adipocytes, which inhibits fat accumulation in adipose tissue and promotes the metabolism of lipids and glucose in other tissues such as muscle (S2). Propionate and butyrate also both activate intestinal gluconeogenesis. Butyrate does so through AMPK, while propionate works through a gut-brain neural circuit involving FFAR3 (261). This glucose then triggers a signal to the brain which normalizes whole body glucose homeostasis (262). In a fasting state, as much as 62% of infused propionate is converted to glucose in the intestine, accounting for 69% of total glucose production (263). This is especially applicable to lower carb diets. Basically, it makes your brain think you are plenty fed with carbs/glucose. When the brain thinks the body is well-fed, energy intensive protein synthesis is supported. It also reduces peripheral gluconeogenesis, sparing amino acids for use in muscle tissue, while improving insulin sensitivity via reduced output of glucose from the liver (262). Short chain fatty acids, especially butyrate, are also direct precursors for ketone formation, obviously handy for ketogenic diets (264, 265). Activation of FFAR2/3 by SCFAs also stimulates the release of the incretin hormone, glucagon-like peptide-1 (GLP-1), enhancing anabolic and anti-catabolic insulin signaling pathways in muscle (266, 267). We will discuss this more, later. Protein Absorption and Efficiency The earliest studies on pro- and prebiotics were done to replace antibiotics for increasing digestion/feed efficiency in livestock. They result in the production of more meat (i.e. muscle mass), in general, and more meat per unit of food given. So, let’s take a look at the mechanisms on how this works, and how it will work for you. As we have briefly discussed, probiotics and prebiotics, via short chain fatty acids, increase the proliferation of intestinal epithelial cells, as well as increasing villus height and crypt depth, expanding total surface area for nutrient absorption. Likewise, increases in the quantity and quality of goblet cells increases mucin, helping to maintain optimal health and function of the intestine. Ultimately, this increases total nutrient digestibility in the intestinal tract (268). SCFAs, and other organic acids such as lactic acid (produced by lactobacillus, thus the name), reduce pH, increasing bioavailability of protein (269). They also enhance the release of digestive proteases, increasing absorption of small peptides and amino acids by enterocytes. (270). Only 80–90% of protein is actually digested and made available as amino acids in the small intestine, and we obviously want it on the high end (271). This inefficiency results in the entry of a good chunk of undigested protein into the large intestine, which we will discuss more in a moment. Once proteins have been digested and absorbed, we get to yet another area where probiotics and prebiotics, via SCFA acids, are useful – namely, in protein sparing. The gut has one of the highest rates of cellular and protein turnover of any tissue in the body. If cellular needs are not met by diet and supplementation, skeletal muscle proteolysis results, with amino acids being funneled from the periphery to the gut (272The liver and the gut account for 20 to 35% of whole-body protein turnover and energy expenditure, and your big brain gets a crack at those before your muscles, as well (273). Up to 50% of dietary amino acids are oxidized in first pass in the gut, with anabolic BCAAs being amongst the most favored (274). Some of this is inevitable, as these amino acids go toward protein structures in the intestines, such as digestive enzymes, mucins, and the physical makeup of the intestinal cells, themselves. But, they are also heavily used for fuel if their favorite food, SCFAs (especially butyrate), are not available (275, 276) . Dietary amino acids are preferred over glucose as intestinal metabolic fuel, and the systemic availability of dietary amino acids is ultimately one of the biggest determinants of the growth rate of lean body tissues such as muscle (277). And, indeed, both probiotics and prebiotics have been shown to enhance the entry of dietary amino acids into systemic circulation. While the increase in digestion and absorption is modest at around 5%, plasma levels are increased by as much as 30% by the protein sparing effect of SCFAs (278, 279). Given the figure of 50% of amino acids being oxidized in first pass in non-pre/probiotic subjects, for a 200lb person on the standard 1g/lb of bodyweight protein intake, we are talking about the equivalent of an extra 30g of protein per day making it to systemic circulation to be available to your muscles! And, there is more. As we mentioned above, 10-20% of protein is unabsorbed in the small intestine and moves on to the large intestine (with plant proteins being more poorly absorbed than animal ones), which leads us to nitrogen/amino acid recycling by the gut microbiota (280, 281). This recycling is not only of the undigested protein, but also amino acids which have entered the ammonia/urea cycle, generally after having been oxidized for fuel, particularly for the metabolic needs of skeletal muscle (282, 283). Glutamine and the BCAAs are favorites, here (284 , 285). Nitrogen/amino acid salvage and recycling by the gut back into the body amino acid pool is quite substantial, being equal to approximately one-half of total dietary intake (286). The gut microbiota’s recycling of ammonia and urea back into amino acids, especially from glutamine, BCAAs, and EAAs has been found to be on the order of 300+mg/kg/day (287, 288). For our 200lb man, this would be another 27 grams of protein per day reclaimed by the healthy and efficient gut to go toward muscle building. Other studies have found in the 15-30g/day range, but this was with smaller people and smaller intakes than bodybuilding and fitness types (289). Lactobacillus have the best research in this regard, though it is an area absolutely begging for more research (290, 291). This nitrogen recycling seems to be of particular importance in the overnight fasting period when food/protein is not being consumed (292). Basically, it helps you stay anabolic 24-7. All in all, this is massive!! Pun intended. Between greater peripheral delivery of amino acids and nitrogen/AA recycling, we are talking as much as 60g of protein a day, for a 200lb person consuming the typical 1g/lb of bodyweight. This is 2 meals worth of extra protein available to promote muscle growth. Finally, data in animals have shown direct correlations of microbial make-up with superior growth and feed efficiency. There is no such data on humans, as they are not grown for food, yet. Families and genera of butyrate producing genera and species including the aforementioned Bacteroides, Roseburia, and Faecalibacterium prausnitzii were all highly represented on the superior growth and feed efficiency side, as you might expect from what we have learned so far (293, 294, 295, 296, 297). Part 4 on Tuesday, June 26th
Bacteria Bifidobacteria and Lactobacillus are by far the most common and well-known probiotic bacteria. They are commercially available and quite affordable. We can also readily manipulate levels of the good bacteria that are not commercially available such as Bacteroides species, Roseburia species, Akkermansia muciniphilia, and Facealbacterium prausnitzii via diet and supplementation of ingredients that ARE available. More on that later. Bifidobacteria Bifidobacteria are significantly lower in type-II diabetics and have been consistently shown to combat the cycle of LPS inflammation, leaky gut, and insulin resistance (49, 50, 51). They are reinforcing on intestinal epithelial cells and mucosa, improving the physical barrier of the intestine, preventing translocation of pathogenic bacteria and LPS from intestine to body tissue (52, 53). They limit pro-inflammatory signals and increase tight junction proteins supporting mucosal recovery, ultimately restoring normal intestinal permeability and preserving gut barrier function in the face of inflammation (54, 55). Bifidobacteria administration quells general colonic inflammation, particularly from LPS and its downstream signal, TNF-alpha (56, 57, 58). In reducing LPS levels, inflammation induced insulin resistance is reversed in the face of a high-fat diet (59, 60, 61). They shift the composition of the microbiota toward that of a lean phenotype, reducing inflammatory activity and insulin resistance while lowering body fat (62, 63). Bifidobacteria are also extremely important for cross-feeding. This is where one bacterial strain produces metabolites that other species and strains can use for fuel (64, 65). This is particularly important for the bacteria that are not commercially available, which we will discuss in detail in a bit. Bifidobacteria produce acetate and oligosaccharides which are then consumed by these acetate utilizing, butyrate and propionate producing bacteria (66). Faecalibacterium prausnitzii is almost fully dependent on acetate. It converts it to butyrate with 85% efficiency, and its growth is enhanced by co-culture with Bifidobacteria (67, 68). Roseburia is also an acetate user, and it is generally required for its growth (69, 70). Combined with bifidobacteria, Roseburia was able to grow in pure complex carbohydrate cultures, which it cannot metabolize on its own, owing to cross-feeding (71). Cross-feeding with Bifidobacterium also modulates the positive effects of prebiotic oligosaccharides on growth of Roseburia and F. prausnitzii by making acetate available (72). And, butyrate production increases mucins, which are fed on by Bacteroides and Akkermansia, two more great, but commercially unavailable bacteria. Lactobacillus Lactobacillus consistently increases tight junction protein formation and improves intestinal barrier function, ultimately inhibiting systemic inflammation from LPS and its downstream pathways (73, 74, 75, 76, 77, 78). They also increase the levels of butyrate and the other short-chain fatty acids (79, 80). Lactobacillus raises levels of Bacteroides, a propionate producer, and another one of the types of great bacteria that we cannot get commercially (81). They promote favorable intestine morphology, improving parameters such as villus height, crypt depth, mucin expression, and the quantity of goblet cells, all things favoring digestive function and efficiency (82). Lactobacillus decrease LPS, systemically, as well as downstream inflammatory markers including TNF-alpha , IL-6, and COX-2, (83, 84, 85, 86). Relatedly, they also reduce expression of TLR-4, which is basically the “LPS receptor” (87, 88). They have also shown improvements of inflammatory colitis, which is essentially an extreme version of a “leaky gut” (89, 90). The reduction in inflammatory responses downstream of the LPS signaling pathway is a consistent finding in studies with Lactobacillus, including decreased adipose tissue inflammation, further evidence of preventing LPS actions outside of the gut (91, 92, 93, 94). In combating these inflammatory pathways, Lactobacillus lower oxidative stress levels, ultimately improving insulin sensitivity (95, 96, 97). They also increase the insulin sensitizing peptide adiponectin (98, 99, 100). Finally, they specifically improve insulin sensitivity against Western-style, obesity promoting diets (101, 102, 103). In even more direct findings on probiotics and body composition improvements, Lactobacillus have been found to protect the testes from oxidative stress, increasing testosterone levels (104, 105). In fact, testosterone levels were 4-8 times higher in aging mice (a model of chronic, low-level inflammation), given Lactobacillus (106). They have been found to increases growth hormone levels and reduce the expression of atrophy inducing genes (107, 108). They increased weight with the same body fat, meaning more lean mass (109). Lactobacillus dose-dependently increased grip strength, muscle fiber number, and endurance swimming while decreasing muscle tissue breakdown (110). They inhibited increased levels of cortisol in response to stress (111). Finally, Lactobacillus feeding stimulates IGF-1 and decreases myostatin (112, 113, 114). Bacillus, another genus of commercial probiotic, increased goblet cell number, villus length, and mucin synthesis in the intestine (115). This would be expected to improve intestinal mucosal cell proliferation and, ultimately, efficiency of nutrient digestion and absorption (116). And, indeed, improved growth performance and enhanced protein utilization has been found with Bacillus (117). Probiotic Combinations You may have noticed that almost no probiotic formulas contain just a single species of bacteria, nowadays. And, if you did not, I will just say that it is for a good reason. They work better in combination. First of all, microbial diversity seems to be good, in and of, itself. Essentially, a diverse gut is a healthy gut (118). Obesity has been associated with a lack of microbial diversity and, as you might expect, lean subjects have greater microbial diversity in the gut (119, 120, 121). Insulin sensitivity is also improved along with diversity increases (122). Finally, in the interesting but not terribly shocking category, exercise increases microbial diversity (123, 124). Combinations also work to specifically create an environment where probiotic bacteria can thrive, thus enhancing their ultimate performance (125). Compared to individual strains alone, combinations greatly increase adhesion to intestinal mucus, which is necessary for most survival, growth, and activity (126, 127). Conversely, they inhibit adhesion and growth of pathogenic bacteria better when in combination (128, 129). However, you do not want to just throw every single commercially available species and strain into a product as so many companies do. They need to be rationally combined. If not, they can interfere with each other’s actions and compete for space and resources (130, 131, 132). But, perhaps the most interesting benefit of probiotic combinations is through the afore-mentioned cross-feeding of the commercially unavailable bacteria we are about to discuss, right now. The Best Probiotics That Money Can’t Buy Unfortunately, several species of bacteria with some of the very best data are not available commercially, due to regulatory issues and well as practical challenges such as stability and viability of the bacteria themselves. Several groups are working on these, but it will happen later rather than sooner, at best. Fortunately, there are a myriad of ways to specifically target and increase these strains using methods that ARE available. So, let’s take a look at these novel wonder-bacteria. Bacteroides Species Bacteroides are butyrate and propionate producing. Levels were 6-fold higher in lean vs. obese subjects, as well as being reduced in obese patients, in general, compared to control populations (133, 134, 135, 136). Levels in Type-2 diabetics were only half that of subjects with normal glucose tolerance (137). Among various species in the Bacteroides genus, B. uniformis reduced bodyweight gain, triglycerides, and adipocyte volume while improving insulin and leptin sensitivity. It also lowered LPS and other inflammatory signals (138). B. fragilis releases a symbiotic immunomodulatory anti-inflammatory factor called Polysacharride A – kind of an anti-LPS (139, 140). This has been shown not just to prevent but to cure experimental colitis, an extreme version of a leaky, inflammatory gut (141). It has also been shown to prevent demyelination of neurons in the central nervous system, indicative of protection against inflammation well outside of the gut (142). Faecalibacterium prausnitzii Faecalibacterium prausnitzii are butyrate producing and considered a physiological sensor and marker of human health (143). It does not get much more important than that. It is lower in the obese and type-2 diabetics (144, 145, 146). Conversely, it is higher in normal glucose tolerance vs. pre-diabetic subjects (147). Faecalibacterium prausnitzii is also negatively correlated with inflammatory markers and sharply decreased in inflammatory bowel diseases (148, 149). It is greatly reduced in ulcerative colitis and less abundant in Crohn’s disease (150, 151). As would be expected from the above, it improves intestinal barrier function (152). Akkermansia muciniphilia Akkermansia muciniphilia is mucin degrading, meaning it feeds on mucins (153). It is also decreased in obesity and type-2 diabetes. Its administration reduced fat mass, adipose tissue inflammation, and enhances insulin sensitivity. Along with this, improved gut barrier function and increased intestinal endocannabinoid levels were seen (154). This species is also inversely related to fasting glucose, waist-to-hip ratio, subcutaneous adipocyte diameter, plasma triglyceride levels, visceral adipose tissue mass, and insulin resistance (155). Along with enhanced glucose tolerance, it reduced adipose tissue inflammation (156). Akkermansia levels are higher in normal glucose tolerance vs. pre-diabetic subjects (157). It decreases inflammatory cytokine production and protected intestinal barrier function in experimental colitis (158). Finally, its levels are reduced in ulcerative colitis (159). Roseburia Species Roseburia species are butyrate producing (160). An increase in this species is associated with decreased body weight, fat mass, insulin sensitivity, and triglycerides -- independent of calorie intake (161). Increased Roseburia correlated with reduced body weight, improved profile of lipid and obesity related gene expression, along with a normalized inflammatory status (162). It is also lower in type-2 diabetes (163). Levels are increased by a Mediterranean diet, as is insulin sensitivity (164). Finally, its levels display an inverse correlation with disease activity in ulcerative colitis (165). High protein/low carbohydrate diets, which are so effective and popular, reduce Roseburia and SCFA levels, making pro- and prebiotics particularly useful with these (166, 167). Prebiotics Prebiotics are the food for our probiotic bacteria, and they are also the substrates that get transformed into super beneficial short-chain fatty acids like butyrate, so we will look at some data on those as well. Prebiotics have come a long way since oat bran and psyllium husks. Beginning with inulin, a huge array of oligosaccharide and glycan type compounds have been found to be fermented and fed on by intestinal bacteria. These newer prebiotics tend to be basically tasteless and dissolve effortlessly, which is quite handy. With the importance of microbial diversity for optimal gut and body health, we want a number of different prebiotics for them to feed on. Likewise, we want to choose the ones that best increase the bacteria we want to increase, rather than just randomly feeding all of them. Let’s briefly look at some data on the positive effects various prebiotics. Increased Good Bacteria and SCFAs Prebiotics, by definition, increase beneficial bacteria, with data being most focused on Lactobacillus and Bifidobacterium, as they were the earliest studied and most common. Bifidogenic potential was the primary measurement for prebiotic activity until about 10 years ago. Lactobacilli are promoted by a wide range of fibers and oligosaccharides (168, 169, 170). They can also ferment sugars, such a sucrose, fructose, and glucose (171). They are stimulated even by flour (172). So, one doesn’t have to put that much effort into getting them to grow. As you would expect from their growth being how prebiotics were defined, Bifidobacteria also grow quite well on a wide range of commercial prebiotics, with 5-10 fold increases in some subjects being noted (173, 174, 175, 176). The much more interesting prebiotic data is the increases found in levels of the aforementioned commercially unavailable butyrate and propionate producing bacteria via the aforementioned cross-feeding. As we have mentioned, and as you will really see later, butyrate production is probably the single most important way that probiotics and prebiotics ultimately work their magic. And, indeed, prebiotics have been found to not only raise Bifidobacterium counts, but do so concomitant with increased Akkermansia and F. prausnitzii (177, 178, 179). They also promote increases in Bacteroides (180, 181, 182, 183). Other studies have found elevated Roseburia, F. prausnitzii, and Bacteroides together with greater butyrate levels, with total SCFA increases as high as 2-3 fold (184, 185, 186, 187). Other prebiotic studies have shown increased propionate production along with Roseburia levels (188, 189). They have also been found to increase butyrate and propionate to go along with raised bifidobacteria and acetate levels – again, suggestive of cross-feeding to butyrate and propionate producing bacteria (190, 191, 192, 193). Mucins Prebiotics administration has shown 2-4 fold mucin elevations, which would create a positive environment for mucin feeders such as Akkermansia, Roseburia, and Bacteroides (194). Another found prebiotic augmentation of mucin production of 6-fold, leading to large elevations in Akkermansia, Roseburia, and propionate (195). Akkermansia is the most well characterized mucin consumer (196, 197). Verrucomicrobia, of which Akkermansia is the primary genus, was increased from .03% to 5.25% by mucin (198). That is a 175-fold increase, if you are counting. Multiple species of Bacteroides are also mucin degrading specialist, as well (198, 199, 200, 201, 202). A species of Roseburia, R. intestinalis also colonizes the mucosal layer and feeds on mucins (203). With these bacteria colonizing the mucus and being close to the epithelium, particularly with the butyrate producers, bioavailability for epithelial cell regeneration and barrier function is enhanced, so they are especially important and effective. Gut Permeability and Inflammation Prebiotics augment intestinal protein junction assembly, decreasing intestinal permeability and preventing loss of gut barrier function (204). Oligosaccharides also directly displayed a microbiota independent increase in tight junction assembly and improved barrier function (205). Prebiotics decrease LPS and increase epithelial cell proliferation (206, 207). They decrease downstream inflammatory markers triggered by LPS (208, 209). Increases in tight junction proteins and improved barrier function inhibited systemic inflammation in adipose tissue (210). Finally, prebiotics protect against stress induced LPS inflammation and activity (211, 212). Insulin Sensitivity and Protein Sparing Along with decreased LPS and inflammation, prebiotics reduced plasma glucose (213). They improved glucose tolerance by reducing oxidative stress and low-grade inflammation (214). Prebiotic inhibition of LPS target Toll-like Receptor 4 (TLR4), and downstream inflammatory affecter TNF-alpha, improved insulin sensitivity (215, 216). They also improve post-prandial blood glucose and insulin levels as well as improving glucose uptake in insulin resistant cells (217, 218, 219). In addition, by supplying SCFAs, the preferred fuel of the enterocyte, prebiotics reduce protein fermentation in the gut (220, 221, 222). This spares amino acids for more useful purposes like building muscle as well as preventing formation of toxic breakdown products (223). We will talk a good bit more about protein sparing, later. Polyphenols as Prebiotics Less well known than with typical prebiotics, polyphenols are also fermented by the gut microbiota. Polyphenols are generally prebiotic for good bacteria (Bifidobacterium, Akkermansia, Bacteroides, and Roseburia), and antibacterial for less favorable and pathogenic ones (224, 225, 226). Fruit/berry based polyphenols seem to be particularly favorable toward Bacteroides and Akkermansia growth compared to other polyphenol sources (227, 228, 229). Lactobacillus lack glycan degrading enzymes, thus do not grow on them particularly well compared to the others, so they are especially targeted to butyrate producers (230). Fermentation of herbs and such containing polyphenols also transforms them, resulting in much higher concentrations of active compounds compared to unfermented (231). This same fermentation is done in the body, but it is highly dependent upon the microbial make-up of the individual’s gut, so it can vary widely from person to person (232, 233). As an example, a fermented herb preparation inhibited LPS mediated inflammatory damage, while the unfermented one was ineffective (234, 235). The same was true for insulin sensitivity (236, 237). So, not only do polyphenols increase good bacteria, but the good bacteria make the polyphenols work better. The prebiotic effect plus the transformation into more active compounds is why polyphenols so consistently show a myriad of great benefits despite supposedly being so poorly bioavailable. Mechanisms of Action With the background information and general overview out of the way, we will now get deeper into the mechanisms of how this affects muscle mass. There are a lot of interacting pathways and systems here, though much of it comes down to inflammation and butyrate, both inside of the gut and outside. First, we will talk about fixing the gut, itself, both the inflammatory signaling (LPS et al) as well as the intestinal barrier that prevents them from escaping. Within the gut, we will also discuss protein sparing and absorption/utilization improvements from a healthy microbiota and gut. Then, we will talk about anabolic and anti-catabolic pathways outside the gut. Part 3 on Thursday, June 21st