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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 (238239). However, butyrate is significantly more potent (240241). 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 (243244). 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 (247248). 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 (249250).

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 (253254). 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 (255256). 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 (258259260). 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 (264265).

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 (266267). 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 (275276) . 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 (278279). 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  (280281).  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 (282283).  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 (287288). 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 (290291). 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 (293294295296297).

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