Short-chain fatty acids (SCFA) epitomize the notion that when we eat, our microbes are eating too. The bacteria in our gut ferment our food, synthesizing three main types of SCFA—acetate, propionate, and butyrate—in roughly a 70:20:10 ratio. The ratio and amount of SCFA produced is contingent upon the type of microbes in the intestine as well as the type of foods eaten. Both, of course, vary from one person to another. Butyrate is the primary fuel supply for the cells of the colon, while propionate and acetate readily cross pass through the wall of the large intestine to be converted into carbohydrates and fats, including cholesterol. SCFA are indirect nutrients, and they have immune and metabolic roles. Acetate, for example, is involved in stimulating the liver to make lipids, while propionate blocks lipid metabolism to favor carbohydrate (gluconeogenesis) synthesis. While butyrate is involved in changing the pH of the colon to prevent bad bacteria from growing, it also suppresses inflammation of the colon. Butyrate keeps NF-κβ (nuclear factor kappa Beta) from driving inflammation.
Microbial synthesized SCFA also have fat-fighting properties. They can latch onto G protein-coupled receptors (GPR43 and GPR41), which are found on immune cells and fat cells. Acetate, for example, is known to suppress intestinal inflammation by docking onto GPR43 in the gut. Omega-3s exert their anti-inflammatory effects, in part, by docking onto the G protein-coupled receptors.
The main way that high-fiber diets reduce appetite and lower body weight is by increasing SCFA binding to GPR41, which then bind to endocrine cells of the gut. Simultaneously, this action reduces intestinal inflammation and fat storage, improves blood-sugar regulation, and improves appetite. Animal and human studies have shown how prebiotic dietary fiber, such as inulin, is able to increase levels of incretins PYY and GLP-1.
A recent study compared levels of SCFA and fecal microbiota between European children and children who live in a rural African village and eat a diet rich in plant fibers with no processed carbohydrates. Among the many differences between these groups, it was observed that children of rural Africa have increased levels of gut microbes, including Bacteroides and Faecalibacterium, which are known to produce healthy SCFA. These children had nearly double the amount of these anti-inflammatory SCFA in their intestines and in a more balanced ratio among the different fatty acids compared to European children. European children had increased acetate-to-propionate ratio; acetate being a main substrate to synthesize cholesterol and other lipids.
Studies in humans suggest that overweight and obese people have an imbalance of SCFA, possibly too much propionate, which is involved in forming fat cells. This may be due to the high saturated-fat content of the diet, which skews the gut microflora balance. Research does suggest that such a diet reduces the number of bacteria in the gut, while decreasing production of healthy SCFA. In contrast, studies suggest that propionate and butyrate may offer protection against obesity by increasing gut satiety hormones and reducing inflammation. Butyric acid and acetic acid, on the other hand, are anti-inflammatory, inhibiting the growth of pathogenic E. coli O157:H7 and decreasing cancer risk.
Lastly, SCFA in the intestine may increase leptin. Fat cells can sense increased SCFA from the diet, leading to increased leptin levels, which may drive inflammation and boost appetite.
Some of the best ways to simultaneously increase the release of fat-fighting intestinal metabolic hormones, such as GLP-1, and balance the gut microflora to prevent absorption of endotoxin and increase healthy levels of SCFA are to consume plant-based polyphenol compounds, prebiotic fiber, probiotics, and pea and whey protein.
The metabolically healthy, fat-fighting Bacteroidetes in the gut contain a significant amount of enzymes that are involved in the breakdown of polyphenol from colorful foods such as berries, green vegetables, and teas.
In contrast, Firmicutes bacteria, which increase in obesity, contain a smaller proportion of enzymes known to degrade these polyphenols. Therefore, these bacteria don’t flourish in the presence of polyphenol foods. Moreover, polyphenols may have suppressive effects on the growth of fat-promoting Firmicutes in the gut. So by eating a diet rich in colorful vegetables and fruits, one is going to favor the proper balance of gut microbes—that is, a high Bacteroidetes/Firmicutes ratio—optimal levels of healthy SCFA, a sufficient gut barrier function, and few endotoxin.
There are no polyphenols in processed cereal grains. By eating processed grains, you enhance the growth of bacteria that thrive on carbohydrates and risk creating inflammation, gastrointestinal permeability, leptin resistance, and obesity.
1) Belly Fat Effect: The Real Secret About How Your Diet, Intestinal Health, and Gut Bacteria Help You Burn Fat
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