Metabolism—The Importance of Short-Chain Fatty Acids
The role of the gut microflora in metabolic function is now well recognized. The amount of energy you harvest from the foods you eat is largely determined by the type of bugs you have in your gut. Let’s look at what this means and why it’s important.
Eating is an interesting process. As you chew your food, salivary enzymes are added to it, turning it into a substance called chyme. This is then swallowed, and the chime moves down to your esophagus and drops into your stomach, where it’s exposed to hydrochloric acid and additional enzymes that pull it apart into its chemical constituents. These are delivered to your small intestine, where most of the nutrient absorption in your body occurs. What’s left over drops into your large intestine, where the vast majority of your gut microflora live. Then your bugs have a feast. One of their favorite foods is fiber. Human beings can’t digest fiber without the help of our flora. That’s why a certain amount of fiber simply passes through us—what our bugs don’t eat comes out in our poo. This is one among many reasons fiber is such an important dietary substance. Without enough fiber in your diet, your flora starve, biodiversity plummets, and dysbiosis can set it. As your gut flora eat fiber, they break it down into short-chain fatty acids (SCFAs), which are extremely important to human health. SCFAs increase the gut’s absorption of water, regrow gut cells, and may provide defenses against colon cancer, inflammatory bowel disease, and more.
The amount of SCFAs your gut bugs produce is incredibly important—as usual, balance is the key. Too few SCFAs and you don’t get their protective benefits. Too many and weight gain, glucose imbalance, and increased triglyceride production may result. As their name indicates, SCFAs are fatty acids that are energy dense and calorie rich. The more short-chain fatty acids you produce, the more calories in your diet. In normal humans, SCFAs provide from 80 to 200 calories per day, depending upon the amount of daily dietary fiber intake. However, certain species of your gut microflora overproduce SCFAs from carbohydrates, so there is interspecies variation in the efficiency of the fermentation of fiber to SCFAs. Remember the Firmicutes bacteria from our discussion about gut bug fingerprints? They’re thought to be fat-forming gut microflora by most scientists today, and the SCFA connection tells us why.
A shift of either 20 percent more Firmicutes or 20 percent fewer Bacteriodetes results in more SCFAs extracted from fiber and absorbed to provide a gain in caloric energy. Why do the Firmicutes bacteria produce more SCFAs? They overdigest the fiber you eat. That means less is left undigested as fiber that comes out in your poo and more SCFAs are absorbed and turned into energy in your body. What happens with the additional energy you can’t use? It gets stored as fat by a complex mechanism, but this is likely the reason SCFAs trigger increased triglyceride production in your liver.
But this isn’t the only way that Firmicutes influence your metabolism. A new study68 shows that this class of bacteria encourages your body to also absorb more of the fat you consume in your diet, creating a double whammy of increased SCFA production and increased dietary fat absorption that has been concretely associated with weight gain.
These facts alone show that the calories in/calories out diet approach is hopelessly out of date—there’s no way for you to accurately track how many calories your gut bugs are harvesting for you. It also shows that the type of food you eat is probably much more important than the amount.
Does this mean you should eat less fiber, so your food won’t be converted into SCFAs? No! When you eat a high-fat, low-fiber diet, your gut bug profile swings toward Firmicutes and ultimately turns into fat. Once these fat-forming bugs take over, what little fiber you do eat is overdigested and turns into more fat—all of which is another step in the wicked cycle of weight gain. Fiber is not your enemy. In fact, it’s one of your dearest dietary friends. You just need to balance it with proper amounts of healthy fats, high-quality protein, and low-glycemic carbs so that it can do its job correctly.
As if runaway Firmicutes and the problems associated with it weren’t enough cause for concern, studies have now shown that your gut microbiota govern another important regulator of lipid metabolism called angiopoietin-like protein 4 (ANGPTL4), also known as Fiaf (fasting-induced adipose factor).69 ANGPTL4 helps regulate the proportion of triglycerides deposited in your adipose tissue (your fat). When too little of it is floating around in your body, more triglycerides are sequestered as fat. Research on ANGPTL4 is in its early days, but studies show that the type of bugs in your gut seems to influence how much of the protein is in your blood. This may be another mechanism by which your flora impact your metabolism and your ability to gain or lose fat.
This is all more evidence that your gut flora have a significant impact on your ability to lose weight.