There are way more bacterial cells living in our gut than the total number of our own cells in our entire body. We are, so to speak, colonized. These gut microbes turn out to be incredibly important. Anyone who has been on antibiotics, which kill many of these bacteria, can attest to the stomach misery caused by upsetting the balance of these little lodgers. Growing evidence suggests that too many of the wrong bugs can cause obesity.
We are born with a pristine intestine, literally sterile. However, it is immediately invaded by the bacteria in mother's milk and environmental bacteria introduced by bottle. The average adult harbors between 1,000 and 1,500 bacterial species, 160 of which constitute the core group or what's called the core microbiota.
Researchers have noticed that altered gut microbiota is associated with diseases that became prevalent in the 21st century. For instance, a reduced diversity of these bugs is seen in inflammatory bowel disease, metabolic syndrome (prediabetes) and obesity. Specifically, the number of Firmicutes was increased, and the number of Bacteroidetes was reduced in obese people compared with lean folks. Interestingly, weight loss by dieting eliminated those differences. These two types of bacteria represent over 90% of all bacterial cells found in the human intestine.
So how do these critters make us fat?
Diet, not surprisingly, has a profound effect on what grows in our gut. Switching from a lean diet to a high-fat Western diet dramatically alters the microbiota in a negative way. These changes are incredibly fast, starting in the first 24 hours of the introducing the new foods.
Once the "bad" bacteria overpopulate, it is easier to absorb calories from the gut. The bugs provide an increased capacity not only to breakdown nutrients, but also make the gut wall more permeable. This allows more nutrient absorption (mostly glucose i.e. sugar). They also exert their influence beyond the gut promoting fat storage throughout the body by a variety of mechanisms including the altering of hormone levels responsible for orchestrating appetite, satiety, and fat metabolism.
What can we do?
We are still learning how best to harness probiotics. Different strains of lactobacilli (gasseri is one) have been shown to decrease fat and the risk for type 2 diabetes by increasing insulin sensitivity. Inulin-type fructans (found in fruits and vegetables) reduced weight, appetite, and blood sugar levels, and increased insulin sensitivity.
But the bottom line remains, if eating meats, make them lean. The fresher and less altered the food, the better, in part because it will have a positive effect on the gut microbiota. Lots of local vegetables, and fresh dairy with live cultures are best for the same reason.
Probiotics are no con job. Just do your homework before collecting bottles of preparations in your medicine cabinet. We are just beginning to understand these microbes.