Scientists have become increasingly interested in how bacteria living in our large intestines influence our health. There are, in fact, more bacteria in our large intestines than there are cells in our bodies. Many diseases have been known for some time to relate to shifts in what’s called the microbiome, or regular population of gut bacteria. Clostridium difficile colitis, for example, occurs when antibiotics reduce the population of “good” bacteria in the microbiome and enable a “bad” bacteria, clostridium difficile, to multiply to the point where it causes disease. Observational studies have begun to show links between certain microbiomes and other diseases including asthma and certain autoimmune diseases. It seems each person’s microbiome is like a fingerprint in that it’s unique to each individual.
A recent observational study now shows a link a between body weight and the diversity of bacteria species in a person’s colon. In this study of twins, the greater the diversity of bacterial species in the study participants’ colons, the lower their levels were of visceral fat (the type of fat most strongly linked to the risk of cardiovascular disease and diabetes). Unfortunately, because this was an observational study, it’s not clear what is causing what. That is, it could be people who have less visceral fat to start with develop greater species diversity in their intestines.
On the other hand, a previous case report described a thin woman who had clostridium difficile colitis that was successfully treated with a fecal transplant (where another person’s stool is injected into the colon of the patient to give them back the “good” bacteria that holds clostridium difficile at bay) and then subsequently and for the first time in her life became obese.
We aren’t yet at the place where we know how to do fecal transplants to treat obesity, but these observations do suggest the possibility that at least part of the cause of obesity in some people relates to the composition of their microbiome. (Interestingly, our microbiomes are profoundly influenced by the microbiomes of our mothers, possibly leading to another way obesity might be passed on from one generation to the next). Given the incredible complexity and redundancy of mechanisms known to regulate weight, it’s becoming increasingly clear that obesity is anything but a simple disorder, and certainly isn’t related only to a person’s eating behavior.