Those of you who know me well know that I enjoy learning things by studying poop, and one of the great things in poop that we can exploit is the remnants of gut bacteria. A healthy bouquet of bacteria is necessary for proper digestion and intestinal function. The bouquets vary from person to person, and the causes and consequences of interpersonal variation in gut bacteria communities aren't very well understood. However, there are some correlations. Studies in mice and humans show that individuals with less diversity in their gut bacteria tend to be obese, and that certain types of bacteria seem to be present more often in obese individuals. This effect could be stimulated in mice by feeding them antibiotics, which kill off portions of the gut bacteria, and the mice treated with antibiotics had more body fat than non-treated mice. In addition to obesity, people with less diversity in their gut bacteria (measured by quantifying the bacterial DNA in stool samples) also were more likely to be resistant to insulin, a symptom of type II diabetes which is also often correlated with obesity.
It has always been assumed that the gut bacteria will "bounce back" or spontaneously re-colonize the gut after an antibiotic regimen or a brief infection, but some people are starting to question this assumption. Post-infectious irritable bowel syndrome is believed to be caused by a disruption of the gut flora following a bout of gastroenteritis. The idea is that the infectious bacteria flush out the good bacteria by competition or flushing with diarrhea. A popular treatment for this imbalance in gut bacteria is to take probiotics, either in pill form or in the form of active culture yogurt and dairy products. However, most of these products only contain one or two species of known gut bacteria. It seems that a healthy digestive system is associated on a diverse bouquet, which could potentially be problematic if we're overloading our guts with only a few species of bacteria, but is unknown as to whether gut bacteria is a factor in causing obesity or if it is somehow a result of obesity.
Pennisi, E. (2011). Girth and the Gut (Bacteria) Science, 332 (6025), 32-33 DOI: 10.1126/science.332.6025.32
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