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Friendly, probiotic bacteria are astonishingly important to the health of their human hosts. Living mostly in the gut, these microbes keep pathogenic bacteria from establishing a foothold in the body. They multiply to provide bulk for stool, easing bowel movement. They convert harmful compounds in our food into nutrients, they synthesize anti-inflammatory agents, they make vitamins, they reduce the toxicity of gluten, and they synthesize sex hormones and neurotransmitters. A growing body of research finds that some strains of bacteria even help to keep us from gaining excess weight.
The problem is that other strains of bacteria may make it easier to put on belly fat than to take it off.
Icky Observations That Led to Discovery
Even though scientists recognized that Lactobacillus bacteria regulate serotonin levels in the 1920's, the observation that gut bacteria could have a relationship to belly fat is much more recent. Scientists noticed that lean lab rats that ate the droppings of obese lab rats developed obesity themselves, but feeding them the feces of lean lab rats helped them return to their normal weight. This disgusting form of a "fecal transplant" showed that there are clear categories of obesity-inducing and obesity-protective gut bacteria in mice.
Humans, however, have different feeding habits, and the roles of different species of bacteria in human obesity are not quite as clear. At first researchers looked for a relatively complicated explanation of the role of gut bacteria in weight control. A series of studies found that a higher ratio of Firmicutes bacteria, the "bad" gut bacteria, to Bacteroidetes bacteria, the "good" bacteria, was associated with obesity. Firmicutes bacteria form tough spores that can survive for long periods in dry conditions, and are the culprits in spoiling wine and beer. Bacteroidetes live in moist environments everywhere, in the soil, in water, on the skin of animals, and the human digestive tract, and can cause disease when the immune system is weak.
Later studies, however, found that the ratio of the two different kinds of bacteria was not as important as the total number of Firmicutes bacteria. However, these studies are correlational, not causational. They don't tell whether Firmicutes bacteria cause obesity or obese people tend to have more Firmicutes bacteria. Endocrinologist Dr. David Bell goes so far as to say that "in humans the changes in gut microbiota are an association with rather than the cause of obesity." However, we don't have to take the esteemed doctor's word for this. We can look at how things we can control affect our weight.
Fecal Transplants, You Will Be Glad to Know, Don't Induce Weight Loss
Physicians in the Netherlands gave 18 obese men who had metabolic syndrome (prediabetes, high cholesterol, high triglycerides, and wide waistlines) fecal transplants from lean donors. The recipients of the healthy donors' gut bacteria did not lose weight, but they had better blood sugar control and lower triglycerides. These benefits stopped about 12 weeks after the fecal transplants, probably because the recipients did not change their diets. If introducing good bacteria can improve metabolic health, then can antibiotics worsen it?