Deep within our digestive tract, a huge colony of microscopic organisms thrives. This ecosystem allows us to digest and absorb nutrients. Now, researchers think the specific types of bacteria that populate the gut may relate to obesity.

“This is an area of intense research and interest, and it’s really moving quickly,” says Dr. Michele Woodley, a gastroenterologist on staff at Missouri Baptist Medical Center. “But in humans right now, the relationship between the bacteria in your gut—and in particular, your weight—isn’t clear.”

Woodley notes that most of the research done to date has involved mouse models. However, these studies yielded fascinating results that may provide important clues to how our intestinal bacteria affect weight.

When groups of obese mice and lean mice are separated from one another and “the skinny mice eat their regular healthy diet and the fat mice eat their regular unhealthy diet; if you look at the stool from each of these mice, they have different bacteria,” Woodley explains. “If you then feed the fat mice a healthy, skinny-mouse diet, they will not lose weight. But if you put the fat mice in the same cage as the skinny mice and then feed them the healthy diet, two things happen: they develop the skinny-mouse bacteria and they lose weight.”

If the previously obese mice, which now have the same type of intestinal bacteria as the lean mice, are again isolated from their naturally lean counterparts but continue to eat a healthy diet, they remain leaner. “But if you feed them a lousy diet, they gain weight; and their bacteria changes again,” Woodley says. “Then when you give them the healthy diet, they no longer lose weight.”

While the rodent studies are intriguing, there is still much to learn, says Dr. Mario Morales, a bariatric surgeon with SSM Weight-Loss Institute. In humans, there has been some preliminary research that suggests that some types of diets, such as those high in fiber and protein and low in carbohydrates, may generate a more diverse bacterial environment that supports a more normal weight.

“Obese individuals tend to have less of a diverse bacterial environment,” he says. “That’s led to the concept that bacteria in the intestines may play a role in weight stability and promotion of an obese profile versus a leaner profile.” Yet scientists have not identified specific microbes that clearly affect obesity.

One theory that could help explain the apparent relationship between gut bacteria and weight involves the role of inflammation in the digestive tract. Diets high in processed carbohydrates and saturated fats are correlated with an increased degree of intestinal inflammation. As the body produces an overgrowth of bacteria needed to break down and absorb a highly processed diet, an inflammatory process begins that may damage other types of bacteria, thus reducing the overall diversity of the bacterial environment.

“People need to understand that what we eat affects the bacteria in our system, which ultimately determines our overall general health,” Morales says. “So you want to assist the bacteria you have in your system, and the best way of doing that is by having a varied diet, particularly including protein, vegetables and fruit. Those food groups seems to promote the most diverse intestinal bacterial environment. Good gut health leads to overall better health.”

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