For the 35 percent of American adults who do daily battle with obesity, the main causes of their condition are all too familiar: an unhealthy diet, a sedentary lifestyle and perhaps some unlucky genes. In recent years, however, researchers have become increasingly convinced that important hidden players literally lurk in human bowels: billions on billions of gut microbes.

Throughout our evolutionary history, the microscopic denizens of our intestines have helped us break down tough plant fibers in exchange for the privilege of living in such a nutritious broth. Yet their roles appear to extend beyond digestion. New evidence indicates that gut bacteria alter the way we store fat, how we balance levels of glucose in the blood, and how we respond to hormones that make us feel hungry or full. The wrong mix of microbes, it seems, can help set the stage for obesity and diabetes from the moment of birth.

Fortunately, researchers are beginning to understand the differences between the wrong mix and a healthy one, as well as the specific factors that shape those differences. They hope to learn how to cultivate this inner ecosystem in ways that could prevent—and possibly treat—obesity, which doctors define as having a particular ratio of height and weight, known as the body mass index, that is greater than 30. Imagine, for example, foods, baby formulas or supplements devised to promote virtuous microbes while suppressing the harmful types. “We need to think about designing foods from the inside out,” suggests Jeffrey Gordon of Washington University in St. Louis. Keeping our gut microbes happy could be the elusive secret to weight control.

Nature. 2006 Dec 21;444(7122):1022-3.

Ley RE1, Turnbaugh PJ, Klein S, Gordon JI.

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