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.
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.
Researchers have long known that the human body is home to all manner of microorganisms, but only in the past decade or so have they come to realize that these microbes outnumber our own cells 10 to one. Rapid gene-sequencing techniques have revealed that the biggest and most diverse metropolises of “microbiota” reside in the large intestine and mouth, although impressive communities also flourish in the genital tract and on our skin.
Each of us begins to assemble a unique congregation of microbes the moment we pass through the birth canal, acquiring our mother’s bacteria first and continuing to gather new members from the environment throughout life. By studying the genes of these various microbes—collectively referred to as the microbiome—investigators have identified many of the most common residents, although these can vary greatly from person to person and among different human populations. In recent years researchers have begun the transition from mere census taking to determining the kind of jobs these minute inhabitants fill in the human body and the effect they have on our overall health.