Are Belly Bugs Making You Fat?

Some stomach bacteria may contribute to obesity, studies show

by Diane Lange
Photograph: Illustrated By Aad Goudappel

In 2006, researchers at Washington University in St. Louis found that both tubby mice and over-weight people have a much higher proportion of one type of gut bacteria than their lean counterparts. Similarly, in a recent study in an Old Order Amish community, University of Maryland scientists learned that 26 species of bacteria are found in different relative amounts in obese people with metabolic syndrome than in their thinner family and neighbors. (This small, close-knit group of Amish was studied because its members eat similar diets and are genetically very much alike.)

Other research shows that the previously obese can differ from their heavier selves: People who undergo gastric bypass surgery—and therefore drop a lot of weight—end up with different bacteria than they had before the surgery.

Does this mean it may be possible to alter your waistline by manipulating your bacterial world? Intriguing animal research points in that direction. When Washington University researchers placed bacteria from fat mice in the bellies of germ-free mice, the germ-free creatures became chubby, too. And Emory University scientists discovered unhealthy bugs made lab mice eat 10 percent more than mice that weren’t infected.

It’s also possible that blocking some effects of bacteria may prevent weight gain. University of Chicago researchers recently found that mice that lacked lymphotoxin, an immune factor in the gut that interacts with bacteria, didn’t gain an ounce after eating a high-fat diet for nine weeks; a group of normal mice that did have lymphotoxin added weight, mostly in the form of fat tissue.

What’s going on? One theory is that certain bacteria contribute to obesity by affecting how excess calories are used either by bacteria or by host cells. Given an assist by their particular bacterial colonies, obese people may be better than thinner ones at deriving energy (calories) from food, says Jane Foster, PhD, an associate professor of psychiatry and behavioral neurosciences at McMaster University.

In other words, possibly because of the bugs in their guts, some people will gain more weight than others when eating the same diet.

The hope is that popping a daily probiotic supplement might change your bacterial popula-tion in a way that neutralizes a tendency to pack on pounds. While this isn’t conventional weight-loss advice, Foster suggests that some people who have long been struggling with the scale try probiotic supplements. Adding a daily dose of one of the Bifidobacterium or Lactobacillus strains may give you an edge, she says. However, exercise and cutting calories is still the bottom line when it comes to trimming yours.

Read Probiotics: A Surprising Way to Beat Stress

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