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Gut reaction: Are intestinal bacteria making us fat?

In her most recent Newsweek column, science reporter Sharon Begley summarizes some of the intriguing new research that suggests that weight gain — and obesity — may not be simply a matter of taking in more calories than you expend:[W]hile the basic m

In her most recent Newsweek column, science reporter Sharon Begley summarizes some of the intriguing new research that suggests that weight gain — and obesity — may not be simply a matter of taking in more calories than you expend:

[W]hile the basic math is right [calories in minus calories out], the meaning of “calories in” isn’t what we’ve been taught, according to a growing pile of studies of chubby mice, obese people, svelte mice, and slim people. The calories that matter are not simply the number printed on grocery items, fast-food menus, and those guilt-inducing signs next to Starbucks’ brownies. The calories that count are those extracted by your digestive enzymes and — as more and more research is showing — the trillions of bacteria in your intestine. People whose gut bacteria are better at digesting fats and carbs than their neighbor’s will absorb all 1,500 calories in a Friendly’s Ultimate Grilled Cheese BurgerMelt, while the neighbor will absorb fewer. So even in people with identical metabolisms, the effects of eating identical foods can be different.

Animal studies suggest that the bacterial “bad guys” — at least, in terms of obesity — are Firmicutes, which, reports Begley, “are more adept at liberating calories from food” than another type of common bacteria found in the gut, the Bacteroidetes. Both obese rats and obese people tend to have more Firmicutes than Bacteroidetes, and when people (and rats) lose weight, the balance between these two bacteria tips away from Firmicutes and toward Bacteroidetes.

Is it possible, therefore, to do the reverse — to influence that bacterial balance in a way that could lead to weight loss? Research offers some “tantalizing” clues, says Begley, but scientists first have to figure out how intestinal bacteria might affect weight. (One possible explanation: by altering the immune system.)

In the meantime, we shouldn’t take this new line of research to suggest that it doesn’t matter what — or how much — we eat. (And beware of companies hawking bacteria-laden weight-loss products; the science has a long, long way to go before it gets to that stage.) But perhaps it’s time to acknowledge that our personal and societal struggles with weight are probably going to require additional new strategies. We still have a lot to learn on this topic.

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Writes Begley:

As scientists work out the details by which out gut bacteria make us fat, health mandarins need to look beyond the simplistic calories in/calories out mantra for explanations of the obesity epidemic. Yes, of course our 21st-century diet (Outback’s 1,565-calorie Bloomin’ Onion, anyone?) and couch-potato ways are among the culprits. But so might be less obviously stupid food choices that encourage the proliferation of the gut bacteria that squeeze every calorie out of what we send their way. “I think the idea that foods, drugs, or other things in our environment might contribute to the epidemic by changing gut microbes is a distinct possibility,” says [University of Cincinnati obesity researcher Randy] Seeley. With little progress being made in the fight against obesity (a new report from the Trust for America’s Health and the Robert Wood Johnson Foundation finds that the prevalence of adult obesity rose in 28 states over the past year and fell in none), bold new ideas are needed more than ever.

(For another take on this research, read this recent L.A. Times article by freelance writer Amber Dance.)