Don’t Just Blame Calories

The grapefruit diet, the Atkins diet, low-fat diets, low-carb diets, the cabbage-soup diet: they and all the other fad diets make the health establishment roll its collective eyes. The only way to lose weight, says every reputable textbook and medical society, is to burn more calories than you consume. And if you are adding pounds, the reason is, pure and simple, that you are consuming more calories than you expend. Weight gain is a straightforward matter of calories in minus calories out, they maintain.

But while the basic math is right, 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.

The bacteria-made-me-fat idea has been gathering steam since 2006. In that year, Jeffrey Gordon of Washington University and colleagues reported in a paper in Nature that obese mice and slim mice have different populations of gut bacteria. Crucially, they showed that the bacteria caused obesity, rather than obesity producing a specific mix of bacteria. When the scientists plucked bacteria called Firmicutes from obese mice, then put them in the bacteria-free guts of mice raised in a sterile environment, the latter bulked up within 10 to 14 days—even though they ate less...