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Can you tell me about the role bacteria play in our lives?

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Can you tell me about the role bacteria play in our lives? I read once about a woman who got a fecal transplant from her obese daughter. Talk about unintended consequences: The woman became obese. Is that possible?

—Art Erickson

The role bacteria play in our lives? More like the role we play in theirs. The human body is made up of 10 trillion cells governed by about 23,000 genes; the microorganisms that reside within it, mainly in the digestive tract, account for 100 trillion cells and about 3 million genes—we're mightily outnumbered in our own innards. This gut microbiota, as they're collectively known, doesn't present a unified front, though: Its constituent species compete for resources, and you may be seeing the results of those skirmishes every time you step on the scale.

As science searches for direct ways to help people lose weight (or at least accumulate it more slowly), scrutiny has turned to those critters deep inside you that affect digestion and fat storage. Or, more frequently, the equivalent critters deep inside mice. Among the key players in gut bacteria research are "germ-free" mice, bred and raised in hermetic isolation to have no microbiota at all. Compared to normal germy mice, germ-free mice have to eat 30 percent more calories to maintain the same body weight, and they don't gain weight even on high-calorie, high-fat diets. A 2004 study found that conventional mice had 42 percent more body fat than their GF peers; when gut microbiota from conventional mice were transplanted into GFs, their body fat jumped up by 60 percent in 10 days. When GF mice receive bacteria transplants from obese mice, they grow obese, while transplanting bacteria from lean mice keeps them lean. And a new mouse study from December suggests that gut bacterial colonies could be responsible for rebound weight gain after dieting—back when human ancestors endured feast-and-famine cycles, hosting bacteria that helped maintain your baseline weight might have been an evolutionary edge.

The microbiota sure seems to be doing something weightwise, then, and it's been suspected for a while that the balance of bacterial species may be a key factor in this. Around 90 percent of the bacteria in the human gut hail from one of two phyla, Bacteroidetes and Firmicutes. The proportional size of each population varies widely, though—genetics, diet and weight all seemingly play a role, as do things like use of antibiotics—and a number of studies suggest that metabolic issues often turn up in tandem with a high ratio of Firmicutes to Bacteroidetes. Obese mice have more Firmicutes and fewer Bacteroidetes, but if you put put those mice on a fat- or carbohydrate-restricted diet for a year, the Bacteroidetes take over. And researchers who measured the energy left over in human poop (by burning it—and you complain about your job) found that a 20 percent increase in the Firmicutes-Bacteroidetes ratio meant an extra 150 calories got absorbed from food daily. An imbalance of bacteria might not only affect fat storage and vitamin metabolism, but also how full we feel after eating.

Though the F-B balance doesn't correlate consistently with obesity itself, there's a stronger association with other obesity-related health problems. Type-2 diabetics seem to have fewer Bacteroidetes than nondiabetics, possibly due to how bacteria in the colon help digest dietary fiber. Obese and pre-diabetic persons ferment fiber into greater amounts of a chemical called butyrate—produced mainly by Firmicutes—which affects liver glucose levels and fat production. More broadly, your gut bacteria balance may influence your potential for developing nonalcoholic fatty liver disease, and could be the root cause for the increase in cancers, especially liver cancer, that disproportionately strike the obese. But tinkering with this stuff can lead to other trouble: A 2015 Austrian paper found that attempts to manipulate the microbiota balance via fasting could cause degradation of the intestinal mucus—aka the stuff that keeps waste matter out of your bloodstream.

OK, OK, so how do you alter your gut-bacteria demographics to shed some pounds? The most extreme option for obese folks is a fecal microbiota transplant from a leaner donor, a therapy still in its experimental stages. As discussed here in a 2014 column on probiotics, introducing bacteria from the stool of a healthy volunteer has worked wonders with patients suffering from certain serious digestive-tract issues. But though studies are underway, researchers haven't yet been able to induce weight loss through the same procedure—and in fact, as Art reports above, there has indeed been a case where a woman's body mass index jumped into the obese range following a fecal transplant from her daughter.

There are, of course, less drastic methods. Claims for the benefits of probiotic supplements struck me as overblown back in 2014; since then, some small-scale studies on probiotics and weight loss have looked more promising, but it's still early. For now, you could always just ditch highly processed foods—a more natural diet means more nutrients get digested in the large intestine, where the "good" bacteria can do their thing more readily. That might take a little discipline, but until the crap-transplant people get their act together, it's probably your best bet.

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