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Instead, it is becoming clear that some people’s guts are simply more efficient than others’ at extracting calories from food. When two people eat the same 3,000-calorie pizza, for example, their bodies absorb different amounts of energy. And those calorie-converting abilities can change over a person’s lifetime with age and other variables.
The question is, why? And is it possible to make changes, if a person wanted to?
If so, the solution will involve the trillions of microbes in our intestines and how they work in concert with another variable that’s just beginning to get attention. The immune system determines levels of inflammation in the gut that are constantly shaping the way we digest food—how many calories get absorbed, and how many nutrients simply pass through.
The relationship between microbes and weight gain has long been overlooked in humans, but people have known about similar effects in animals for decades. After World War II, antibiotics became affordable and abundant for the first time. Farmers began giving the drugs to their livestock—for example, to treat a milk cow’s infected udder—and noticed that animals who got antibiotics grew larger and more quickly.
This led to a flood of patent applications for antibiotic-laden foods for all sorts of livestock. In 1950, the drug company Merck filed a patent for “a method of accelerating the growth of animals” with “a novel growth-promoting factor” that was, simply, penicillin. Eli Lilly patented three new antibiotics to mix into the feed of sheep, goats, and cattle because the microbe-killing agents “increased feed efficiency.” In the ensuing decades it became standard practice to give livestock copious doses of antibiotics to make them grow faster and larger, even though no one knew why this happened, or what other effects the practice might have.
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In 2006, Jeffrey Gordon, a biologist at Washington University in St. Louis, reported that the microbiomes of obese mice had something in common: Compared with their lean counterparts, the heavier mice had fewer Bacteroides and more Firmicutes species in their guts. What’s more, biochemical analyses showed that this ratio made the microbes better at “energy harvest”—essentially, extracting calories from food and passing it into the body. That is, even when mice ate the same amount and type of food, the bacterial populations meant that some developed metabolic problems, while others didn’t. Similar bacterial patterns have since been confirmed in obese humans.
What’s more, Gordon found, the microbiome associated with obesity is transferable. In 2013, his lab took gut bacteria from pairs of human twins in which only one twin was obese, then fed the samples to mice. The mice given bacteria from the obese humans quickly gained weight. The others did not.
Gut bacteria are also transferred between humans, in the form of fecal transplants, as an experimental treatment for serious infections like Clostridium difficile. In one study, obese patients who received transplants from lean donors later had healthier responses to insulin.
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