James Hamblin writes:

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.

Researchers have only recently shown that these antibiotics kill off some of the microbes that occur normally in the gut and help livestock, and people, digest food. By breaking down nutrients and helping them pass through the walls of the bowel, these microbes serve as a sort of gatekeeper between what is eaten and what actually makes it into the body.

Killing them is not without consequences. Just as antibiotics are associated with faster growth in cattle, a decrease in diversity in the human microbiome is associated with obesity. As the usage of animal antibiotics exploded in the 20th century, so too did usage in humans. The rise coincides with the obesity epidemic. This could be a spurious correlation, of course—lots of things have been on the rise since the ’50s. But dismissing it entirely would require ignoring a growing body of evidence that our metabolic health is inseparable from the health of our gut microbes. [Continue reading…]