What do the trillions of microbes in your gut actually do?

The microbes in your gut do several jobs that are well proven, and many more that are still guesses. The proven ones: they ferment the fibre you can't digest into short-chain fatty acids your body absorbs, they help make vitamin K and some B vitamins, they help train your immune system, and they defend you simply by taking up all the room and food, so harmful newcomers can't settle in.

The numbers are big. Your body holds an estimated 38 trillion bacterial cells, about as many as its own 30 to 40 trillion human cells, and together they carry around 100 times more genes than you do. Most live in the colon, while the acidic stomach and small intestine hold relatively few species. Your community starts at birth and looks adult by about age two.

The best evidence for their defensive role is what happens when they're wiped out. Antibiotics can clear the way for a nasty bacterium called Clostridioides difficile. For people whose infection keeps coming back, transplanting stool from a healthy donor works in about 85 to 90 percent of cases. The idea is ancient: a Chinese physician, Ge Hong, recorded donor feces as a remedy in the 4th century.

Now the honest caveat. Headlines link gut microbes to mood, depression and the brain, but most of that work comes from animals, often unusual germ-free mice, and small human studies. Researchers still can't say whether microbial changes cause disease, result from it, or both.