The organ you didn't know you had

In 2007 the National Institutes of Health launched the Human Microbiome Project to catalog the trillions of bacteria, fungi and other organisms that live in and on a human body, most of them in the gut. The findings reorganized medicine. The gut's microbes outnumber your own cells, they weigh about as much as your brain, they make vitamins, train the immune system, regulate inflammation, and produce compounds that reach the brain. They're an organ. And unlike your other organs, you feed this one directly, three times a day, and what you feed it changes what it becomes within days.

What the microbes want is fiber. Not the fiber on a cereal box, but the whole range of plant fibers that human digestion can't break down and that bacteria can: in beans, whole grains, vegetables, fruit, nuts and seeds. The guideline is about thirty grams a day. Most people in the West eat about fifteen. The research links the gap to everything from inflammation to mood, and the fix is the plainest on the shelf: more plants, more kinds of them. One large study found that people who ate thirty different plant foods a week had markedly more diverse microbiomes than people who ate ten.

It's a Healing key because the gut is where a great deal of healing turns out to happen, and because it's the organ the table keys have been feeding all along without naming. The Mediterranean plate, the daily dozen, the greens: they were microbiome diets before anyone had the word.