BDNF
A protein that helps brain cells grow and connect. Exercise raises it in your blood. What it does in your head is harder to show.
BDNF is a protein that helps brain cells survive, grow and form the new connections behind neuroplasticity. The letters stand for brain-derived neurotrophic factor, roughly "nourishment for nerve cells, made in the brain". Exercise raises it: across 29 studies and 1,111 people, one session pushed up the level in the blood, and regular training lifted the resting level a little. Evidence: solid That's blood, though, not brain. Nobody can easily sample a living man's brain, so blood stands in, and how faithfully it stands in is unclear. Evidence: decent
You'll read that exercise uses BDNF to grow you new brain cells. That's oversold. Whether adults make new brain cells at all is disputed. In 2018 two teams published opposite answers: Sorrells and colleagues, in Nature, found no sign of new cells forming in adults; Boldrini and colleagues, in Cell Stem Cell, found them still forming. So "exercise gives you new brain cells" is a maybe at best. Evidence: worth trying
The firmer result is size. In a year-long trial with 120 older adults, walking grew the hippocampus, the brain's memory centre, by about two per cent, while the stretching group's shrank. Evidence: decent BDNF is a plausible reason. Not a proven one. Evidence: decent
Why it matters
Pills sold to raise BDNF rest on a few small, short blood studies, none showing a better brain. Evidence: decent The thing that does raise it is free: walk three times a week, or move hard. You don't need the chemistry settled to do either.