Time was, vitamins were used mostly in hospital wards to treat malnourishment: vitamin B3 for pellagra, vitamin C for scurvy, vitamin D for rickets. Now, though,

we gulp down pills for a performance boost—or at least a sense of control over our health—and the $28 billion dietary-supplement industry has been happy to oblige. But it’s hard to prove the benefits of supplements, because that’s exactly what they do: They supplement. For most users, this means that a pill will contribute only tiny improvements. And that in turn means evidence of those gains is likely to be drowned out by statistical noise, whether a scientist is studying cancer prevention or exercise enhancement.

In general, you should just get your nutrients from food. But if you are looking to prevent disease or increase performance, evidence—both positive and negative— is trickling out of the lab. In the matrix below, we plot how well a few supplements actually work against the strength of the evidence for or against common claims. Decide for yourself if that spendy pill is worthwhile.