How Supplement Evidence Is Evaluated

Dietary supplements are a large global industry, and the strength of evidence behind individual products varies greatly. Researchers distinguish between laboratory and animal studies, observational studies that find associations, and randomized controlled trials that can test cause and effect.

Systematic reviews and meta analyses, such as those published by the Cochrane collaboration, pool trial results to estimate overall effects. Many compounds that look promising in cell culture, such as resveratrol and curcumin, have proven difficult to translate into consistent human benefits, partly because of poor absorption.

Large trials have sometimes found unexpected harms, as when beta carotene supplements increased lung cancer risk in smokers. In many countries supplements are regulated more like foods than medicines, so products are not required to prove effectiveness before sale..