Free radicals are reactive molecules produced during normal metabolism and in response to pollution, smoking, and UV exposure. Antioxidants — including vitamins C and E, beta-carotene, selenium, and polyphenols — can neutralise them before they damage cells.
The strongest evidence is for dietary patterns rich in antioxidant foods, not individual supplements. Large trials of isolated antioxidant supplements have generally shown no clear benefit for preventing heart disease or cancer in the general population. High-dose beta-carotene supplements were actually associated with increased lung cancer risk in smokers in two landmark trials.
A varied diet built around vegetables, fruit, wholegrains, legumes, nuts, and seeds provides a broad mix of antioxidants alongside fibre, vitamins, and minerals — and that whole-diet effect is what the evidence consistently supports.