Do you actually need vitamin supplements?

The evidence on routine multivitamin use in healthy, adequately nourished adults is consistently underwhelming. Large randomized trials and systematic reviews have generally failed to demonstrate reductions in cardiovascular disease, cancer, or mortality, and the US Preventive Services Task Force has concluded that evidence is insufficient to recommend most supplements for chronic disease prevention in this population, while recommending against beta-carotene and vitamin E specifically due to evidence of harm or lack of benefit. The reason is straightforward: supplementing a nutrient you are not deficient in generally does nothing, and nutrients in food come in combinations that isolated pills do not replicate.

Specific groups genuinely benefit and the recommendations are well established. Folic acid before and during early pregnancy substantially reduces neural tube defects. Vitamin B12 for people following vegan diets, older adults with reduced absorption, and those on certain medications. Vitamin D for people with limited sun exposure or documented low levels, though the enthusiasm for universal supplementation has outrun the trial evidence. Iron for those with diagnosed deficiency, which should be diagnosed rather than assumed since excess iron is harmful. Calcium where dietary intake is inadequate. Prescribed supplementation after bariatric surgery or for malabsorption conditions.

Two cautions about the category. Supplements are regulated far more loosely than drugs, with manufacturers responsible for their own safety substantiation and the FDA generally acting only after problems appear, and independent testing has repeatedly found products whose contents do not match their labels, including some containing undeclared pharmaceutical ingredients. Look for third-party verification marks from USP or NSF as a partial safeguard. And supplements interact with medications in clinically significant ways, with vitamin K affecting warfarin, St. John's wort reducing the effectiveness of numerous drugs including oral contraceptives, and high-dose fish oil affecting bleeding risk, so anything you take belongs on the list you give your physician and pharmacist.