Supplements: what the evidence supports

The supplement industry is very large, is regulated as food rather than as medicine in most jurisdictions, and therefore does not have to demonstrate that products work before selling them.

The ones with a genuine case

Short and specific.

Folic acid before conception and in early pregnancy, which prevents neural tube defects and has strong evidence.

Vitamin D, recommended at population level in many countries during winter months and year-round for at-risk groups, on the basis of widespread insufficiency and bone health.

Vitamin B12 for people on vegan diets, where deficiency is otherwise likely and consequences are neurological.

Iron where deficiency is documented, and not otherwise, since iron overload is harmful.

Calcium and vitamin D in specific groups at risk of fracture, particularly with low dietary intake.

Specific formulations for defined conditions, such as those used in intermediate age-related macular degeneration.

Supplementation after bariatric surgery or with malabsorption, which is medically necessary.

And creatine for resistance training performance, which has an unusually solid evidence base among sports supplements.

The ones that disappoint in trials

Where large studies have failed to confirm observational promise.

Multivitamins in adequately nourished people, where trials have not shown mortality or cardiovascular benefit.

Antioxidant supplements, where some trials have found harm — including increased lung cancer in smokers taking beta-carotene, a finding that ended a major trial early.

Omega-3 supplements for primary cardiovascular prevention in the general population, where large trials have been largely neutral, in contrast to eating fish.

Vitamin C for preventing colds in the general population.

Most products marketed for immunity, energy, detox, hair, joints and hormones.

And collagen, where the literature is growing, small and predominantly industry-funded.

Why observational evidence misleads here

The recurring pattern.

People who take supplements differ systematically from people who do not — in income, education, diet, exercise and health-seeking behaviour.

Low levels of a nutrient frequently accompany illness rather than causing it, which produces associations that disappear when tested.

And correcting a deficiency helps while adding more to someone replete does not, which explains most of the pattern in this field.

Safety, which is not assumed

Supplements are not automatically safe because they are available without prescription.

High-dose vitamin D causes hypercalcaemia, and cases of toxicity from online products are documented and increasing.

Vitamin A in high doses is teratogenic and is why liver and retinol supplements are restricted in pregnancy.

Iron overdose is a serious poisoning, particularly in children.

St John's wort interacts with a long list of medicines including contraceptives, anticoagulants and antidepressants.

High-dose fish oil and vitamin E affect bleeding.

Biotin interferes with several laboratory assays including thyroid and cardiac markers, producing misleading results that have caused real clinical errors.

And products bought from unregulated sources have repeatedly been found to contain undeclared pharmaceuticals, particularly in sports and weight loss categories.

The regulation gap

Which explains the market.

Because supplements are regulated as foods, manufacturers do not have to prove efficacy before sale, and enforcement is generally reactive.

Health claims are restricted in many jurisdictions, which is why marketing uses structure-function language and implication rather than direct claims.

Independent testing organisations exist and certify content and purity for some products, which is worth looking for if buying anything.

The cost

Worth calculating.

A daily multivitamin, a fish oil, a probiotic and whatever else is currently fashionable amounts to a meaningful monthly sum, sustained over years.

The same money spent on food — vegetables, pulses, fish, fruit — buys nutrients in a form the evidence supports.

Which is the comparison the industry never makes.

How to decide

The questions worth asking.

Is there a documented deficiency or a specific indication?

Is there trial evidence in people like me, or only observational data?

What would I notice if it were working?

Does it interact with anything I take?

And would the money be better spent on food, sleep or exercise, which is generally the answer.

General information only, not medical or nutritional advice. Consult a qualified clinician or pharmacist before taking supplements, particularly alongside prescribed medication.