Vitamin D and Supplements Worth Considering
The supplement industry is large and lightly regulated relative to medicines, and marketing claims consistently run ahead of the evidence.
A short list of supplements has reasonable support in defined circumstances, and most people taking a broad multivitamin are unlikely to derive measurable benefit from it.
Vitamin D
Synthesised in skin exposed to sunlight, which means people in northern latitudes produce little or none during winter months.
Public health bodies in several countries recommend supplementation during autumn and winter, and year-round for people with limited sun exposure, darker skin, or who cover most of their skin.
This is among the better-supported general recommendations, and typical recommended amounts are modest. Very high doses are not more beneficial and can cause harm through elevated calcium.
Others with reasonable evidence in specific groups
Folic acid before and during early pregnancy, which has strong evidence for reducing neural tube defects and is recommended for anyone who might become pregnant.
Vitamin B12 for people following a vegan diet, since it is not reliably available from plant sources.
Iron where deficiency has been demonstrated by testing. Supplementing without deficiency is unhelpful and excess iron is harmful.
Calcium and vitamin D together in some older adults at risk of fracture, typically on clinical advice.
Where evidence is weaker than the marketing
Broad multivitamins in well-nourished populations have not demonstrated meaningful benefit in large trials for most outcomes.
Antioxidant supplements in high doses have in some trials shown neutral or adverse effects, which is a useful caution against assuming more is better.
Many popular botanical supplements have limited high-quality trial evidence, and product content frequently differs from labelling in independent testing.
Interactions and cautions
Supplements interact with medicines. St John's wort affects the metabolism of numerous drugs including anticoagulants and hormonal contraceptives.
High-dose biotin interferes with several laboratory immunoassays, producing misleading thyroid and cardiac test results.
Fat-soluble vitamins accumulate, so excess is possible in a way it is not with most water-soluble ones.
Tell your doctor and pharmacist what you take, including supplements, because they are frequently omitted from medication lists.
A reasonable position
Test rather than guess where deficiency is suspected. Supplement where there is a defined reason, at sensible doses, from a reputable manufacturer.
Diet remains the better route for most nutrients, and no supplement compensates for an otherwise poor one.
This article is general information and not medical advice. Consult a qualified healthcare professional before starting supplements, particularly alongside medication.
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