Vitamin D and supplements: what has backing and what doesn't
Introduction
The supplement aisle of any pharmacy is a lesson in marketing. Dozens of bottles promise energy, immunity, focus, joints and rest, with a confidence the evidence rarely supports. And in among all that sit a few products that genuinely do have backing in specific situations, which makes separating wheat from chaff harder.
Vitamin D is the most interesting case of all, because it contains both sides of the problem. On one hand, its deficiency is genuinely common and correcting it matters: it's essential for absorbing calcium and maintaining bone. On the other, it has become a kind of cure-all credited with effects on cancer, depression, immunity and longevity that large trials haven't confirmed.
It's worth understanding why this happens, because the pattern repeats with other nutrients. Observational studies find again and again that people with low vitamin D levels have worse health. But when you move to trials where people are supplemented and compared with placebo, the benefit in the general population doesn't appear. The most plausible explanation is that a low level is usually a marker of something else — less outdoor activity, obesity, chronic illness — rather than the cause of the problem.
This article covers vitamin D with that honesty, then reviews the rest of the aisle: what to take, what not to, and what to ask before buying anything.
A deficiency deserves correction. That doesn't mean supplementing someone without a deficiency produces any benefit.
The 12 keys to vitamin D and supplementation
1. Vitamin D behaves more like a hormone than a vitamin. It's synthesized in the skin by ultraviolet B radiation and activated in the liver and kidneys. Its receptors are in practically every tissue, which explains the enormous number of hypotheses about its effects and also why so many haven't been confirmed.
2. Deficiency is common and has identifiable causes. High latitudes, winter, little sun exposure, darker skin in less sunny countries, older age, obesity — vitamin D is sequestered in adipose tissue — malabsorption and certain medications. If several apply to you, the probability of deficiency is high.
3. Measure before supplementing, if there's a reason to measure. Screening the whole healthy population isn't indicated, but testing people with risk factors or compatible symptoms is. Measuring avoids both ways of getting it wrong: leaving a real deficiency untreated and supplementing for years someone who doesn't need it.
4. What's well established is bone. Vitamin D allows calcium to be absorbed from the gut; without it, intake doesn't reach the bone. Severe deficiency causes rickets in children and osteomalacia in adults. In older people with deficiency and low calcium intake, combined supplementation reduces fracture risk. That's its solid ground.
5. The large trials have been disappointing beyond that. The VITAL trial, with more than twenty-five thousand participants, found no reduction in cancer or cardiovascular events with vitamin D supplementation in the general population. It's an important result because it contradicts decades of expectation based on observational studies.
6. In respiratory infections the benefit exists but is modest and depends on deficiency. Meta-analyses suggest a small reduction in the risk of acute respiratory infection, concentrated in those who started with low levels. It's a real effect and also far smaller than headlines suggest.
7. More isn't better: it's toxic. Vitamin D is fat-soluble and accumulates. Mega-doses can produce hypercalcemia, with nausea, weakness, kidney problems and arrhythmias. There have been cases of poisoning from badly dosed or unsupervised supplements. High doses are prescribed, not self-administered.
8. Sun is the main source, with caveats. A few minutes of exposure on arms and face several times a week is enough for many people in spring and summer, but it depends on latitude, season, time of day, pigmentation and age. In winter, at high latitudes, UVB radiation is insufficient no matter how much you go outside. And sun protection remains necessary: burning isn't a strategy.
9. Diet supplies little on its own. Oily fish, egg yolk, liver and fortified foods are the main sources, and in most dietary patterns they don't cover requirements without sun or a supplement. It's one of the few vitamins where food alone rarely solves the problem.
10. The supplements with real backing are few. Vitamin B12 in vegan diets and in older people or those on long-term metformin or antacids. Folic acid before and during pregnancy, with solid evidence in preventing neural tube defects. Iron in demonstrated deficiency. Iodine in pregnancy depending on the region. Vitamin D in deficiency. That list is short, and it's short for good reasons.
11. Antioxidants in pill form have failed repeatedly. It's one of nutrition's most counterintuitive findings: diets rich in antioxidant-containing foods are associated with better health, but isolated supplements don't reproduce the effect, and some have shown harm. High-dose beta-carotene increased lung cancer incidence in smokers in two large trials. A food isn't the sum of its compounds.
12. Supplements interact more than people think. Calcium interferes with iron absorption and with some antibiotics. St John's wort reduces the efficacy of contraceptives, anticoagulants and antidepressants. High doses of vitamin E or omega-3 can increase bleeding risk with anticoagulants. Always take the full list of what you take to your appointment, supplements included.
How to choose if you decide to supplement
Prefer vitamin D3 (cholecalciferol) over D2: it's more effective at raising levels. Take it with a meal containing some fat, since it's fat-soluble. And bear in mind that supplements are regulated as foods, not medicines, in most countries: that means less control over actual content and contaminants. Independent verification seals are a reasonable quality signal.
The signs of a product that doesn't deserve your money
It promises to fix several unrelated things. It leans on testimonials rather than trials. It uses phrases like "detoxify", "alkalize" or "boost your immune system" with no specified mechanism. It includes a long list of ingredients in minimal doses — so-called label decoration — to look comprehensive. Or it requires you to buy its own proprietary test to know you need it.
The case of multivitamins
They're the best-selling supplement and one of the worst supported. In people with a reasonably varied diet, trials show no benefit in mortality, cancer or cardiovascular disease. They make sense in specific situations — malabsorption, bariatric surgery, very restrictive diets, certain life stages — and little sense as a generic insurance policy.
A final thought
Vitamin D neatly teaches the difference between correcting a deficiency and chasing a benefit. If you have risk factors, measure it and correct it if it's low: that has backing and is worth doing, above all for bone. What the evidence doesn't buy is the idea of supplementing everyone in the hope of protection against cancer or cardiovascular disease. And that criterion applies to the rest of the aisle: few products, for specific indications, at known doses, and tell your doctor. The money left over goes considerably further on fish, vegetables and a pair of trainers.
References
- Manson, J. E. et al. (2019). Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine, 380(1), 33–44.
- The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group (1994). The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. New England Journal of Medicine, 330(15), 1029–1035.