Vitamin D is one of the most talked-about nutrients in health media, and one of the most misunderstood. It is genuinely under-supplied in some groups and over-supplemented in others who probably do not need it. Knowing where you stand means understanding both who is really at risk and what the evidence does, and does not, support.
Who is actually at risk
Vitamin D shortfall is common worldwide, affecting an estimated billion people. It is most likely in people with darker skin, older adults, those living far from the equator, and people who are mostly indoors. Doctors usually call it a deficiency when a blood test falls below 20 ng/mL. Severe, long-term shortfall causes the bone diseases rickets in children and softening of the bones in adults (Holick, 2007).
The numbers are striking. In a US survey, about 42% of adults were below that level, with much higher rates in Black Americans (82%) and Hispanic Americans (69%). The reason is skin pigment: it filters the sunlight that makes vitamin D in the skin, so people with darker skin make less from the same sun (Forrest & Stuhldreher, 2011).
What to do about it
Because few foods contain much vitamin D, sunlight and supplements do most of the work, and sunlight is unreliable in winter at higher latitudes. UK health guidance is a good template: most people should consider a daily 10-microgram (400 IU) supplement through the darker months (roughly October to March), and people at higher risk, those with darker skin, who rarely go outside, who cover their skin, or who live in care homes, should take it year-round (NHS).
That dose is conservative and safe for most adults: enough to prevent deficiency, nowhere near toxic levels. The NHS advises not exceeding 100 micrograms (4,000 IU) a day without medical supervision, since very high long-term doses can raise blood calcium to harmful levels.
If you have darker skin, are over 65, are pregnant, or are housebound, year-round supplementation is the clear advice. A blood test is the only way to know your actual level, and a doctor can arrange one if you are in a high-risk group (NIH ODS).
Where the evidence is strongest: bone health
The case for vitamin D and bones is among the strongest in nutrition. Beyond preventing those bone diseases, a higher dose helps protect older adults from fractures. A pooled analysis of trials found that taking about 800 IU a day or more cut the risk of hip fracture by roughly 30% in adults over 65, while lower doses did not (Bischoff-Ferrari et al., 2012). For the wider role of vitamins in women's health, see our guide to metabolism vitamins for women.
Where the evidence is weaker
Vitamin D is often sold as protection against cancer, heart disease, and more. For people who are not short on it, that does not hold up. A large trial of nearly 26,000 adults found that vitamin D pills did not lower the risk of cancer or heart problems compared with a dummy pill (Manson et al., 2019). The honest position: fixing a real shortfall matters, but topping up levels that are already fine does not seem to add protection against these diseases.
Common myths
"Vitamin D supplements protect everyone from cancer and heart disease." Not in people who are not deficient. The large trial above found no reduction in cancer or cardiovascular events from supplementing (Manson et al., 2019).
"You can get enough vitamin D from food." Very few foods contain much. Oily fish, egg yolks, and fortified products help, but food alone rarely meets needs, especially in winter (NIH ODS).
"A tan means you have enough." Melanin lowers the skin's vitamin D production, so people with darker skin need much more sun for the same amount, which is exactly why deficiency rates are higher in darker-skinned people at northern latitudes (Forrest & Stuhldreher, 2011).
The bottom line
Vitamin D deficiency is genuinely common, especially with darker skin, limited sun, or older age. A modest daily supplement in winter, and year-round for high-risk groups, is well supported and safe. Going beyond that in the hope of preventing cancer or heart disease is not backed by the evidence. Fix a real deficiency; do not chase ever-higher levels. Browse our nutrition guides for more.
Frequently Asked Questions (FAQ)
Who is most at risk of vitamin D deficiency? People with darker skin, older adults, those who rarely get outdoors or cover most of their skin, and anyone living far from the equator during winter. Diet alone rarely covers it, because few foods contain much vitamin D.
How much vitamin D should I take? Public health guidance suggests around 10 micrograms (400 IU) a day, especially in winter, and year-round for high-risk groups. Do not exceed 100 micrograms (4,000 IU) a day without medical advice.
Do vitamin D supplements prevent cancer or heart disease? No. A large trial found vitamin D did not reduce cancer or cardiovascular disease in people who were not deficient. Correcting a real deficiency matters; topping up normal levels does not appear to add protection.
Can I get enough vitamin D from food? Rarely. Oily fish, egg yolks, and fortified foods provide some, but most people cannot reach adequate levels from food alone, especially in winter.
How do I know if I am deficient? A blood test (25-hydroxyvitamin D) is the only way to know. If you are in a high-risk group, a doctor can arrange one before or after you start supplementing.
Medical disclaimer: This article is for informational purposes only. Always consult a qualified healthcare provider before making changes to your health routine.
Sources
- Holick MF. Vitamin D deficiency. New England Journal of Medicine, 2007 — PubMed. https://pubmed.ncbi.nlm.nih.gov/17634462/
- Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research, 2011 — PubMed. https://pubmed.ncbi.nlm.nih.gov/21310306/
- Bischoff-Ferrari HA, et al. A pooled analysis of vitamin D dose requirements for fracture prevention. New England Journal of Medicine, 2012 — PubMed. https://pubmed.ncbi.nlm.nih.gov/22762317/
- Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease (VITAL). New England Journal of Medicine, 2019 — PubMed. https://pubmed.ncbi.nlm.nih.gov/30415629/
- NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
- NIH Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
All sources accessed 31 May 2026.
Sophia Martinez, MS, CNS
A wellness researcher focused on what the evidence actually says.
