Supplements & evidence · Free guide
Vitamin D: A Small Nutrient With a Big Job
What vitamin D does, who benefits most, and how much to take
Explore vitamin D: benefits, daily needs & safety
What it does. Vitamin D helps your body absorb calcium and keep bones strong. It also plays a part in normal muscle, nerve, and immune function. Food, fortified drinks, and supplements can provide it. Your skin makes vitamin D after sun exposure, too. Unprotected sun exposure is not a safe way to treat low levels (NIH ODS).
Who can run low. Older adults, people with limited sun exposure or darker skin, and people with obesity or conditions that affect fat absorption may be more likely to have low levels. These are reasons to pay attention to intake, not automatic reasons to take high-dose supplements (NIH ODS).
What the research shows
Trials differ in who they include, doses, and follow-up, so findings can be mixed and may not answer every person's situation.
- Bones: An earlier review found modestly fewer fractures in older adults taking daily vitamin D plus calcium, but not vitamin D alone. A broader 2026 review found little to no clinically meaningful benefit for fractures or falls in most adults studied. Evidence remains limited for people at high risk or in residential care (JAMA Netw Open 2019; BMJ 2026).
- Colds and respiratory infections: An early review suggested a modest benefit, most apparent in people with very low starting levels taking daily or weekly doses. A larger 2025 review found no statistically significant reduction overall and no clear benefit by age, starting level, dose, or schedule. Vitamin D is not established cold-prevention treatment (BMJ 2017; 2025 review).
- Autoimmune disease: One large trial found fewer diagnoses during 5.3 years of vitamin D in middle-aged and older adults. A seven-year follow-up found this difference was no longer apparent two years after supplements stopped; it does not establish a lasting prevention benefit or a recommendation for everyone (BMJ 2022; 2024 follow-up).
- Cancer: Vitamin D has not been shown to prevent cancer. A 2023 review of vitamin D3 trials found no statistically clear reduction in cancer deaths overall. A daily-dose subgroup suggested a possible benefit, but that finding does not establish a cancer-prevention dose (Ageing Res Rev 2023).
What you need each day. For most adults through age 70, the recommended total intake is 600 IU a day; from age 71 onward, it is 800 IU a day. This includes food, fortified drinks, and supplements. It is not a default supplement dose. Treatment for documented low levels may use a different, clinician-guided plan (NIH ODS).
More is not always better. The adult upper limit is 4,000 IU a day from all sources. This is a limit, not a target. Children's upper limits vary by age. Higher doses and treatment for low levels need clinician guidance. A pharmacist can help check for medication interactions (NIH ODS).
Do you need a blood test? Routine vitamin D testing is not recommended for otherwise healthy adults. A clinician may recommend testing when there is a specific medical reason. Discuss supplements and vitamin D testing with your healthcare provider; they can help decide what is appropriate for your health and review possible medication interactions (2024 Endocrine Society guideline).
The bottom line. Aim for enough vitamin D, not as much as possible. A supplement can fill an intake gap or be part of a clinician's treatment plan. A dose used in a study is not a personal recommendation.
Sources and further reading
Explore vitamin D: benefits, daily needs & safety
General health information, not a diagnosis or a substitute for professional care.