Vitamin D
Worth it if you're deficient or get little sun; doesn't improve performance
What it is
A fat-soluble vitamin found in few foods and made in the skin under the sun's ultraviolet rays. It helps the body absorb calcium and is needed for normal bone mineralisation.
Benefits
Enough vitamin D prevents rickets in children and osteomalacia in adults (NIH). Deficiency is linked to a higher risk of bone injuries and viral respiratory infections (AIS). Supplements do not improve exercise performance (ISSN); data on muscle function and recovery are equivocal and probably depend on your starting level (IOC).
Dose
The US allowance for adults is 15–20 mcg (600–800 IU) a day depending on age (NIH); Europe sets 15 mcg a day (EFSA). AIS: 1000 IU a day if you are not deficient but at risk; with diagnosed insufficiency, 2000 IU a day for 1–2 months, then a repeat blood test.
When to take it
The IOC suggests 1000 IU a day from autumn to spring to maintain your level. Fat in a meal improves vitamin D absorption, though some is absorbed without it (NIH). The sources give no advice on time of day.
Who it's for
People with a low level on a blood test or little sun: indoor training, covering clothing, regular sunscreen, living above 35° latitude (AIS). Also at risk: people with dark skin, older adults, people with obesity and those with conditions that limit fat absorption (NIH).
Who can skip it
Healthy adults with no risk factors: the Endocrine Society does not recommend routine supplementation for healthy adults aged 19–74 (NIH). The USPSTF finds the evidence insufficient to weigh the benefits and harms of single vitamins for preventing heart disease and cancer.
Forms
There are two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Both raise blood 25(OH)D, but D3 raises it more and keeps it up longer (NIH ODS). A 2012 meta-analysis: D3 is more efficacious than D2; the difference was significant with infrequent large doses and disappeared with daily dosing. D3 is typically made from lanolin from sheep wool; an animal-free version from lichen exists.
Safety
The upper level for adults is 100 mcg (4000 IU) a day at both NIH and EFSA. Excess causes hypercalcaemia: nausea, vomiting, muscle weakness, kidney stones; in extreme cases, kidney failure and arrhythmias. Toxicity almost always comes from supplements. Vitamin D interacts with orlistat, statins, steroids and thiazide diuretics: if you take any of these, discuss the supplement with your doctor (NIH).
This is a summary of public sources, not medical advice. If you have a health condition, are pregnant or take medication, talk to a doctor before taking a supplement.
Sources
- Vitamin D — Fact Sheet for Health Professionals . NIH Office of Dietary Supplements, 2025.
- AIS Sports Supplement Framework — Group A . Australian Institute of Sport, 2026.
- Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate . European Food Safety Authority, 2023.
- IOC consensus statement: dietary supplements and the high-performance athlete . International Olympic Committee / British Journal of Sports Medicine, 2018.
- ISSN exercise & sports nutrition review update: research & recommendations . International Society of Sports Nutrition, 2018.
- Vitamin, Mineral, and Multivitamin Supplementation to Prevent Cardiovascular Disease and Cancer: Preventive Medication . U.S. Preventive Services Task Force, 2022.
- Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis . American Journal of Clinical Nutrition, 2012.
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