Supplement
Vitamin D
Also known as: Vitamin D3, cholecalciferol
Correcting a deficiency matters; a performance boost if you're already replete is unproven.
- Evidence
- Silver· human data
- Sources cited
- 1
- Last reviewed
- 22 July 2026
What it is
Vitamin D is a fat-soluble vitamin your skin makes from sunlight and you get in small amounts from food. It's essential for bone health, immune function and muscle, and deficiency is common, especially in winter at northern latitudes.
What the evidence shows
Correcting a deficiency. Gold· human data That low vitamin D is bad for bone and general health, and that fixing a genuine deficiency is worthwhile, is well established. This is the real reason to care about vitamin D.
A performance or strength boost when you're already replete. Silver· human data Most apparent "performance" benefits come from correcting a deficiency. Topping up someone whose levels are already adequate has not been shown to reliably improve strength or endurance.
Testosterone and body-composition claims. Unproven Marketed hard, but the effect (where any) again traces back to correcting a deficiency, not to supplementing replete people.
Who it suits
People at real risk of deficiency (little sun exposure, darker skin, covering up, northern winters) ideally guided by a blood test. If you're replete, there's little to gain.
Practical notes
Test, don't guess. A simple blood test tells you whether you actually need it, and D3 (cholecalciferol) raises levels more effectively than D2. Read the safety box. This is a vitamin where far more is genuinely worse, not better.
Sources
Frequently asked questions
- Should I take vitamin D?
- If you're deficient, which is common in winter at northern latitudes, correcting it is worthwhile for bone and general health. If your levels are already adequate, extra vitamin D is unlikely to improve performance.
- Does it boost strength or testosterone?
- Claims of strength or testosterone gains mostly come from correcting a deficiency, not from topping up someone who is already replete. In replete people the performance evidence is weak.
- How would I know if I'm low?
- A simple blood test. Risk factors include little sun exposure, darker skin, covering up, and northern winters. Testing beats guessing at a dose.
- D3 or D2?
- D3 (cholecalciferol) is generally preferred as it raises blood levels more effectively than D2.