Vitamin D
Preliminary· human dataAlso known as: Vitamin D3, cholecalciferol
Correcting a deficiency matters; a performance boost if you're already replete is unproven.
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. Well-supported· 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. Preliminary· 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. Marketing claim 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 — 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.
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Written and reviewed by Mathew Beale — MSc Biotechnology, University of Reading.
Last reviewed: