Vitamin D

Does it work?

• -Only if you're actually low. Topping up a deficiency restores bone strength, muscle function, and immune health — all of which matter for training. But if your blood level is already normal, megadosing does nothing for strength or recovery. The fix is a blood test, not a guess.

How much · when

• -Get a 25(OH)D blood test first. Below 50 nmol/L (20 ng/mL) is deficient; 50-75 is suboptimal. Above that, a supplement adds nothing.
• -If you're low, 1,000-2,000 IU/day of D3 is enough for most people to reach a healthy level over a few months. Severe deficiency may need a higher short-term dose from a doctor.
• -Take it with a meal that has some fat — it absorbs better than on an empty stomach.
• -Highest-risk groups: indoor/winter training, northern latitudes, darker skin, and anyone who covers up or rarely gets sun. Test if that's you.

The catch

• -In people who already have normal levels, trials show no gain in strength, power, or recovery — even when supplements push blood levels higher.
• -More is not better. Very high doses don't add benefit and some trials hint they can slightly worsen muscle performance.
• -It's a deficiency fix, not a performance enhancer. The 'low vitamin D hurts athletes' headlines are about correcting a shortfall, not about dosing past normal.

Safety

• -Don't megadose blind. Long-term intake well above ~4,000 IU/day without testing can build toward toxicity (high blood calcium, kidney issues).
• -Deficiency is common but not universal — a cheap blood test tells you whether you need it at all.
• -If you take calcium, blood pressure, or heart medication, check with a doctor before high-dose D.

Key research

• IOC consensus statement: dietary supplements and the high-performance athleteBr J Sports Med · 2018 · Position stand
• Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysisLancet Diabetes Endocrinol · 2018 · Meta-analysis
• Effect of vitamin D supplementation on upper and lower limb muscle strength and muscle power in athletes: A meta-analysisPLoS One · 2019 · Meta-analysis
• Effects of vitamin D3 supplementation on strength of lower and upper extremities in athletes: an updated systematic review and meta-analysis of randomized controlled trialsFront Nutr · 2024 · Meta-analysis
• Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant dataBMJ · 2017 · Meta-analysis

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