Does vitamin D improve strength and performance?

Only if you are genuinely deficient — and "deficient" means far lower than the marketing implies.

This one surprised me when I went through the evidence. The familiar story is "vitamin D fixes your strength if you're low." The reality is thinner than that.

What the biggest studies show

The largest synthesis — 54 randomized trials covering 8,747 people — found supplementation performed slightly worse than placebo on muscle function. Not better. Slightly worse.

In athletes specifically, the meta-analyses are null. There is essentially no ergogenic effect in people who already have adequate levels.

A large trial in adults with baseline levels around 33 nmol/L — a range many labs would flag as "insufficient" — found zero change in strength after four months of supplementation.

Where the benefit might be real

In frank deficiency, below about 25 nmol/L, the picture changes: a meta-analysis that split its trials by baseline status found a large pooled improvement in hip strength there, against nothing at all above 25 nmol/L. That is the honest kernel of truth in the claim — but be clear how thin it is. That deficient-subgroup result rests on just two small trials. The null half of the same analysis is far better supported than the positive half.

Note the gap either way: whatever benefit exists lives below 25 nmol/L, while the vast supplement market is aimed at people in the "insufficient" 25-50 range, where the evidence does not support a strength effect at all.

Practical read

If you have a real, tested deficiency, correcting it is worth doing — for your health generally, not just your squat. If your levels are adequate and you're taking vitamin D to lift more, the evidence says you're not getting anything for it.

Get tested rather than guessing. That's the whole answer.