Vitamin D for muscle strength: why supplements alone don't work

A deficiency you don't feel — until it's too late

Vitamin D is popularly known as the "sunshine vitamin" and is mostly associated with bone health. Less well known is the role it plays in muscle tissue: muscle fibers carry vitamin D receptors, and a deficiency is linked to reduced muscle strength and a higher risk of sarcopenia — the gradual loss of muscle mass and strength with age. In a country like Belgium, at a latitude where skin barely produces enough vitamin D from UVB exposure between October and March, this is not a marginal problem.

How widespread is the deficiency, really?

The ODIN project, the largest European data collection on vitamin D status to date (Cashman et al., American Journal of Clinical Nutrition, 2016), analyzed blood values from 55,844 people across 19 European countries. The figures:

• 13% of the population stayed below 30 nmol/L year-round (severe deficiency).
• Using the broader cutoff of 50 nmol/L (the threshold most guidelines use for "insufficient"), that rose to 40.4%.
• The season makes a large difference: between October and March, 17.7% fell below 30 nmol/L, compared with 8.3% between April and September.

In other words: anyone training, cycling, or running through the winter without ever thinking about it has a real chance of running structurally low — with no visible symptom at all.

What happens to your muscles with a deficiency — and what doesn't

This is where it gets interesting, and more nuanced than most supplement marketing suggests. A 2021 network meta-analysis (Prokopidis et al., published in Nutrients) pooled 9 randomized trials in 1,420 people with sarcopenia and compared different combinations of protein, vitamin D, and exercise:

• Vitamin D alone produced no convincing evidence in this analysis. The authors are explicit about it: the evidence for vitamin D as a standalone intervention is weak.
• The combination of protein, vitamin D, and exercise did significantly improve grip strength: a mean difference of 3.86 kg (95% CI 0.52–7.21) compared with an iso-caloric control group.
• Protein plus vitamin D (without structured exercise) shortened the time needed for five chair stands — a direct measure of functional leg strength — by an average of 1.32 seconds (95% CI −1.98 to −0.65, based on 2 trials with 324 participants).
• For muscle mass itself, the authors found no significant differences between the combinations.

The researchers' conclusion is sober: there is currently no standardized therapeutic dose for sarcopenia, and vitamin D does not work as an isolated intervention. It adds value on top of an adequate protein intake and a training stimulus — the same pattern we saw with creatine.

So what's a sensible approach?

Based on the above, three building blocks come together:

1. Get your 25(OH)D level measured with a blood test, especially if you train through winter or spend little time outdoors. That's the only way to distinguish "deficient" from "probably fine."
2. Build in enough protein — see our article on protein for endurance athletes for concrete guidelines by body weight.
3. Keep the training stimulus in place. Vitamin D and protein amplify a training effect, they don't replace it — the same principle that applies to strength training after 65.

A common maintenance dose for a confirmed deficiency typically falls between 800 and 2,000 IU per day, but the exact dose depends on your baseline value, body weight, and any other conditions — this is exactly the kind of decision better made with a doctor or based on a blood test than on what's printed on the bottle.

Safety

Vitamin D is fat-soluble and accumulates in body fat; unlike water-soluble vitamins, prolonged overdosing (typically only above 4,000 IU/day sustained over a long period, without medical supervision) can lead to excessively high blood calcium levels. That's a different risk category than, say, creatine or protein, and one more reason to base dosing on a measured value rather than a one-size-fits-all number.

How this fits into a coaching plan

At HealthyDNA, we never look at vitamin D in isolation from the rest of your blood panel — the same way we approached creatinine and biological age. A 25(OH)D value included in a blood test gives you a concrete starting point instead of a guess, and lets us tailor dosage and follow-up to your situation — including how it interacts with your protein intake and training schedule.

Want to know whether your vitamin D level is where it should be? Get in touch for a free intake conversation.

This article is general information and does not replace medical advice.

Sources: Cashman et al. (2016), "Vitamin D deficiency in Europe: pandemic?", American Journal of Clinical Nutrition (ODIN project, 55,844 participants, 19 European countries); Prokopidis et al. (2021), "The Optimal Strategy of Vitamin D for Sarcopenia: A Network Meta-Analysis of Randomized Controlled Trials", Nutrients, 13(10), 3589.