Most people know vitamin D comes from sunlight — but if you train hard, spend long hours indoors, or live somewhere sunny and still avoid the midday heat, your levels may be lower than you’d expect. Vitamin D contributes to the maintenance of normal muscle function, which makes it one of the few micronutrients athletes genuinely need to keep an eye on.
What vitamin D actually does (and what it doesn’t)
Vitamin D is a fat-soluble nutrient that behaves more like a hormone than a classic vitamin. Its authorised functions under EU food law are specific and worth knowing:
- Vitamin D contributes to the maintenance of normal muscle function.
- Vitamin D contributes to the normal function of the immune system.
- Vitamin D contributes to the maintenance of normal bones.
- Vitamin D contributes to normal absorption and utilisation of calcium and phosphorus.
- Vitamin D contributes to normal blood calcium levels.
You’ll see bolder claims elsewhere — that vitamin D “boosts testosterone”, “supercharges immunity”, or “improves athletic performance”. None of those are authorised health claims in the EU, and the evidence for them is far thinner than the headlines suggest. We’ll stick to what the science actually supports.
Where does vitamin D come from?
Sunlight
Your skin synthesises vitamin D3 (cholecalciferol) when UVB rays hit it. In practice, this means bare skin — not through glass, not when you’re wearing SPF 50. Even in Malta and Cyprus, where sunlight is abundant most of the year, many people end up deficient because they sensibly avoid the harshest midday sun, work indoors, or apply sunscreen consistently. Deficiency is more common in sunny Mediterranean countries than the weather map would suggest.
Food
Dietary sources exist but are limited:
- Oily fish (salmon, sardines, mackerel) — the best natural sources.
- Egg yolks — modest amounts.
- Fortified foods — certain cereals, dairy alternatives and some milks carry added vitamin D.
- Liver contains vitamin D, though it’s rarely eaten in large quantities.
Getting adequate vitamin D from food alone is genuinely difficult — the amounts in most diets fall well short of recommended intakes.
How much do you need?
Two figures are useful here:
- The EU Nutrient Reference Value (NRV) — the number you’ll see as “% NRV” on supplement labels — is 5 µg (200 IU) per day. This is a regulatory labelling figure, not a nutritional target.
- EFSA’s Adequate Intake for adults is 15 µg (600 IU) per day — the amount considered sufficient to maintain healthy blood levels (a serum 25-hydroxyvitamin D of at least 50 nmol/L) when sun exposure is minimal.
The gap between 5 µg and 15 µg tells a story: the NRV on labels is older, set before more recent dietary reference values. If you’re relying on a supplement and checking its label, don’t assume hitting 100% NRV is enough. For reference, the NHS advises adults not to exceed 100 µg (4,000 IU) daily from supplements — that’s a safety ceiling, not a target.
Who is most at risk of deficiency?
Vitamin D deficiency is remarkably common. Athletes are not automatically protected — and some training patterns actively increase risk:
- Indoor athletes — gym-goers, swimmers, martial artists, gymnasts.
- Early-morning or evening training — when UVB levels are low even outdoors.
- Darker skin — more melanin means slower synthesis at the same sun exposure.
- Consistent sunscreen use — sensible for skin health, but it reduces synthesis.
- Winter months — in many European locations UVB is insufficient from roughly October through March.
- Higher body fat — vitamin D is fat-soluble and may be held in adipose tissue.
The only reliable way to know your status is a blood test (serum 25-hydroxyvitamin D). Symptoms of deficiency — fatigue, low mood, bone aches — are vague and easy to misattribute.
Vitamin D and muscle function: what the evidence shows
The authorised claim — “Vitamin D contributes to the maintenance of normal muscle function” — reflects solid mechanistic evidence. Muscle tissue carries vitamin D receptors, and correcting a deficiency restores normal neuromuscular function. What’s less established is whether loading up beyond sufficiency adds any performance benefit for people who already have adequate levels. Research there is mixed and not strong enough to claim vitamin D improves athletic performance or strength. Sufficiency matters; megadosing probably doesn’t add much.
The same goes for immunity: vitamin D contributes to the normal function of the immune system — that’s the evidence base. “Prevents colds” or “fights off illness” are not claims the science supports reliably enough to make, and they aren’t authorised in the EU. Honest is better than hopeful.
D3 vs D2: which form?
Most supplements use D3 (cholecalciferol) — the same form your skin produces. It’s the better-studied form and generally more effective at raising and maintaining blood levels than D2 (ergocalciferol, from plant or yeast sources). If you’re comparing products, D3 is the one to look for.
Practical notes for athletes
- If you train mostly indoors, consider a daily D3 supplement — particularly from autumn through spring.
- Year-round supplementation is reasonable even in southern Europe if you regularly avoid midday sun or use high-SPF protection.
- Vitamin D works alongside calcium and magnesium, so it’s worth seeing it as part of a wider mineral picture — see our magnesium for athletes guide.
- Because it’s fat-soluble, take vitamin D with a meal containing some fat for better absorption.
- A GP or sports nutritionist can order a blood test. If you’re going to supplement meaningfully, knowing your baseline is worth it.
Does CapyFuel sell vitamin D?
Not currently — our range focuses on whey protein, creatine, pre-workout and BCAAs. Vitamin D is a nutrient most people should cover through diet, sensible sun exposure and a straightforward standalone supplement from a pharmacy, and we’d rather tell you that than invent a product angle. What we can help with is the training and recovery stack vitamin D sits alongside — if muscle and recovery are on your mind, our guide to supplements for muscle gain and the rest and recovery guide are good next reads.
Related: Magnesium for athletes · Omega-3 for athletes · Electrolytes & hydration · Supplements for muscle gain.
Frequently asked questions
How much vitamin D should an athlete take per day?
EFSA sets the adequate intake for adults at 15 µg (600 IU) per day — the amount needed to maintain healthy blood levels when sun exposure is limited. The 5 µg figure shown as 100% NRV on supplement labels is a labelling reference, not a nutritional target, so check the actual microgram amount on any product. If in doubt, a blood test tells you where you stand.
Does vitamin D improve athletic performance or strength?
Improving performance or strength is not an authorised health claim for vitamin D in the EU, and the evidence in people who already have adequate levels is inconsistent. What is established is that vitamin D contributes to the maintenance of normal muscle function — so correcting a deficiency matters, but supplementing beyond sufficiency likely doesn't add a performance edge.
Can I get enough vitamin D from sunlight in Malta or Cyprus?
In theory yes, but in practice deficiency is common even in sunny countries. If you train indoors, apply sunscreen, avoid the midday sun or have darker skin, synthesis may be lower than you'd expect. Year-round supplementation is a reasonable precaution for many athletes in the region, especially those whose training keeps them out of peak UVB hours.
Is D3 better than D2 in supplements?
D3 (cholecalciferol) is the same form your skin produces from sunlight and is generally more effective at raising and sustaining blood levels than D2 (ergocalciferol). Most quality supplements use D3. If you're comparing products, check the form listed on the label — it's usually stated alongside the microgram amount.