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Vitamin D occupies an odd position in sports nutrition. It is one of the few supplements with a genuinely strong case behind it — and that case is far narrower than the marketing around it implies. The honest summary is two sentences: if you are deficient, correcting it matters. If you are not, taking more does not appear to make you faster.
What it does
Vitamin D is really a hormone rather than a vitamin, and receptors for it appear throughout the body, including in muscle. It governs calcium absorption and bone mineralisation, plays a role in immune function, and is involved in muscle function — which is where the athletic interest comes from.
The defining feature for cyclists is how we get it: most of it is made in the skin from UVB exposure, with diet a distant second. Oily fish, egg yolks and fortified foods contribute, but for most people at northern latitudes food alone does not cover a winter.
Why cyclists are a higher-risk group
Above roughly 40° latitude — most of northern Europe, the UK, Canada and the northern US — winter sunlight is too weak for meaningful skin synthesis for several months of the year, regardless of how much time you spend outside.
The cyclist's specific version of the problem. The months when sunlight is scarcest are also the months when the riding moves to a trainer in a garage.
Winter kit covers everything but a small part of the face. Sunscreen, applied properly, also reduces synthesis — which is not a reason to skip it.
More melanin means less vitamin D produced for the same sun exposure, so the risk of low status at high latitude is meaningfully greater.
It is worth being clear that athletes are not a magically protected group. Reviews of vitamin D status in athletes find that prevalence of low status broadly tracks the general population, with some athletic groups reporting substantial proportions below the usual thresholds. Being outdoorsy in summer does not immunise you against February.
What the evidence actually supports
This is where the useful distinction lives, and it is the one most supplement copy blurs.
A meta-analysis of randomised controlled trials found that supplementation improved measures of strength and power in athletes who started out insufficient. Bone health is the other well-established reason to care, and it matters especially to cyclists, whose sport does very little for bone density on its own.
The same body of work finds that pushing an already-sufficient athlete higher does not produce further gains in maximal strength and power. A systematic review of supplementation in elite athletes found effects on aerobic and anaerobic performance measures inconsistent at best. More is not better; enough is better than not enough.
That asymmetry is the whole practical message. Vitamin D is not an ergogenic aid. It is a deficiency you would rather not have.
The bone point deserves its own paragraph
Cycling is non-weight-bearing. Unlike runners, cyclists get very little mechanical loading stimulus for bone, and low bone mineral density is a recognised concern in the sport — particularly for riders who also spend time in an energy deficit, where the two problems compound. If you read one companion article to this one, make it low energy availability and REDs, and the practical response is the boring one: strength training for cyclists loads bone in a way that riding never will.
What to actually do
- Get tested if you are in a risk group. A blood test for 25(OH)D is inexpensive and turns guesswork into a number. This is a conversation for a doctor, particularly because thresholds and units differ between countries and laboratories.
- Do not take large doses on a hunch. Vitamin D is fat-soluble and accumulates; very high intakes over time can cause harm. This is not a "take more just in case" nutrient.
- Get sun in summer, sensibly. Short exposures on bare arms and legs outside peak burning hours build stores for later. Burning is not a training strategy, and skin cancer risk is real.
- Eat the food sources anyway. Oily fish a couple of times a week is good for several reasons, of which this is only one.
- If you supplement, do it on advice and re-test. The point is to move a number into range, and the only way to know that happened is to measure it again.
Test, then treat — with a doctor
Everything above is background for a conversation with a medical professional, not a substitute for one. If you are also chronically fatigued, vitamin D is only one of several things worth checking — iron and ferritin covers the other common one, with the same "get a test first" warning.
In perspective
Vitamin D is worth ten minutes of attention once a year and no more. It will not transform your riding. What it can do is remove a quiet handicap you did not know you had — and, more importantly for a cyclist, look after bone that the sport itself does nothing to protect.
Once that is sorted, the things that will actually make you faster are the unglamorous ones: consistent training, enough food, and enough sleep. Moveee will help with the first of those — have a look at your plan — and no supplement on the market competes with it.
Sources 2
Where this article summarises a study, the study itself is linked — not a write-up of it.
- 1 Wyatt PB, Reiter CR, Satalich JR, et al. Effects of Vitamin D Supplementation in Elite Athletes: A Systematic Review · Orthopaedic Journal of Sports Medicine · 2024
- 2 Sist M, Zou L, Galloway SDR, Rodriguez-Sanchez N. Effects of vitamin D supplementation on maximal strength and power in athletes: a systematic review and meta-analysis of randomized controlled trials · Frontiers in Nutrition · 2023
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