Vitamin D and Endurance Sport: Spotting a Deficit

Vitamin D is the only "vitamin" the body can make for itself — provided skin meets enough UVB radiation. That is exactly where the problem starts in endurance sport: we train early in the morning, late in the evening, indoors through the winter, and in summer we cover up with sunscreen or long sleeves. The result is blood levels that sit below the target range for a large share of endurance athletes in central Europe between November and March — often without them noticing anything.

Why vitamin D matters for endurance athletes

Vitamin D is not a performance booster in the classic sense; it is a regulator. It feeds into calcium and phosphate metabolism, governs bone mineral density, influences protein synthesis in muscle and modulates the immune system. For an endurance athlete those are precisely the four areas where a deficiency does damage over months.

The mechanism runs through the vitamin D receptor, which is present in almost every cell in the body — muscle cells, immune cells, bone cells. When the hormone is missing, those processes run more slowly and less precisely. A level of 30 nmol/l does not mean the body fails immediately; it means several systems are operating under harder conditions. Across a training season, that is exactly the difference that shows up as "somehow nothing is moving forward".

What a deficiency costs in training

• Infection susceptibility: Endurance athletes already train close to the limits of what the immune system tolerates. Remove vitamin D as a modulator and infections cluster right in the build phase — and every lost training block costs several weeks of fitness. Related reading: immune system and endurance training.
• Bone health: Running imposes high mechanical loads. Reduced bone density does not turn that into an acute problem, but it removes your negotiating margin — stress fractures rarely come from a single run, but from months of insufficient recovery. More in stress fractures in runners.
• Muscle strength and recovery: In studies of athletes with a deficiency, supplementation improved jump power, maximal strength and in some cases recovery speed after hard sessions. In those already well supplied the effect disappeared — a deficiency is a brake, not a missing extra.
• Blood formation and oxygen transport: Vitamin D takes part in regulating iron metabolism. A deficiency can encourage iron deficiency and vice versa — two issues that frequently appear together (iron deficiency in endurance athletes).
• Mood and training motivation: The seasonal cluster of low drive in the dark months is partly tied to vitamin D status. Less appetite for sessions means less training volume in practice.

Order matters here: vitamin D explains no miracles and no mysteries. If performance is missing, sleep is poor and the legs feel heavy, a deficiency is one candidate among several — not automatically the answer. But it is a candidate you can rule in or out with a single blood draw.

Why endurance athletes in particular are affected

Vitamin D is produced in the skin by UVB radiation with a wavelength between 290 and 315 nanometres. In central Europe that radiation only reaches the ground in relevant amounts between roughly April and September, and even then only around midday. A runner doing an easy session at six in the morning gets practically no UVB at that time — the sun is too low and the atmosphere filters out the effective part of the spectrum.

Three sport-specific factors add to this. First, clothing: long tights, arm warmers and caps cover almost the entire skin surface in winter. Second, indoor training — cyclists on the turbo, swimmers in the pool and treadmill sessions deliver zero UVB. Third, sunscreen: applied correctly and consistently, SPF 30 clearly reduces vitamin D production. None of these is a mistake in itself. Together they explain why ambitious athletes are not automatically better supplied than someone who never exercises.

Athletes with darker skin are a special case: more melanin substantially lengthens the exposure time required. Anyone living and training in central Europe should factor that into how they read their blood level.

Which values count — and what each number means

What gets measured is 25-hydroxyvitamin D, abbreviated 25(OH)D. That is the storage value, not the active hormone — and the only figure worth judging by. Labs in Austria and Germany usually report nmol/l, while international literature often uses ng/ml. The conversion factor is 2.5: 50 nmol/l equals 20 ng/ml.

• Below 30 nmol/l (12 ng/ml): pronounced deficiency. This is no longer about performance but about basic function — supplementation is medically warranted.
• 30 to 50 nmol/l (12–20 ng/ml): insufficient supply. For endurance athletes this is the band where most subclinical effects have been described.
• 50 to 125 nmol/l (20–50 ng/ml): the target range for athletes. Bone, muscle and immune system are supplied here with no detectable downside.
• Above 125 nmol/l (50 ng/ml): no proven added benefit, and the risk of hypercalcaemia rises at clearly higher levels. Higher is not better.

Timing matters: someone testing in September after a sunny summer sees a completely different number than in February. For athletes, a late-winter draw (February/March) is most meaningful — that is where the true low point shows. A test at the height of summer says little about how you will be supplied through the training winter.

Supplementation: dose, form and timing

With a confirmed deficiency, 1,000 to 2,000 IU (25–50 µg) per day is a common starting point; in pronounced cases higher doses — sometimes as a weekly megadose — are prescribed after medical consultation. Vitamin D is fat-soluble: it should be taken with a meal containing fat, otherwise absorption drops noticeably. Daily dosing has a slight edge over weekly in studies because it produces steadier levels.

On form: cholecalciferol (vitamin D3) is the body's own variant and outperforms plant-derived D2 when it comes to raising blood levels. Combinations with vitamin K2 are widespread, but an added benefit for healthy athletes is not established — anyone eating a balanced diet with leafy greens covers vitamin K anyway. Calcium does not need supplementing as long as the diet is sound.

Retest after eight to twelve weeks: without a follow-up measurement, supplementation is flying blind. Anyone still below 50 nmol/l after three months needs a higher dose or a search for the underlying cause — not simply more pills on a hunch. And one rule always holds: taking a supplement is no substitute for proper diagnostics. Fatigue, performance decline and frequent infections have many possible causes, and a blood panel brings order to them (see blood tests for endurance athletes).

The honest limits

Vitamin D is popular because it is easy to measure and easy to swallow. That ease invites exaggeration. The evidence is sober: with a deficiency there is a benefit from correcting it, while at an adequate level more vitamin D produces no measurable performance gain. There are no solid data showing that high doses turn a well-supplied runner into a faster one.

What actually works stays unglamorous: training volume and distribution, enough sleep, sufficient energy intake from carbohydrate and protein, a reasonable iron status. Vitamin D belongs in that package — as a component, not a shortcut. Anyone who knows their status and corrects it through the winter half of the year has removed an unnecessary brake. That alone is reason enough.

If you want to keep an eye on load and recovery, the Peakora training glossary is a useful reference — it explains terms such as TSS, TSB, supercompensation and training breaks in short form.

How Peakora handles this

Peakora is a training planner, not a health app — and will stay that way. The engine estimates load via TSS and tracks form through CTL, ATL and TSB. If you respond below par to planned sessions over several weeks, that shows up as a visible fatigue curve — the cue to get your blood level checked. The tool does not replace a diagnosis, but it does tell you when a diagnosis is worth pursuing.

Concretely: an unplanned cluster of infections, several weeks without progress despite rising load, or conspicuously slow recovery after hard sessions are all reasons to get a blood draw. After that the plan keeps running — load, volume and intensity distribution follow the same logic as before, only without the undetected brake.

Frequently asked questions about vitamin D and endurance sport

Are endurance athletes more likely to be vitamin D deficient?

Yes. Anyone who trains mainly indoors, works indoors through the daylight hours or covers their skin consistently out of caution about sun exposure produces very little vitamin D of their own for months. Endurance athletes in central Europe are frequently below 50 nmol/l through the winter half of the year.

Do you notice a vitamin D deficiency in training?

Usually not directly. Typical signs are non-specific: muscle soreness that lingers longer than usual, frequent colds in autumn and winter, fatigue despite adequate sleep, and performance plateaus with no obvious cause. A blood test settles the question.

How much vitamin D should an endurance athlete take?

With a confirmed deficiency, 1,000 to 2,000 IU per day is a reasonable starting point, and more after medical advice in pronounced cases. Target levels are 50 to 125 nmol/l (20–50 ng/ml) — staying well above that brings no performance benefit.

Can you get enough vitamin D from food alone?

Barely. Oily fish, egg yolk and fortified foods together usually supply only 100 to 300 IU per day. That covers part of the basic requirement, but it does not replace the body's own production via UVB light, and certainly not supplementation when a deficiency exists.

Does vitamin D improve endurance performance?

Only indirectly. Correcting a deficiency can be expected to improve muscle strength, reduce infection frequency and stabilise bone health. If you are already adequately supplied, higher doses produce no measurable performance gain.