Vitamin D in Winter: Who Needs It and What It Won't Do for Performance

Every winter, vitamin D does the rounds in triathlon clubs. Someone's coach recommends a high-dose spray, someone else has had a blood test that came back low, and a podcast suggests it boosts testosterone, power and immunity all at once. Vitamin D is genuinely important, and many athletes in northern countries do run low over winter. But the evidence splits clearly into what it does for health and what it doesn't do for performance.

This article covers why winter matters, how common low vitamin D is in athletes, what supplementation does and doesn't do for strength, illness and bone, and how much is safe. It isn't a diagnosis or a prescription: if you have symptoms or a medical condition, a doctor should decide what you need.

Why winter matters

Most of your vitamin D comes from your skin, which makes it when exposed to ultraviolet B (UVB) light from the sun. Food provides relatively little: oily fish, egg yolks, red meat and fortified foods contribute some, but it's hard to get enough from diet alone.

At northern latitudes, the sun is too low in winter for enough UVB to reach the ground. The NHS states that "between October and early March we do not make enough vitamin D from sunlight." For a UK triathlete, that's almost exactly the off-season and base-building period.

Winter training makes this worse:

• More indoor sessions. Turbo rides, treadmill runs, gym sessions and pool swims all happen out of the sun.
• Training in the dark. Early-morning and evening sessions before and after work mean little daylight exposure.
• Covered skin. Even outdoor winter sessions involve tights, long sleeves, gloves and hats.

Your body can store vitamin D from summer, but stores fall over the winter months, so levels are usually lowest in late winter and early spring.

How vitamin D is measured

Blood tests measure 25-hydroxyvitamin D, written 25(OH)D. It's reported in nanomoles per litre (nmol/L) in the UK, or nanograms per millilitre (ng/mL) in the US. To convert, 1 ng/mL is roughly 2.5 nmol/L.

There's no universal agreement on the cut-offs. Different studies and organisations use different thresholds:

• Below 25 nmol/L is widely treated as deficient.
• Below 50 nmol/L is often used as insufficient or inadequate.
• Above 75 or 80 nmol/L (30 to 32 ng/mL) is the higher threshold some sports researchers use for "sufficient".

This matters when you read headline numbers. A study that uses 80 nmol/L as its cut-off will find far more "inadequate" athletes than one that uses 50 nmol/L.

How common is low vitamin D in athletes?

A meta-analysis of 2,313 athletes

The broadest picture comes from a 2015 systematic review and meta-analysis in Sports Medicine by Farrokhyar and colleagues. It pooled 23 studies covering 2,313 athletes, with a mean age of 22.5 years and 76% male.

• 56% had inadequate vitamin D, defined as 25(OH)D below 32 ng/mL (about 80 nmol/L). The 95% confidence interval was 44% to 67%.
• Winter and spring increased the risk (risk ratio 1.85) compared with summer and autumn.
• Indoor sports had a higher risk than outdoor sports (risk ratio 1.19).
• Inadequacy was higher in the UK and the Middle East than elsewhere.

Keep the threshold in mind. Under 80 nmol/L is a generous definition of inadequacy, so this figure isn't the proportion of athletes who are clinically deficient. The athletes were also young and mostly male, and none of the included studies were in triathletes.

A UK winter study

A 2013 study in the Journal of Sports Sciences by Close and colleagues measured vitamin D in 61 non-supplemented UK professional athletes and 30 age-matched healthy non-athletes during winter.

• 62% of the athletes (38 of 61) and 73% of the non-athletes (22 of 30) had 25(OH)D below 50 nmol/L.

So even by a stricter threshold, most of these people were low by winter, and being an athlete didn't protect them. The athletes were professionals and the sports and sex split aren't given in the abstract, so treat it as a snapshot rather than a prevalence estimate for age-group triathletes.

What vitamin D does in the body

Vitamin D's best-established job is regulating calcium and phosphate, which makes it central to bone health. It also acts on other tissues, including muscle and the immune system.

A 2018 review by Owens, Allison and Close in Sports Medicine summarised the athlete research. They noted that vitamin D's role in calcium balance and bone health is well established in non-athletes, but that studies in athletes have consistently failed to link blood levels with bone health, possibly because training itself stimulates bone. They also highlighted possible roles in muscle recovery and infection risk, while stressing that much remains uncertain.

That pattern, clear in deficiency and much less clear above it, runs through the rest of the evidence.

What it won't do: strength and performance

The meta-analysis

The most direct test is a 2019 systematic review and meta-analysis of randomised controlled trials by Han and colleagues in the Journal of the International Society of Sports Nutrition. It included 5 RCTs with 163 healthy athletes (86 given vitamin D3, 77 given a placebo).

• Blood levels rose. Athletes who started insufficient reached sufficient levels after daily supplementation for more than 4 weeks.
• Strength didn't improve significantly. There was no significant effect on one-repetition maximum bench press, maximal quadriceps strength or overall strength. The pooled standardised mean difference was -0.75, with a 95% confidence interval from -1.82 to 0.32, which crosses zero.

The authors called for larger, better-designed trials across different sports and populations. Five trials is a small evidence base, and strength isn't the same as endurance performance. But there's no sign here that topping up vitamin D adds strength to an athlete who isn't deficient.

The trial that did find an effect

The Close study also included a placebo-controlled trial. Athletes took 5,000 IU of vitamin D3 a day or a placebo for 8 weeks.

• In the vitamin D group, 25(OH)D rose from 29 to 103 nmol/L on average, and 10 m sprint and vertical jump both improved significantly.
• In the placebo group, levels didn't change significantly (53 to 74 nmol/L), and neither did sprint or jump.

This is often quoted as proof that vitamin D improves performance. Three cautions apply. The vitamin D group started deficient, with an average of 29 nmol/L, so the result concerns correcting deficiency. The abstract doesn't report the group sizes. And sprint and jump aren't endurance outcomes. Inference: correcting a genuine deficiency may help muscle function, but there's no evidence that pushing levels higher in someone who is already sufficient improves triathlon performance.

What it might do: respiratory infections

Winter is also cold-and-flu season, which is where vitamin D's immune role becomes relevant.

The 2017 analysis

A 2017 meta-analysis of individual participant data in the BMJ by Martineau and colleagues pooled 25 randomised controlled trials with 11,321 participants aged 0 to 95. These weren't athletes.

• Overall, vitamin D reduced the odds of at least one acute respiratory infection (adjusted odds ratio 0.88).
• In people starting below 25 nmol/L, the effect was much larger (odds ratio 0.30).
• In people starting at 25 nmol/L or above, there was a smaller benefit (odds ratio 0.75).
• Daily or weekly dosing reduced risk (odds ratio 0.81), but trials using large one-off "bolus" doses showed no significant benefit.
• Serious adverse events didn't differ between vitamin D and placebo.

The 2021 update

A larger 2021 meta-analysis by Jolliffe and colleagues in The Lancet Diabetes & Endocrinology included 46 trials and 75,541 participants. It again found a reduction in risk (odds ratio 0.92), but the authors described it as small: 61.3% of people taking vitamin D had at least one respiratory infection, compared with 62.3% on placebo. Daily doses of 400 to 1,000 IU showed a clearer effect (odds ratio 0.70). Unlike the 2017 analysis, this one found no significant difference by starting vitamin D level.

Inference: regular, modest daily supplementation may slightly reduce your chance of a winter respiratory infection, with any benefit probably largest if you are deficient. It won't make you immune. TriForward's "Train Through a Cold? The "Neck Check" Rule and When to Rest" covers what to do when a cold arrives anyway.

What it might do: bone

Bone is where vitamin D matters most for triathletes. TriForward's "Bone Stress Injuries in Triathletes" lists vitamin D deficiency among the risk factors reported in elite triathletes.

The strongest trial is a 2008 randomised, double-blind, placebo-controlled study by Lappe and colleagues in the Journal of Bone and Mineral Research. It included 5,201 female US Navy recruits during 8 weeks of basic training.

• The supplement was 2,000 mg of calcium and 800 IU of vitamin D a day, against a placebo.
• Stress fractures were diagnosed in 5.3% of the supplemented group and 6.6% of the placebo group, a 20% lower incidence.
• Among the 3,700 who completed the study, the reduction was 21%.

This is a large, well-designed trial, but it has limits for triathletes. The recruits were young women, mostly untrained, doing sudden high-volume marching and running. The supplement combined calcium and vitamin D, so the effect of vitamin D alone isn't known. Inference: the result supports making sure your vitamin D and calcium intake are adequate during a winter run build, not taking large doses.

Who is most likely to be low?

The research and NHS guidance point to some groups at greater risk:

• People with darker skin, including those of African, African-Caribbean or south Asian background, who make less vitamin D from the same sun exposure.
• People who are rarely outdoors, such as those who train mostly indoors and work in offices.
• People who cover most of their skin when outdoors.
• Indoor-sport athletes, as shown in the Farrokhyar meta-analysis. In winter, many triathletes become indoor athletes.

Owens and colleagues added a caution about skin colour. Black athletes often have low measured 25(OH)D without showing problems, possibly because differences in vitamin D binding protein leave more vitamin D available to tissues. They argued that, without signs of a problem, routine screening could lead to inappropriate supplementation.

How much, and how much is too much?

UK advice

The NHS states that government advice is for everyone to consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. People at high risk of low vitamin D are advised to take it all year round. The NHS gives the conversion as 1 microgram equals 40 IU.

The upper limit

The NHS also states: "Do not take more than 100 micrograms (4,000 IU) of vitamin D a day as it could be harmful." Taking too much over a long period can cause calcium to build up in the body, which can weaken bones and damage the kidneys and heart. You can't overdose from sunlight.

Owens and colleagues reported that some athletes take very large doses, and that their own laboratory data suggest high-dose supplementation may be problematic. More isn't better.

Where athletes go wrong

The Close trial used 5,000 IU a day for 8 weeks in deficient athletes under supervision. That's above the NHS limit for self-supplementing, and it's a research dose, not a recommendation. If a blood test shows you are deficient, a doctor may prescribe a higher dose for a set period. That's different from buying a high-strength product and taking it all winter.

A triathlon scenario

The following example is illustrative, not drawn from a study. A 44-year-old triathlete in Manchester trains mostly before work: turbo sessions in the garage, a pool swim at 6.30am and dark-evening runs. By February he feels flat and wonders whether low vitamin D is the reason. A teammate suggests a high-strength 5,000 IU product.

A sensible approach, based on the evidence above, might be:

• From October to March, take a standard 10 microgram (400 IU) daily supplement, as the NHS advises for everyone.
• Don't add a high-strength product on top, and check that other supplements, such as multivitamins or fish oils, aren't also adding vitamin D.
• Get outdoors in daylight when possible, such as a weekend ride or lunchtime run, though winter sun won't make much vitamin D at UK latitudes.
• Take the fatigue to a GP if it persists. Low iron, low energy availability, poor sleep, illness and simply too much training are all more likely causes of feeling flat than vitamin D alone.

Common mistakes

• Treating vitamin D as a performance supplement. The evidence in athletes who aren't deficient doesn't support it.
• Megadosing. Taking high-strength products for months, or stacking products, can exceed the safe upper limit.
• Bolus dosing. Large monthly or one-off doses showed no benefit for respiratory infections in the 2017 analysis, while daily or weekly doses did.
• Self-testing and self-treating. Home tests use different thresholds and labs. Interpreting a result, and deciding on a treatment dose, is a job for a clinician.
• Blaming vitamin D for every winter slump. Fatigue and illness have many causes, including under-fuelling, which TriForward's "Under-Fuelled, Not Undertrained" covers.
• Forgetting about the rest of bone health. Vitamin D works alongside calcium, adequate energy intake and loading, as TriForward's "Lifting for Bone" explains.

What we don't know

• Whether supplementation improves endurance performance. No trials have tested vitamin D on triathlon, cycling or running performance in age-group athletes.
• The right threshold for athletes. Studies use cut-offs from 50 to 80 nmol/L, and there's no agreement on what level is optimal for training.
• Effects in women and masters athletes. Most athlete studies are in young men. The bone trial was in young female recruits taking calcium as well.
• Whether reduced infections translate into more consistent training. The respiratory infection trials weren't in athletes and didn't measure training days lost.
• Individual differences, including how skin colour, genetics and binding proteins affect what a blood test means.

References

• Farrokhyar F, Tabasinejad R, Dao D, Peterson D, Ayeni OR, Hadioonzadeh R, Bhandari M. Prevalence of vitamin D inadequacy in athletes: a systematic review and meta-analysis. Sports Medicine, 2015;45(3):365–378. doi:10.1007/s40279-014-0267-6
• Close GL, Russell J, Cobley JN, Owens DJ, Wilson G, Gregson W, Fraser WD, Morton JP. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function. Journal of Sports Sciences, 2013;31(4):344–353. doi:10.1080/02640414.2012.733822
• Han Q, Li X, Tan Q, Shao JF, Yi M. Effects of vitamin D3 supplementation on serum 25(OH)D concentration and strength in athletes: a systematic review and meta-analysis of randomized controlled trials. Journal of the International Society of Sports Nutrition, 2019;16(1):55. doi:10.1186/s12970-019-0323-6
• Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Medicine, 2018;48(Suppl 1):3–16. doi:10.1007/s40279-017-0841-9
• Martineau AR, Jolliffe DA, Hooper R, Greenberg L, Aloia JF, Bergman P, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 2017;356:i6583. doi:10.1136/bmj.i6583
• Jolliffe DA, Camargo CA, Sluyter JD, Aglipay M, Aloia JF, Ganmaa D, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. The Lancet Diabetes & Endocrinology, 2021;9(5):276–292. doi:10.1016/S2213-8587(21)00051-6
• Lappe JM, Cullen DM, Haynatzki G, Recker RR, Ahlf RL, Thompson K. Calcium and vitamin D supplementation decreases incidence of stress fractures in female navy recruits. Journal of Bone and Mineral Research, 2008;23(5):741–749. doi:10.1359/jbmr.080102
• NHS. Vitamin D. Vitamins and minerals, nhs.uk (page last reviewed 3 August 2020; accessed 29 October 2026). https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/