Vitamin D in winter: should you take it? What the studies say

Every autumn brings the same question: should you take vitamin D to get through the winter? Studies provide a two-part answer. Yes, your levels drop in winter, and a modest daily dose is enough to maintain them. But raising them back up probably won't make you either stronger or sick less often — and a massive dose all at once did more harm than good.

In winter, levels drop, even in the sun

In the UK, in the middle of winter, 62% of 61 professional athletes and 73% of 30 healthy adults taking no supplements had a blood vitamin D (25(OH)D) level below 50 nmol/L. In Northern Europe, in February-March, the median level was 29.4 nmol/L in 199 adolescent girls and 40.7 nmol/L in 221 older women; those taking a vitamin D supplement had a higher level.

The country's sun does not protect against everything. In Tunisia, in January and February, out of 150 unsuplemented elite athletes, 55.3% were between 25 and 50 nmol/L and 14.7% below 25 nmol/L. Those who trained indoors had a significantly lower level than those who trained outdoors. Being a woman and being under 18 also increased the risk.

In 225 endurance athletes followed for four winter months, the proportion of those whose status was insufficient or deficient went from 38% at the start to 55% at the end.

How much to take to maintain it

Two trials directly addressed the question, for 22 winter weeks, with 0, 5, 10 or 15 µg of vitamin D3 per day. In 238 adults aged 20 to 40, keeping the level above 50 nmol/L in 97.5% of participants required 28 µg per day; above 25 nmol/L, 8.7 µg were enough. In 225 adults aged 64 and over, the figures are close: 24.7 µg per day for the 50 nmol/L threshold, 8.6 µg for the 25 nmol/L one. In both trials, the requirement varied depending on the sun exposure the previous summer.

In athletes, the doses tested are higher. A meta-analysis of 13 trials on 532 athletes shows that 3,000 to 5,000 IU per day raise an insufficient level to a sufficient one, even in winter beyond the 45th parallel. In 30 athletes, 20,000 or 40,000 IU per week were enough to exceed 50 nmol/L in everyone after 6 and 12 weeks.

More vitamin D, more strength? Not proven

This is what we hope for in the gym, and the two reviews in the batch say no. The 2017 meta-analysis: out of the seven trials that measured performance, none found an effect after 12 weeks, even though the level had gone back up well. The 2023 one, on 11 trials and 436 athletes: no significant effect on maximal strength or power. In those starting below 75 nmol/L, the estimated effect on strength was small (0.25 for the upper body, 0.26 for the lower) and remained compatible with zero.

Individual trials go both ways. On the positive side, 24 elite ballet dancers gained 18.7% in isometric strength and 7.1% in vertical jump with 2,000 IU per day for four winter months, and got injured less — but the control group, which took nothing, only had seven dancers. In the British study, 5,000 IU per day for 8 weeks improved 10 m sprint and vertical jump. In 35 taekwondo practitioners lacking vitamin D, four weeks at 5,000 IU per day improved peak anaerobic power and a knee strength measurement, but not the rest of the tests.

On the negative side: the dose-response trial in 30 athletes changed nothing for the bench press, leg press or vertical jump; in 117 Chinese students, one winter month at 1,000 IU per day raised the level without changing jump or grip strength; in 117 children aged 4 to 8, 20 winter weeks at 10 or 20 µg per day did not increase muscle strength.

Colds and flu: an effect that fades

The 2021 meta-analysis, covering 43 trials and 48,488 people, found a small decrease in the risk of acute respiratory infection (odds ratio 0.92), more pronounced with daily intake (0.78) and doses of 400 to 1,000 IU per day. Its 2025 update adds six trials and 19,337 people, bringing the total to 61,589: the estimated effect barely moves (0.94), but it is no longer significant, and it no longer varies according to age, baseline level, dose, or dosing frequency.

Among winter trials, a placebo-controlled trial in schoolchildren from December to March found 10.8% of type A influenza under 1,200 IU per day versus 18.6% under placebo. In Taiwanese children aged 2 to 5 years, one month at 2,000 IU per day did not significantly reduce influenza or enterovirus infections. In athletes, 25 taekwondo practitioners showed fewer upper respiratory tract infection symptoms the higher their level rose—a correlation in a small sample size. In 225 endurance athletes, those who were deficient had more upper respiratory tract infections, and more severe ones; this is an observational study, which does not say whether a supplement would have prevented them.

The winter blues: little evidence

In 2,117 women aged 70 and over, 800 IU of vitamin D per day (with calcium) for six months, started between May and October, did not improve psychological well-being. A trial of 86 people suggests that vitamin D maintains a normal response to a laboratory-induced stress, whereas the placebo group, whose levels dropped, reacted longer; neither serotonin nor cortisol moved.

No large dose all at once

The idea of a single large dose "for the whole winter" has been tested. In 2,256 women aged 70 and over, a single dose of 500,000 IU per year, given in autumn or winter, caused more falls (+15%) and more fractures (+26%) than placebo. In 46 elite athletes, 70,000 IU per week raised a metabolite (24.25(OH)₂D) that remained elevated after stopping; the authors deem high doses potentially harmful and propose lower, more frequent doses.

Every figure in this article comes from a study you can open: the underlined mentions lead to the summary of the work it is drawn from, written from the abstract alone. Several sports trials involve 24 to 46 people; winter requirements depend on sun exposure in summer, so trial doses do not apply as-is to everyone. Informational content, which does not replace the advice of a healthcare professional.