Key points
Winter skin makes almost no vitamin D. At latitude 42° (Boston), winter sun did not trigger its synthesis in skin samples from November to February, and at the latitude of Edmonton (about 53°) from October to March (Webb AR J Clin Endocrinol Metab 1988. PMID 2839537).
Low levels are common. Among 55,844 Europeans, 13% had a 25(OH)D level below 12 ng/mL (30 nmol/L), 17.7% in winter; 40.4% had a level below 20 ng/mL (50 nmol/L) (Cashman KD Am J Clin Nutr 2016. PMID 26864360).
Fewer acute respiratory infections. In a meta-analysis of 46 studies, daily vitamin D intake lowered the risk of acute respiratory infections (Jolliffe DA Lancet Diabetes Endocrinol 2021. PMID 33798465).
Fewer autoimmune diseases. In the VITAL study, men from age 50 and women from age 55 took 2000 IU (50 µg) a day for about 5 years: autoimmune diseases developed in 123 people versus 155 on placebo — 22% fewer (Hahn J BMJ 2022. PMID 35082139).
Lower cancer mortality. In a meta-analysis of 5 studies, cancer mortality with vitamin D was 13% lower, mainly with daily intake (Keum N Ann Oncol 2019. PMID 30796437).
Why we run short in winter
Vitamin D is made in the skin by ultraviolet B rays (UVB). In winter the sun is low, its rays pass through a thick layer of atmosphere, and almost all UVB is lost along the way. The farther north, the longer the “vitamin D winter”: on clear days there was no synthesis for 4 months at the latitude of Boston and for 6 at the latitude of Edmonton (Webb AR J Clin Endocrinol Metab 1988. PMID 2839537). Kyiv, Warsaw, Berlin and London lie at 50–53°, that is, closer to Edmonton.
In winter in Europe, a level below 12 ng/mL (30 nmol/L) was twice as common as in summer: 17.7% versus 8.3%. In people with dark skin, a low level was 3–71 times more common than in people with light skin (Cashman KD Am J Clin Nutr 2016. PMID 26864360).
Few foods are rich in vitamin D, so people get it both from fortified foods and from supplements (Demay MB J Clin Endocrinol Metab 2024. PMID 38828931). Oily sea fish and fish oil contain the most.
How much you need
The basic recommended intake. 600 IU (15 µg) a day up to age 70 and 800 IU (20 µg) after 70 — this is intake from food and supplements combined. It assumes minimal sun exposure and provides a 25(OH)D level of at least 20 ng/mL (50 nmol/L) in 97.5% of people (Ross AC J Clin Endocrinol Metab 2011. PMID 21118827). In winter, when there is no sun, reaching it is especially important.
Who needs more than the recommended intake. In its 2024 recommendations, the Endocrine Society conditionally suggests supplementation for children and adolescents aged 1–18, people over 75, pregnant women and people with high-risk prediabetes. It does not advise healthy adults under 75 to take more than the recommended intake to prevent disease (Demay MB J Clin Endocrinol Metab 2024. PMID 38828931). A dose above the recommended intake is chosen by a doctor.
Which doses were studied. In the subgroup of studies that gave 400–1000 IU (10–25 µg) a day for up to 12 months, the risk of acute respiratory infections was lower, although the role of the dose itself has not been proven (Jolliffe DA Lancet Diabetes Endocrinol 2021. PMID 33798465).
How to take it so it works
Every day, not in infrequent large doses. In the 2021 meta-analysis, protection against infections was found with daily intake (Jolliffe DA Lancet Diabetes Endocrinol 2021. PMID 33798465). If vitamin D is indicated, the Endocrine Society suggests that non-pregnant people over 50 take it daily rather than in infrequent large doses (Demay MB J Clin Endocrinol Metab 2024. PMID 38828931).
With the most substantial meal of the day. Vitamin D dissolves in fat. In 17 patients whose levels rose poorly on treatment (30.5 ng/mL, or 76 nmol/L, at baseline), after they switched to taking it with their largest meal, 25(OH)D rose by an average of 57% over 2–3 months — to 47.2 ng/mL (118 nmol/L); there was no control group (Mulligan GB J Bone Miner Res 2010. PMID 20200983).
About pairing it with K2. Vitamin D helps the body absorb calcium, and vitamin K2 takes part in the work of proteins that bind calcium in bone. Studies have not yet proven that adding K2 to vitamin D benefits everyone; when the pair is justified is covered in the article Vitamin D and K2 in Hashimoto’s thyroiditis: a biological pair.
What the studies showed
| What was studied | Who or what | Result |
|---|---|---|
| Sun and skin in winter, 1988 | Skin samples at 42° and about 53° | No synthesis: November–February and October–March |
| Levels in Europe, 2016 | 55,844 people | Below 12 ng/mL (30 nmol/L) in 13%, in winter in 17.7% |
| Infections, meta-analysis of 25 studies, 2017 | 10,933 people | Lower risk, especially with daily or weekly intake |
| Infections, 2021 meta-analysis | Data from 43 studies, 48,488 people | Lower risk with daily intake |
| Autoimmune diseases, VITAL, 2022 | 25,871 people, 2000 IU (50 µg) a day | 123 vs 155 cases, −22% |
| Cancer mortality, meta-analysis, 2019 | 5 studies | −13% |
| Taking with food, 2010 | 17 patients, no control group | Level +57% over 2–3 months |
Not all expectations were confirmed: in the same VITAL study, vitamin D did not reduce the rate of cancer or cardiovascular events (Manson JE N Engl J Med 2019. PMID 30415629), the meta-analysis found no reduction in cancer incidence (Keum N Ann Oncol 2019. PMID 30796437), and protection against infections is small overall, and its dependence on the baseline level was not confirmed in 2021 (Jolliffe DA Lancet Diabetes Endocrinol 2021. PMID 33798465).
Do you need a test
Without established indications, the Endocrine Society in its 2024 recommendations does not advise routine 25(OH)D testing for anyone — including pregnant women, people over 75, people with obesity and people with dark skin: there are no studies proving that such testing is beneficial (Demay MB J Clin Endocrinol Metab 2024. PMID 38828931). The indications for testing are determined by a doctor.
If you are being treated for hypothyroidism or autoimmune thyroiditis, vitamin D doses and monitoring are covered in the article Vitamin D in hypothyroidism and thyroiditis: doses, injections and monitoring.
Who should be more careful
- With kidney disease and kidney stones, sarcoidosis, high blood calcium or parathyroid disease, vitamin D is taken only in agreement with a doctor.
- High doses are not taken without monitoring: a higher 25(OH)D level was not consistently linked to greater benefit, and for some outcomes observational data showed a higher risk at both low and high levels (Ross AC J Clin Endocrinol Metab 2011. PMID 21118827).
- During pregnancy, the dose is chosen by a doctor.
Bottom line
In winter at our latitudes the skin makes almost no vitamin D, so it is worth reaching the recommended intake with food and supplements: every day, with the most substantial meal. In studies, daily intake lowered the risk of acute respiratory infections and autoimmune diseases. More than the recommended intake is only for children, people over 75, pregnant women and people with high-risk prediabetes, as prescribed by a doctor. You can work out a regimen that fits your health at a consultation.
This material is for information only and does not replace a consultation with a doctor. A vitamin D dose above the daily recommended intake is chosen by a doctor.
