Key points
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[1]). 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[2]).
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[6]). Oily sea fish and fish oil contain the most.
How much you need
How to take it so it works
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[9]), the meta-analysis found no reduction in cancer incidence (Keum N Ann Oncol 2019. PMID 30796437[5]), 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[3]).
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[6]). 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[7]).
- 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.
References
- Webb AR J Clin Endocrinol Metab 1988. PMID 2839537
- Cashman KD Am J Clin Nutr 2016. PMID 26864360
- Jolliffe DA Lancet Diabetes Endocrinol 2021. PMID 33798465
- Hahn J BMJ 2022. PMID 35082139
- Keum N Ann Oncol 2019. PMID 30796437
- Demay MB J Clin Endocrinol Metab 2024. PMID 38828931
- Ross AC J Clin Endocrinol Metab 2011. PMID 21118827
- Mulligan GB J Bone Miner Res 2010. PMID 20200983
- Manson JE N Engl J Med 2019. PMID 30415629
Key facts
- At latitude 42° (Boston), winter sun did not trigger vitamin D 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).
- Among 55,844 Europeans, 13% had a 25(OH)D level below 12 ng/mL (30 nmol/L), and in winter 17.7% (Cashman KD, Am J Clin Nutr 2016).
- Daily vitamin D intake lowered the risk of acute respiratory infections (Jolliffe DA, Lancet Diabetes Endocrinol 2021).
- 2000 IU (50 µg) a day for about 5 years: autoimmune diseases in 123 people versus 155 on placebo, 22% fewer (Hahn J, BMJ 2022).
- The recommended intake from food and supplements combined is 600 IU (15 µg) a day up to age 70 and 800 IU (20 µg) after 70; it assumes minimal sun exposure (Ross AC, J Clin Endocrinol Metab 2011).
