Vitamin D in Winter: Who Needs a Supplement and How Much

By late October the midday sun over most of Europe sits low in the sky. It can still feel warm on your face, and on a bright day you might eat lunch outside, but the light reaching your skin carries very little of the ultraviolet B that skin uses to make vitamin D. Depending on how far north you live, that stays true until somewhere around March.

Vitamin D is one of the few supplements that public health bodies suggest healthy adults take, at least for part of the year. It has also been sold as protection against nearly everything, and the largest trial so far didn’t back up the two claims made most often. The two fit together once you look at who was low to begin with.

Why Winter Sunlight Doesn’t Count

Skin makes vitamin D from a cholesterol-like compound when UVB reaches it. When the sun is low, its light crosses more of the atmosphere and the ozone layer soaks up most of the UVB on the way. In 1988, Ann Webb and colleagues measured how well sunlight converted that compound at different times of year in two cities. In Boston, at 42°N (about the latitude of Rome), skin made no vitamin D from the sun from roughly November to February. In Edmonton, at 52°N, level with Amsterdam and Berlin, the gap ran from October to March.

Webb, A.R., Kline, L. & Holick, M.F. (1988). “Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin.” The Journal of Clinical Endocrinology & Metabolism, 67(2), 373-378.

Most of Europe lies north of 42°N, so Paris, London, Munich and Copenhagen all spend the winter in that zone. A rough rule of thumb: when your shadow is longer than you are tall, the sun is too low for much UVB to reach you. The body keeps some vitamin D in reserve from the summer, so levels drift down over the winter and tend to bottom out near its end rather than dropping in November.

Kevin Cashman and colleagues pulled together data from population studies across Europe and found that about 13% of people had blood levels below 30 nmol/L, the usual cut-off for deficiency, averaged across the year, and the share was higher in winter. The risk is greater with darker skin, which needs more sun to make the same amount, in older people, and in anyone who spends most days indoors or covered up.

Cashman, K.D. et al. (2016). “Vitamin D deficiency in Europe: pandemic?” The American Journal of Clinical Nutrition, 103(4), 1033-1044.

In 2016 the UK’s Scientific Advisory Committee on Nutrition published the report behind current British advice: everyone should consider 10 micrograms (400 IU) a day in autumn and winter, and people who rarely get sun on their skin should take it all year. The dose is modest, set so that most people stay out of the deficient range even when the sun contributes nothing.

What the Big Trials Showed

The bigger promises come from observational studies, where people with low vitamin D have more cancer and more heart disease. That link could easily run the other way, since being ill tends to keep people indoors. The VITAL trial tested it directly, giving 25,871 adults in the US either 2,000 IU of vitamin D a day or a placebo for a median of 5.3 years. The supplement did not lower the rate of invasive cancer, or of major cardiovascular events such as heart attacks and strokes. Most of the volunteers weren’t deficient at the start, though, so what VITAL mainly shows is that extra vitamin D doesn’t prevent these diseases in people who already have enough.

Manson, J.E. et al. (2019). “Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease.” New England Journal of Medicine, 380(1), 33-44.

Respiratory infections are where the trial evidence looks more encouraging. Adrian Martineau’s group pooled the individual data of about 11,000 people from 25 randomised trials and found that vitamin D supplements reduced acute respiratory infections such as colds and flu. The overall effect was small. It showed up with daily or weekly doses rather than large monthly ones, and it was biggest in people who had been deficient. Alongside VITAL, that fits a nutrient that matters when you are short of it and adds little once you aren’t.

Martineau, A.R. et al. (2017). “Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data.” BMJ, 356, i6583.

Why a Blood Test Beats Guessing

The test measures 25-hydroxyvitamin D, written 25(OH)D, which reflects roughly the past few weeks. Labs report it in either nmol/L or ng/mL (divide nmol/L by 2.5 to convert, so 30 nmol/L is 12 ng/mL). Below 30 nmol/L is deficient by almost any standard. Above it, guidelines disagree about what counts as enough, so go through your result with your doctor.

Testing also guards against the opposite mistake. Vitamin D builds up in the body, and too much of it raises blood calcium, which over time can damage the kidneys and heart. The European Food Safety Authority sets the upper limit for adults at 100 micrograms (4,000 IU) a day from food and supplements combined, and some high-dose capsules sold online exceed that on their own. If you have kidney disease or take medicines that affect calcium or vitamin D, talk to your doctor before going beyond the standard winter dose.

If you start a supplement this winter, the likeliest problem is forgetting it on busy mornings. Lamplit’s supplements log is free: you note what you took, the dose and when, and a few weeks of entries will show whether “daily” really meant daily. If you get tested, add the 25(OH)D result to the lab tests log as well. A test next spring is far easier to read when you can compare it with this autumn’s.

Start your winter vitamin D log in Lamplit. It’s free and takes less than a minute.