Step outside on a clear November afternoon and the sun can feel generous enough, but for a good part of the year, across much of North America and Europe, the light reaching the ground can’t make your skin produce vitamin D, however long you stand in it.
That’s the reasoning behind the supplement advice health authorities repeat every autumn. It’s also part of why vitamin D became one of the most popular supplements in the world, often for reasons the evidence doesn’t support.
Why the sun stops working in winter
Your skin makes vitamin D when ultraviolet B light, the shorter-wavelength part of sunlight, hits a cholesterol-based compound just under the surface. The liver and kidneys then convert it, in two steps, into the active form the body uses to absorb calcium and keep bones mineralized.
When the sun sits low in the sky, its light takes a longer, slanted path through the atmosphere, and the ozone layer filters out most of the UVB on the way. Above roughly 37 degrees north (a line running near San Francisco, Richmond and southern Spain), the amount of UVB that gets through falls sharply between about November and February.
Researchers measuring this in Boston found that skin exposed on cloudless winter days produced essentially no vitamin D at all. Farther north, in Seattle, Toronto, London or Berlin, the gap runs longer, roughly October through March.
The body stores some vitamin D in fat tissue and draws on it slowly, so levels usually reach their lowest point in late winter.
What official guidance recommends
In the UK, the NHS advises everyone to consider a daily 10-microgram supplement from October to March. That’s 400 IU, since one microgram equals 40 international units. In the US, the recommended intake from all sources is 600 IU a day for adults up to 70 and 800 IU after that, and most people fall short on food alone.
Supplements on store shelves often come in much higher doses, with 2,000, 5,000 and even 10,000 IU capsules sitting side by side. Both the NHS and US authorities put the safe upper limit for adults at 4,000 IU (100 micrograms) a day.
Who is most likely to run low
For some people the risk of deficiency is real, and the case for supplementing is stronger, sometimes year-round:
- Older adults, whose skin makes vitamin D less efficiently and who often spend less time outdoors. Endocrine Society guidelines specifically back supplements for adults 75 and over.
- People with darker skin. Melanin works as a natural sunscreen, so it takes considerably longer in the sun to make the same amount.
- Anyone who rarely gets outside, including care-home residents, people who are housebound and night-shift workers, as well as people who keep most of their skin covered outdoors.
- Pregnant and breastfeeding women, babies and young children, whose needs are higher. Breast milk and infant formula vary in how much they supply.
- People with conditions that affect fat absorption. Crohn’s, celiac disease and some weight-loss surgeries can reduce how much vitamin D the body absorbs from food or pills.
Food helps a little. Oily fish like salmon, sardines and mackerel are the best natural source, and a single portion of salmon can cover most of a day’s needs. Egg yolks, red meat and liver contribute smaller amounts. In the US, fortified milk, cereals and some orange juice fill much of the gap. Even so, most adults need either sunlight or a supplement to reach the recommended intake.
Where the evidence gets more modest
Over the past twenty years, vitamin D picked up a reputation that went well beyond bones. Observational studies kept finding that people with low levels had more heart disease, cancer, depression and infections, and it looked as if supplements might prevent all of it.
The large trials that followed mostly didn’t bear that out. One of the biggest followed more than 25,000 healthy American adults for over five years. People taking 2,000 IU a day had no fewer cancers, heart attacks, strokes or fractures than those taking a placebo, even among participants who started with low levels.
Low vitamin D often turns out to be a sign of poor health rather than its cause: people who are ill, inactive or mostly indoors tend to have less of it.
In guidelines updated in 2024, the Endocrine Society recommends against routine vitamin D blood tests for healthy people and says most adults under 75 don’t need doses above the standard daily intake. For a healthy adult, a winter supplement keeps levels from dropping into deficiency during the months when sunlight can’t help. The trials found no protection against other diseases beyond that.
The risk of taking too much
Vitamin D is fat-soluble, so the body doesn’t flush out the excess the way it does with vitamin C. High doses taken for months can raise the amount of calcium in the blood, causing nausea, vomiting, excessive thirst, frequent urination, confusion and weakness. Over time, too much calcium can damage the kidneys and harden blood vessels.
Toxicity from sunlight or food is essentially unheard of. It almost always comes from supplements, usually high-dose ones taken for a long time or combined with other products that also contain vitamin D. Check your multivitamin and any calcium supplement before adding a standalone capsule on top.
When to talk to a doctor
A blood test makes sense if you have symptoms that point to genuine deficiency, such as persistent bone pain, muscle weakness or frequent falls. It’s also worth discussing if you have osteoporosis, kidney or liver disease or a malabsorption condition, or if you take medications known to affect vitamin D, including some anti-seizure drugs and long-term steroids. In those cases the right dose can differ substantially from the general advice, and your doctor should set it.
What to do this winter
For most adults living at northern latitudes, a daily supplement of around 400 to 1,000 IU from October through March is enough. Eat oily fish when you can, and keep going outside, since daylight still helps sleep, mood and exercise in the months when it can’t make vitamin D. You can skip the megadoses and, unless you’re in one of the groups above, the blood test. In spring, once the sun climbs high enough again, your skin takes over.