Articles · Nutrients
Vitamin D in winter: can the sun still do the job?
Why the winter sun makes little or no vitamin D across much of the world, which foods carry some, and who is advised to take a supplement.
· 4 min read
On a bright winter day, it feels as if the sun must be doing you good. It lifts the mood, certainly. But for vitamin D, the low winter sun in much of the world does very little. Why is that, and what can you do about it?
Where does vitamin D come from?
Unlike most vitamins, vitamin D is something the body makes itself, in the skin, when it's hit by a particular kind of sunlight: ultraviolet B (UVB). Food provides a smaller share.
The body needs vitamin D to absorb calcium and keep bones and muscles healthy. Too little for too long can lead to soft, weak bones: rickets in children, osteomalacia in adults.
Think about it: When did you last spend time outdoors in daylight, with your face and arms uncovered, in the middle of the day?
Why winter sun doesn't make much
UVB has to pass through the atmosphere at a steep enough angle to reach us. When the sun sits low in the sky, as it does in winter far from the equator, most UVB is filtered out before it reaches the ground.
A classic study measured this with skin samples left in the midday sun: in Boston, about 42° north, they made no vitamin D from November to February. Further north, in Edmonton (52°), the gap ran from October to March. In the sun of Los Angeles (34°) and Puerto Rico (18°), they still made it in midwinter. In the UK, health advice says the sun isn't strong enough from about October to early March.
Even in summer, how much you make depends on the time of day, your skin colour, your age, how much skin is uncovered and sunscreen. Older skin makes less, and darker skin needs more sun to make the same amount.
Impression, Sunrise, Claude Monet, 1872, Musée Marmottan Monet, Paris. A pale orange sun over the misty harbour of Le Havre: the painting that gave Impressionism its name. Astronomers who studied the sun's position and the tides in it think Monet painted it on a November morning. Lovely light, but not much vitamin D. Public domain · Wikimedia CommonsWhich foods carry vitamin D?
Few foods contain much, but some help:
- Oily fish, such as salmon, trout, mackerel, sardines and herring: a portion can provide most of a day's needs.
- Eggs: the yolk carries a little, about 1 microgram per egg.
- Mushrooms exposed to UV light: some are grown or treated this way, and the label says so.
- Fortified foods: some milks, plant drinks, spreads and breakfast cereals have vitamin D added. Check the label.
For most adults in the United States the recommended intake is 15 micrograms (600 IU) a day, and 20 micrograms (800 IU) over 70. That's hard to reach from food alone unless you eat oily fish often.
Should you take a supplement?
In the UK, the NHS advises everyone to consider a daily supplement of 10 micrograms (400 IU) in autumn and winter, and all year round for people who are rarely outdoors, cover most of their skin, or have dark skin. Other countries have similar advice; ask your doctor or pharmacist what applies where you live.
More isn't better: very high doses taken for a long time can raise calcium in the blood to harmful levels. For adults, the upper limit is 100 micrograms (4,000 IU) a day unless a doctor advises otherwise.
Try this: Put a tin of sardines or mackerel on your shopping list this week. On toast with lemon, it's a five-minute lunch with a good dose of vitamin D.
Think about it: Does anyone in your family spend most of the winter indoors, perhaps an older relative? They may benefit most from a supplement.
Quick quiz
Why does the skin make little vitamin D in winter far from the equator?
With the sun low in the sky, most of the UVB light that the skin needs is filtered out by the atmosphere before it reaches the ground.
Which food is one of the richest natural sources of vitamin D?
Oily fish, such as salmon, trout, mackerel, sardines or herring.
Is it safe to take very high doses of vitamin D "just in case"?
No. Long-term high doses can raise blood calcium to harmful levels. Adults shouldn't exceed 100 micrograms (4,000 IU) a day without medical advice.
Sources
General information, not medical advice. If you have a health condition, are pregnant or take medication, ask a doctor or dietitian what is right for you.