Vitamin D in a British winter

We spend a lot of our time talking people out of supplements. Most of what is sold is either unnecessary in a decent diet, present at doses too low to matter, or addressing a problem the buyer does not have.

Vitamin D is the exception, and the reason has nothing to do with how well anyone eats.

The geography

Most of your vitamin D comes from sunlight, specifically UVB, acting on your skin. The UK sits between roughly 50 and 60 degrees north. Between October and March, the sun does not get high enough in the sky here for meaningful UVB to reach the ground — which means that for about half the year, however long you stand outside, your skin cannot make vitamin D.^1,2

There is very little of it in food. Oily fish is the best source, with smaller amounts in egg yolks, red meat, and fortified foods such as some cereals and spreads. Even a good diet with fish twice a week does not reliably cover the winter.

So this is not a supplement compensating for poor eating. It is a supplement compensating for living at this latitude, which no amount of cooking fixes.

Why it matters

Vitamin D regulates how much calcium and phosphate your body absorbs, which makes it fundamental to bones, teeth and muscles. Severe deficiency causes rickets in children and osteomalacia — soft, painful bones — in adults.¹

You will also have seen headlines connecting vitamin D to immunity, mood, and a long list of other conditions. Some of that research is genuinely interesting and much of it is not yet settled, and we would rather not oversell it. The bone and muscle case is solid and sufficient on its own.

The numbers

Everyone over one year

10 micrograms (400 IU) a day through autumn and winter. That is the SACN recommendation and the NHS advice, and it is what a standard, inexpensive supplement contains.^1,2

All year round, for some people

• Anyone who is rarely outdoors — including people who are housebound, in a care home, or working nights.
• Anyone who usually covers most of their skin when outside.
• People with brown or black skin, including those of African, African-Caribbean and South Asian background, who may not make enough vitamin D from summer sunlight.¹

Babies and young children

• Breastfed babies from birth to one year: 8.5 to 10 micrograms a day.
• Formula-fed babies: no supplement needed while having more than 500ml of formula a day, as it is already fortified.
• Children aged one to four: 10 micrograms a day, all year round.

Pregnancy and breastfeeding

10 micrograms a day, throughout. If you are eligible for the Healthy Start scheme, the vitamins are provided free — worth checking, because take-up is much lower than eligibility.¹

How much is too much

Vitamin D is stored in the body, so unlike vitamin C you can take too much. Very high intakes over long periods can raise blood calcium, which can damage the kidneys and heart.

The NHS advises that adults should not take more than 100 micrograms (4,000 IU) a day, with lower limits for children — 50 micrograms for ages one to ten, and 25 micrograms for infants under one.¹

You will see 2,000 IU, 4,000 IU and even 10,000 IU products on the shelf. Unless a doctor has tested you and told you to take a higher dose, there is no advantage to going above 400 IU, and one bottle of high-strength capsules plus a multivitamin plus a fortified cereal can add up faster than people expect. Check what is already in what you take.

What we'd do this week

Buy one bottle of 10µg vitamin D, and put it somewhere you cannot miss it — next to the kettle, or beside the toothbrushes. Get the children's drops at the same time if that applies.

Then stop thinking about it until March. It is, genuinely, the cheapest and best-supported thing on any list we write.