Vitamin D in winter: why levels drop and what to eat

Vitamin D levels tend to fall in winter because most of our vitamin D is made in the skin from summer sunlight, and between October and early March the sun in northern countries is too low for that to happen. Food helps a little, but very few foods contain much. That is why the NHS advises everyone in the UK to consider a daily supplement of 10 micrograms during autumn and winter.

Why levels drop in winter

Your skin makes vitamin D when it is exposed to ultraviolet B (UVB) light from the sun. In the UK and countries at a similar latitude, the sunlight from about late March or April to the end of September is strong enough for most people to make what they need from time outdoors.

From October onwards the sun sits lower in the sky, UVB is filtered out by the atmosphere, and the skin makes little or none. Shorter days, more time indoors and more clothing all add to it. The body can store some vitamin D from summer, but for many people those stores run down by late winter.

What vitamin D does

Vitamin D helps the body absorb calcium and phosphate from food, which keeps bones, teeth and muscles healthy. A long-term lack can lead to soft, weak bones: rickets in children and osteomalacia in adults. It also has a role in normal immune function, although claims that extra vitamin D prevents colds or other illness are not well supported.

How much you need

Labels often use international units (IU) instead: 10 micrograms is 400 IU, and 15 micrograms is 600 IU. The figures differ because they come from different methods and assumptions about sunlight, not because the bodies disagree about what vitamin D does.

The foods with the most vitamin D

Only a handful of foods contain useful amounts. These approximate values come from the NIH Office of Dietary Supplements, based on USDA food data. A 3 ounce portion is about 85 g.

Amounts vary a lot between wild and farmed fish, and between brands of fortified foods, so treat these as rough guides.

A few points worth knowing:

• Oily fish is the best natural source. Salmon, trout, mackerel, herring and sardines all contain vitamin D, along with omega-3 fats. Our guide to omega-3 foods covers how often to eat them.
• Fortification depends on where you live. Most milk in the US and Canada is fortified with vitamin D; most cows' milk in the UK is not. Some breakfast cereals, fat spreads, yoghurts and plant milks are fortified. The label will say.
• UV mushrooms are the main plant source. Ordinary mushrooms contain very little, but some are exposed to UV light during growing and are sold that way. Again, check the pack.
• Eggs and red meat add small amounts. Useful, but not enough on their own.

Even a diet rich in these foods usually falls short of 10 micrograms a day in winter, which is the reason for the supplement advice.

When to consider a supplement

The NHS advises that everyone over the age of four in the UK should consider taking 10 micrograms of vitamin D a day during autumn and winter. It advises taking one all year round if you:

• Are rarely outdoors, for example if you are housebound or live in a care home
• Usually wear clothes that cover most of your skin when outside
• Have dark skin, for example if you have an African, African-Caribbean or South Asian background, because your skin makes less vitamin D from sunlight

Babies and young children have their own advice, and many receive vitamin D through infant formula or vitamin drops. A health visitor, pharmacist or GP can tell you what applies to your child.

Can you have too much?

Yes, from supplements. Taking high doses for a long time can cause too much calcium to build up in the body, which can weaken bones and damage the kidneys and heart. The NHS says adults should not take more than 100 micrograms (4,000 IU) a day. You cannot get too much vitamin D from sunlight, and it is very unlikely from food alone.

Some people should not take vitamin D without medical advice, including people with certain kidney conditions or high calcium levels. If you take other medicines or multivitamins, ask a pharmacist so you do not double up.

Questions people ask

Does winter sun through a window help?

Not much. Most window glass blocks UVB, which is the part of sunlight the skin uses to make vitamin D. Daylight is still good for your mood and body clock, so a walk outside in winter is worth it for other reasons.

Can a sunbed top up vitamin D?

The NHS does not recommend sunbeds for this. They raise the risk of skin cancer, and a supplement is a safer way to cover winter.

Should I get my vitamin D level tested?

Most people do not need a test before following the general advice. A doctor may suggest one if you have symptoms such as bone pain or muscle weakness, or a condition that affects how you absorb nutrients.

Photograph: AI-generated image

Filed under vitamin d, winter, oily fish, supplements.