Vitamin D in Winter: Do You Need a Supplement, and How Much?
From autumn to early spring, sunlight in northern countries is too weak for your skin to make vitamin D. Here is what the NHS and Harvard's nutrition experts say about how much you need and where the limit is.
Your skin makes vitamin D when sunlight hits it. In winter, in much of the northern hemisphere, there is not enough sunlight to do the job. That is why health bodies talk about vitamin D every time the days get short.
Here is what the NHS and the Harvard T.H. Chan School of Public Health say about how much you need, who is most at risk and where the safe ceiling sits.
The short version
- The NHS says everyone should consider a daily 10 microgram (400 IU) supplement from October to early March.
- Harvard lists the US recommended dietary allowance as 600 IU (15 mcg) a day for adults 19 and older, and 800 IU (20 mcg) for adults over 70.
- Both put the upper limit at 4,000 IU (100 mcg) a day for adults.
- Food helps, but few foods are rich in vitamin D. Oily fish and egg yolks are among the better sources.
Why winter is the problem
Harvard explains that UVB rays in sunlight let the skin make vitamin D3. Several things cut that production: winter, high northern latitudes, darker skin, older age, full clothing, limited time outdoors and sunscreen. Harvard says correctly applied sunscreen can cut absorption by more than 90%.
Harvard gives a striking example of latitude. People in Boston, Edmonton and Bergen cannot make enough vitamin D from the sun for about four, five and six months of the year respectively. Window glass blocks UVB too, so sitting by a window or in a car does not help.
The NHS puts the UK's season in simple terms. From about late March or early April to the end of September, most people can make all the vitamin D they need from sunlight on their skin. For the rest of the year, the sun is too weak.
How much do you need?
The two sources use different yardsticks, so it helps to line them up.
The NHS says children aged one and over and adults need 10 micrograms, or 400 IU, a day, including pregnant and breastfeeding women. It advises everyone to consider a daily 10 mcg supplement from October to early March.
Harvard cites the US recommended dietary allowance: 600 IU (15 mcg) a day for adults 19 and older, and 800 IU (20 mcg) for adults over 70. Harvard also notes that for most people a supplement is the easiest way to get enough.
Which number applies to you depends on where you live and what your doctor advises. If a doctor recommends a different amount, follow that advice.
Who is more likely to fall short
The NHS says some people should consider a supplement all year round, not only in winter. That includes people who are rarely outdoors, such as those who are frail or housebound, people living in an institution like a care home, and people who usually cover most of their skin outdoors. It also includes people with dark skin, including those of African, African-Caribbean or South Asian background.
Harvard lists further risk factors: little sun, lactose intolerance, a vegan diet, inflammatory bowel disease, obesity and gastric bypass surgery.
What about food?
Food can contribute, but it rarely covers the whole need. The NHS names oily fish such as salmon, sardines, trout, herring and mackerel, as well as red meat, egg yolks and fortified foods like some fat spreads and breakfast cereals. It says UK cows' milk is generally not a good source because it is not fortified.
Harvard puts fatty fish and fish liver oils at the top, with beef liver, egg yolk and cheese in smaller amounts. Some mushrooms contain vitamin D2, and mushrooms exposed to intense UV light can contain more. Fortified orange juice, dairy and plant milks and cereals also count.
The gap can be wide. Harvard cites US survey data in which women aged 51 to 71 had a median intake of 308 IU a day from food and supplements together, but only 140 IU from food alone.
What happens when levels stay low
Both sources point to the bones. The NHS says deficiency can cause rickets in children and bone pain from osteomalacia in adults. Harvard says prolonged deficiency causes rickets in children and osteomalacia, a softening of the bones, in adults.
The upper limit matters
More is not better. The NHS says adults should not take more than 100 micrograms (4,000 IU) a day because it could be harmful. Long-term excess can cause hypercalcaemia, too much calcium in the blood, which can weaken bones and damage the kidneys and heart. For children, the NHS lists lower limits: 50 mcg (2,000 IU) for ages 1 to 10, and 25 mcg (1,000 IU) for infants under 12 months.
Harvard says toxicity mostly comes from supplements, not from food or sun, and advises against more than 4,000 IU a day from supplements unless a doctor is supervising. Signs of toxicity include loss of appetite, weight loss and an irregular heartbeat. Harvard also reports that in one trial, very large infrequent doses of 500,000 IU once a year increased falls and fractures.
The NHS adds that you cannot overdose on vitamin D from sunlight, but you should still protect your skin in strong sun.
Which form to pick
Harvard says D3 (cholecalciferol) tends to raise and sustain blood levels better than D2 (ergocalciferol). Check the label for the dose in micrograms or IU, and keep to the amount you intend to take.
When to talk to a doctor
The NHS advises consulting a doctor if you have a medical condition that may limit how much vitamin D you can safely take, or if you are unsure. That also applies if you are pregnant or breastfeeding, or giving supplements to a child. Note that the NHS page cited here carries an older review date, so check the current guidance for your country before changing what you take.
For most people, the practical takeaway is simple: in the months when the sun cannot do the work, a modest daily supplement, a few vitamin D-rich foods and a respect for the upper limit cover the basics.
