Ask the Editor: Do I Need More Vitamin D in Winter?

Our question this time is a seasonal one. The days are drawing in, and a reader wants to know whether they should be taking more vitamin D over the winter.

The short answer is no. The amount of vitamin D you need does not change with the season. What changes is where it comes from. In summer the sun may cover some of it. In winter, food and any supplement you already take carry more of the load. Below we explain why the number stays put, what the darker months change, and when it makes sense to speak to a doctor or pharmacist.

The number does not change

According to the NIH Office of Dietary Supplements, the recommended amounts (RDAs) are:

• 15 mcg (600 IU) a day for ages 1 to 70, including during pregnancy and breastfeeding
• 20 mcg (800 IU) a day for adults over 70
• 10 mcg (400 IU) a day for infants under 12 months, which is an Adequate Intake rather than an RDA

If you are switching between labels, 1 mcg of vitamin D equals 40 IU.

The reason these figures do not rise in the winter months is worth knowing. The Food and Nutrition Board set them on the assumption that people get very little sun. The recommendation already expects almost no help from sunlight. So the winter answer is the same as the answer in July: 600 IU for most adults, 800 IU over 70.

What changes in winter is the sun

Your skin can make vitamin D when UVB radiation reaches it. How much it makes depends on many things, including the season, the time of day, the length of the day, cloud cover, smog, how much melanin is in your skin and whether you wear sunscreen. Older people and people with dark skin make less vitamin D from sunlight.

Shorter, greyer days mean less of that UVB. One detail surprises a lot of people: UVB does not pass through glass. Sitting in a bright window seat on a crisp January afternoon feels pleasant, but it does not produce vitamin D.

The NIH fact sheet is candid that all these variables, together with uncertainty about how much sun is actually needed, make it difficult to set guidelines on how much sun exposure is enough. That is one more reason the recommendations are built around food and supplements.

It is also why US survey data cited by NIH can seem puzzling at first. Most people in the United States take in less than the recommended amount from food and supplements, yet most have sufficient blood levels by the Food and Nutrition Board’s thresholds. Sun exposure is one reason. In winter, that cushion gets thinner, so what you eat matters more.

Where to find vitamin D in food

Few foods contain vitamin D naturally. Among the best sources are:

• The flesh of fatty fish such as trout, salmon, tuna and mackerel
• Fish liver oils
• Smaller amounts in beef liver, egg yolks and cheese

In American diets, most vitamin D comes from fortified foods such as milk and breakfast cereals. Almost all US milk is fortified with about 3 mcg (120 IU) per cup. Some plant milks are fortified too, so it is worth checking the label. As a rough illustration, a cup of fortified milk covers about a fifth of the 600 IU most adults need, which shows why a winter diet built only on everyday foods can fall short without some planning.

Supplements and the upper limit

Plenty of people take a vitamin D supplement. We will not suggest a product or a dose here, because the right choice depends on your diet, health and circumstances. Our guide on when and how to take vitamin D covers the practical side, including food timing, the different forms and blood tests.

What we can share is the ceiling. NIH lists a Tolerable Upper Intake Level, which counts vitamin D from all sources combined:

• 25 mcg (1,000 IU) for 0 to 6 months
• 38 mcg (1,500 IU) for 7 to 12 months
• 63 mcg (2,500 IU) for 1 to 3 years
• 75 mcg (3,000 IU) for 4 to 8 years
• 100 mcg (4,000 IU) from age 9, including during pregnancy and breastfeeding

Too much vitamin D almost always comes from supplements, not the sun. Experts do not believe that too much sun causes vitamin D toxicity, because the skin limits how much it makes. Doses above the upper limit belong only under medical supervision. The same idea applies to other supplements people reach for in winter. Our page on magnesium dosage and the upper limit explains how that one works.

Who should talk to a doctor or pharmacist

According to NIH, some groups are more likely to have inadequate vitamin D status:

• Breastfed infants
• Older adults
• People with limited sun exposure, such as those who are homebound or whose clothing or work keeps their skin covered
• People with dark skin
• People with conditions that limit fat absorption
• People with obesity or who have had gastric bypass surgery

If you are in one of these groups, or you suspect your intake from food is low, a doctor or pharmacist can advise you. They can also tell you whether a blood test makes sense for you. This is especially worth doing before you change anything you already take.

The takeaway

Vitamin D in winter does not call for a bigger number. It calls for a closer look at your food and your habits, because the sun is doing less. To see the recommended amount and upper limit for your age, sex and life stage, try our nutrient lookup. For the practical details of taking it, our vitamin D guide above is the place to start.