Vitamin D in Northern Winters: What the Research Says

If you live in Canada or the northern United States, your skin makes little or no vitamin D from sunlight for several months each winter. That is the single most useful fact about vitamin D for northern readers: from about late October to early March, food and, for some people, a supplement become the main sources. Below is what the research says about why this happens, who is most affected, how much vitamin D adults need, and how to talk with your doctor about it.

Why winter sunlight falls short

Your skin produces vitamin D when it is exposed to ultraviolet B (UVB) light. In winter, the sun sits lower in the sky, and its rays travel through more of the atmosphere before they reach the ground. Most of the UVB is filtered out along the way.

A classic study by Webb, Kline and Holick tested this directly. They found that winter sunlight in Boston (about 42 degrees north) did not produce vitamin D in skin samples from November through February. In Edmonton (about 52 degrees north), the "vitamin D winter" stretched from October through March. Statistics Canada notes the same pattern for the country as a whole: during fall and winter, from late October to early March, very few UVB rays reach the surface, so Canadians may not make enough vitamin D from the sun during those months.

Sitting by a sunny window does not help much either, because ordinary window glass blocks most UVB.

How common is low vitamin D?

Low levels are not rare. In national testing from 2012 to 2013, Statistics Canada found that about a third of Canadians had blood levels of vitamin D below 50 nanomoles per litre (nmol/L), the level generally considered adequate for bone health in most people. About one in ten were below 30 nmol/L, the range linked with deficiency.

Researchers measure vitamin D status with a blood test for 25-hydroxyvitamin D. The U.S. National Institutes of Health Office of Dietary Supplements (NIH ODS) explains that levels of 50 nmol/L (20 ng/mL) or more are adequate for most people, while levels below 30 nmol/L (12 ng/mL) are too low for bone and overall health.

Why vitamin D matters

Vitamin D helps the body absorb calcium and maintain normal levels of calcium and phosphate, which bones need to stay strong. Over time, too little vitamin D can contribute to thin, brittle bones. According to the NIH ODS, vitamin D also plays roles in muscle function and the immune system. Scientists continue to study its links with many other aspects of health, and results in those areas are still mixed, so it is best to see vitamin D as one important nutrient among many rather than a cure-all.

Who is most likely to run low in winter

Some people have a harder time keeping their vitamin D up, especially in the darker months:

• Older adults. Aging skin makes vitamin D less efficiently. MacLaughlin and Holick showed that skin from older adults produced far less of the vitamin D precursor than skin from younger adults under the same light.


• People with darker skin. More melanin means the skin needs more sun exposure to make the same amount of vitamin D.


• People who spend most of their time indoors, including many older adults living in care homes, shift workers and office workers.


• People who cover most of their skin outdoors or use sunscreen consistently (which remains wise for skin cancer prevention).


• People with conditions or surgeries that affect fat absorption, such as some digestive disorders, since vitamin D is fat-soluble.


• People with higher body weight, because vitamin D is stored in fat tissue and may be less available in the blood.

How much vitamin D do adults need?

Health Canada and the U.S. National Academies use the same Dietary Reference Intakes for vitamin D:

• Ages 1 to 70: 600 IU (15 micrograms) per day


• Over 70: 800 IU (20 micrograms) per day


• Tolerable upper intake level for adults: 4,000 IU (100 micrograms) per day from all sources

Health Canada has also advised that, in addition to following Canada's Food Guide, everyone over the age of 50 should take a daily vitamin D supplement of 400 IU. Your own needs may differ, and a health care provider may recommend a different amount based on a blood test, your medical history or medications you take. Do not take more than the upper limit unless your doctor has advised it and is monitoring you.

Food sources to lean on in winter

Few foods are naturally rich in vitamin D, but a handful are worth knowing:

• Fatty fish such as salmon, trout, sardines and mackerel


• Egg yolks


• Mushrooms that have been exposed to UV light (check the label)


• Fortified foods. In Canada, cow's milk and margarine are fortified with vitamin D, and many plant-based beverages and some yogurts are too.

A serving of salmon can provide a large share of a day's needs, while a cup of fortified milk provides roughly 100 IU. Because typical diets fall short, the NIH ODS notes that many people do not get the recommended amounts from food alone, which is one reason supplements are common in northern countries.

D2 or D3?

Supplements come in two main forms: vitamin D2 (ergocalciferol), usually made from yeast or fungi, and vitamin D3 (cholecalciferol), the same form your skin makes. Both are absorbed well. A 2012 meta-analysis by Tripkovic and colleagues found that vitamin D3 was more effective than D2 at raising and maintaining blood levels of 25-hydroxyvitamin D. For people following a vegan diet, D3 made from lichen is now available.

Vitamin D is fat-soluble, so taking it with a meal that contains some fat may help absorption.

Talk with your doctor first

Before you start a vitamin D supplement, or change the amount you take, talk with your doctor or pharmacist. This is especially important if you:

• Take prescription medications. Some, such as certain steroids, weight-loss drugs and anti-seizure medications, can affect vitamin D levels, and vitamin D can interact with others.


• Have kidney disease, a history of kidney stones, or a condition that affects calcium levels.


• Are pregnant or breastfeeding.

Never stop or change a prescribed medication on your own because you have started a supplement. Your doctor can also tell you whether a blood test makes sense for you; routine testing is not recommended for everyone.

A simple winter plan

• Know your season. If you live in Canada or the northern United States, assume your skin makes little vitamin D from about late October to early March.


• Eat for vitamin D. Include fatty fish once or twice a week, eggs, and fortified milk or plant beverages.


• Ask about a supplement. Especially if you are over 50, have darker skin or spend most of your time indoors, ask your health care provider whether a daily supplement is right for you and how much to take.


• Stay under the upper limit unless your doctor advises otherwise.


• Get outside when you can. Winter walks may not make much vitamin D, but daylight and movement still support overall well-being, and spring sun will return.

The bottom line

For people in northern climates, vitamin D is a seasonal issue. The winter sun simply is not strong enough to keep levels up for several months of the year, and about a third of Canadians already have levels below what is considered adequate. A food-first approach, a conversation with your doctor about supplementation, and attention to safe upper limits can help you get through the darker months well.