Vitamin D and Your Immune System: What the Research Shows

Vitamin D is best known for bones, but it also plays an important role in your immune system. That matters most in autumn and winter, when vitamin D levels drop for many people in Canada and the northern US, the same months when colds and flu circulate.

How vitamin D works with your immune system

Many immune cells, including T cells, B cells and macrophages, have vitamin D receptors, and some can activate vitamin D themselves. Vitamin D helps these cells respond to germs and also helps keep immune responses balanced, so they do their job without over-reacting. Health Canada recognizes that vitamin D helps maintain normal immune function.

What the research on supplements found

One of the best-known analyses, published in the BMJ in 2017, pooled data from 25 randomized controlled trials with about 11,000 participants. It found that vitamin D supplements modestly reduced the risk of acute respiratory infections overall. The clearest benefit was in people who started with low vitamin D levels, and in those who took vitamin D daily or weekly rather than in occasional very large doses.

Later studies have been mixed, and vitamin D is not a guarantee against getting sick. The takeaway is practical: keeping your vitamin D in a healthy range, especially if you tend to run low, is a sensible part of supporting your immune system.

Why levels drop in winter

Your skin makes vitamin D from UVB sunlight. Above roughly 37 degrees latitude, which includes all of Canada and much of the northern US, the winter sun is too weak for meaningful vitamin D production from about November to February. Add more time indoors and heavier clothing, and levels can fall significantly by late winter.

Who benefits most from paying attention?

• people living in northern regions, especially in winter;
• older adults;
• people with darker skin;
• people who spend most of the day indoors or cover most of their skin;
• people with higher body weight or conditions that affect fat absorption.

Five ways to support your immune system this winter

1. Check your vitamin D level with a blood test, and talk to your provider about how much you need. Our guide to vitamin D deficiency explains the numbers.
2. Take vitamin D consistently, daily with a meal, rather than in occasional large doses.
3. Get enough sleep, 7 to 9 hours a night.
4. Eat a varied diet with fatty fish, eggs, vegetables, fruit and foods rich in zinc and selenium. See minerals and immunity.
5. Keep moving, ideally outdoors in daylight when you can.

This article is general information, not medical advice. Talk to your doctor or pharmacist before taking vitamin D supplements, especially if you have kidney disease, high calcium levels or take medication.