Vitamin D is sometimes called the "sunshine vitamin" for good reason — your skin produces it when exposed to direct sunlight. But here in Cambridge, from roughly October through April, the sun is too far away and too low in the sky for our skin to make meaningful amounts. Add in the fact that vitamin D can't penetrate through glass, and most of us spend our daylight hours indoors, and it's no surprise that seasonal deficiency is extremely common in our region.
What Vitamin D Actually Does
Vitamin D does much more than support bone health. It plays a direct role in immune function by priming your T cells — the "soldiers" of your immune system — to recognize and respond to invading pathogens. Without adequate vitamin D, these cells remain dormant when your body needs them most. This is one reason cold and flu season aligns so closely with the months when our vitamin D levels are lowest.
Research has also identified vitamin D as a meaningful factor in mood and mental health. Studies have found that vitamin D levels are a good predictor of depression risk, particularly in younger women. It takes months to build up adequate vitamin D stores, which means waiting until you feel run down in January is already too late. It also plays essential roles in calcium absorption, bone density, muscle function, and reducing chronic inflammation.
What You Can Do
Supplementation through the darker months is the most practical solution for most people in Ontario. Here's what the evidence supports:
- Supplement with 2,000–4,000 IU of vitamin D3 daily from October through April. D3 (cholecalciferol) is the form your body makes naturally and is better absorbed than D2.
- Take vitamin D with a meal that contains fat — since it's fat-soluble, absorption improves significantly when consumed with dietary fat.
- Get outside during the summer months. Even 15–20 minutes of direct sun exposure on your arms and face (without sunscreen) a few times per week helps build your stores. Remember, sunlight through a window doesn't count.
- Eat vitamin D-rich foods as a complement to supplementation: fatty fish (salmon, sardines, mackerel), egg yolks, and fortified foods. These help, but generally aren't enough on their own in northern latitudes.
- Ask your healthcare provider to test your 25-hydroxyvitamin D levels. This simple blood test tells you where you stand and helps determine the right dose for your body. Optimal levels are generally considered to be between 75–150 nmol/L.
- Vitamin D supports calcium absorption, so ensure you're also getting adequate calcium and magnesium — the three work together for bone and muscle health.
While 2,000–4,000 IU daily is a common and generally well-tolerated range for adults, the right dose for you depends on your baseline levels, body weight, and individual factors. People with kidney disease or certain medical conditions should use vitamin D under medical supervision. Always discuss supplementation with your healthcare provider.
If you've been feeling more tired, more susceptible to illness, or lower in mood during the darker months, vitamin D deficiency may be playing a role. We can test your levels, recommend the right dose, and see how it fits into your overall health picture.