Vitamin D in a Canadian Winter: Dose, Food, Testing

If you live above 44° North latitude — Toronto, Ottawa, Calgary, Vancouver, almost any major Canadian city — your skin makes almost no vitamin D from October through April. This is not a guess; it's a function of atmospheric physics. Vitamin D synthesis requires UVB radiation at a specific narrow wavelength of 290-315 nm, and when the winter sun sits low on the horizon, that wavelength is absorbed by the deeper atmosphere it traverses before reaching the ground. By the time the photons get to your forearms, the UVB portion has been scrubbed out Holick 2011.

The practical consequence: a Canadian winter is a low-vitamin-D environment by default. The 2016 Cashman pan-European survey put about 13% of Europeans below the deficiency threshold of 30 nmol/L of serum 25-hydroxyvitamin D, with the worst rates in winter and at higher latitudes Cashman 2016. Canadian survey data tell a similar story. The fix is not exotic: a modest daily supplement, modestly fortified food, and a clear-eyed view of which questions a blood test actually answers.

Why winter blocks the UVB band

UVB radiation is the higher-energy portion of ultraviolet light. It's what triggers the photochemical conversion of 7-dehydrocholesterol in your skin into pre-vitamin D3, which then thermally isomerises to vitamin D3 over the next day or so. The UVB band — 290-315 nm — is also the band most aggressively scattered and absorbed by the ozone layer and by air molecules along an oblique solar path.

In summer at 44° North, the sun rises high enough at midday that UVB makes it through to your skin in usable doses. In winter the solar angle never gets above about 25°, the path through the atmosphere is much longer, and the UVB is essentially gone by the time the light hits the ground. You can stand outside on a brilliant January day for hours and produce zero vitamin D. The visible light is fine; the UVB band is not.

This is why latitude matters. Below about 35° North — Los Angeles, Atlanta, Casablanca — winter UVB is reduced but still meaningfully present. Above 44° — Toronto, Boston, Munich — it is essentially absent for six months.

The dose the trials actually use

Health Canada's adequate intake is 600 IU/day for adults under 70 and 800 IU/day for those over 70, with a tolerable upper intake of 4000 IU/day Health Canada 2012. These figures are conservative public-health guidance designed to prevent overt deficiency in most adults.

The trial literature consistently uses higher doses to push serum 25-hydroxyvitamin D into the broadly accepted sufficiency range (50-75 nmol/L). The 2022 Bouillon review summarised this: most adults need 800-2000 IU/day to maintain that range through a high-latitude winter, with individual responses varying by body weight, baseline status, and skin pigmentation Bouillon 2022.

The 2018 Pludowski guidelines (Central European consensus) recommend 1000-2000 IU/day for the general adult population in northern winters and up to 4000 IU/day for adults with obesity or malabsorption Pludowski 2018. The 2011 Endocrine Society guideline (US) gives 1500-2000 IU/day as the typical adult requirement Holick 2011.

The practical synthesis: 1000-2000 IU/day of vitamin D3 (cholecalciferol) is the dose backed by the strongest trial evidence for a Canadian adult through winter. Heavier individuals, anyone with darker skin, and the elderly may need the upper end of that range or slightly higher. Talk to your doctor before exceeding 4000 IU/day.

Why D3 beats D2

Two supplemental forms exist. Vitamin D3 (cholecalciferol) is the form your skin makes and the form that occurs in animal products and lichen-derived supplements. Vitamin D2 (ergocalciferol) is plant-derived, mostly from yeast and mushrooms exposed to UV light. They share most of the metabolic pathway but differ in pharmacokinetics.

The 2012 Tripkovic meta-analysis directly compared the two and found D3 raised serum 25-hydroxyvitamin D about 1.7-fold more efficiently than D2 per IU consumed, and the rise was more stable over time Tripkovic 2012. D2 is also less stable on the shelf. Unless you have a specific reason to avoid animal-derived D3 (vegan strictness, supplement-availability constraints), D3 is the better choice.

Food sources that actually contribute

Diet alone will not solve a Canadian winter. But food contributes meaningfully when stacked with a modest supplement.

Fatty fish: the strongest natural source. A 100 g serving of cooked Atlantic salmon supplies roughly 500-700 IU. Sardines, mackerel, and rainbow trout are in the same range. Cod liver oil tops 1000 IU per tablespoon, though it also carries a significant vitamin A load.

Egg yolks: roughly 40 IU per yolk, with eggs from hens raised on vitamin-D-enriched feed or with outdoor access running higher.

Fortified foods in Canada: fluid cow milk is fortified by law to 35-45 IU per 100 mL. Most plant-milk alternatives match that fortification. Margarine is fortified to 530 IU per 100 g. Some yogurts and orange juices add vitamin D voluntarily.

UV-exposed mushrooms: the only meaningful plant source, but the D2 form they provide is less effective.

Realistic Canadian winter food intake adds maybe 200-400 IU/day for most adults. That leaves an 800-1600 IU gap to close with a supplement.

When a blood test is actually worth it

For a typical healthy adult, routine 25-hydroxyvitamin D testing is not recommended by Canadian Task Force or US Preventive Services Task Force guidelines. The reason is straightforward: the answer doesn't change the recommendation. Almost any healthy adult above 44° North will be at the low end of the range in winter, the corrective dose is well-tolerated, and the cost of universal testing is high.

Testing is worth ordering if:

• You have osteoporosis or fracture risk and you and your physician are calibrating treatment
• You have malabsorption — Crohn's, celiac, post-bariatric surgery, chronic pancreatitis — that would impair vitamin D absorption
• You have darker skin combined with high-latitude living; melanin reduces UVB conversion efficiency and risks compound
• You're symptomatic — bone pain, proximal muscle weakness, unexplained falls in the elderly
• You're on long-term anticonvulsants or glucocorticoids, which accelerate vitamin D metabolism

Ask for serum 25-hydroxyvitamin D (also called 25-OH-D or calcidiol) — not the active 1,25-dihydroxy form, which is tightly regulated and a poor measure of stores.

What about respiratory infections and mood?

The strongest non-skeletal claim with reasonable trial support is for respiratory infection. The 2017 Martineau individual-participant-data meta-analysis pooled 25 RCTs and 11,000 participants and found vitamin D supplementation reduced acute respiratory tract infection risk, with the biggest effects in those starting from baseline deficiency and on daily (not bolus) dosing Martineau 2017. The absolute risk reduction was modest; the effect was real.

The cardiovascular, cancer-prevention, and mood claims are weaker. The large VITAL trial (Manson 2019) randomised 26,000 US adults to 2000 IU/day or placebo for five years and found no reduction in cancer or major cardiovascular events Manson 2019. The 2017 Autier umbrella review came to similar conclusions on most non-skeletal outcomes — the observational signals do not survive randomised testing Autier 2017.

The DO-HEALTH trial in older European adults found 2000 IU/day didn't meaningfully change clinical outcomes when added to a multi-component intervention, though the supplement alone is reasonable insurance against deficiency-related fall and fracture risk Bischoff-Ferrari 2020.

The honest summary: vitamin D corrects deficiency, supports bone health, and modestly reduces respiratory infections. It is not a wonder drug for the heart, the brain, or cancer.

Safety and the upper limit

Health Canada sets the tolerable upper intake at 4000 IU/day for adults. Toxicity (hypercalcemia, kidney involvement) has been documented in case reports at chronic daily intakes above about 10,000 IU/day for months. The safety margin between the recommended 1000-2000 IU/day dose and a problematic dose is large — this is a forgiving supplement, but not infinitely so.

People at higher caution: those with sarcoidosis or other granulomatous disease (which can dysregulate vitamin D metabolism), severe kidney disease, hypercalcemia, or on high-dose calcium supplementation. In those cases, dose under physician supervision.

Practical takeaways

• October through April in Canada: assume cutaneous synthesis is zero. Supplement.
• Adult winter dose: 1000-2000 IU/day of vitamin D3 (cholecalciferol).
• Take with food — fat improves absorption. The fattiest meal of the day works.
• Eat fatty fish twice a week if you can — salmon, mackerel, sardines.
• Don't bother testing unless you have a specific risk factor; the dose is right regardless.
• Don't exceed 4000 IU/day without a physician's reason to.
• Summer: short, regular sun exposure with bare forearms and face is enough at 44° North between May and September. Sun protection still matters for skin-cancer prevention.

Related: the 20-supplement evidence buy guide; magnesium — what the evidence shows.

References