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

If you live above 44° North latitude — Ottawa, Calgary, Vancouver, almost any major Canadian city, with Toronto just south of the line at about 43.7° — 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 response is not exotic: attention to food and fortified products, a conversation with your clinician or pharmacist about whether a supplement makes sense for you, and a clear-eyed view of which questions a blood test actually answers.

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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. Farther north — Toronto, Boston, Munich — it is largely absent through the winter months, and the winter gets longer the farther north you go.

The dose the trials actually use

Health Canada's recommended dietary allowance is 600 IU/day for adults up to 70 and 800 IU/day from age 71, with a tolerable upper intake of 4000 IU/day Health Canada 2012. These figures are conservative public-health guidance, set to meet the needs of nearly all healthy adults assuming minimal sun exposure.

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) gave 1500-2000 IU/day for adults at risk of deficiency Holick 2011; the society has since revised that guidance.

What this means for you: these are research and guideline figures, not a personal prescription. People with higher body weight, darker skin, or older age tend to run lower, and whether you need a supplement, and how much, is a question for your clinician or pharmacist, who can take your diet, medications and health history into account. Intakes above the 4000 IU/day upper limit belong only under medical supervision.

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 by an average of about 15 nmol/L more than D2 across the pooled trials Tripkovic 2012. But that advantage was concentrated in trials using a single large bolus dose; when D2 and D3 were compared as daily doses — the way most people take vitamin D — the difference between the two forms was not statistically significant. D2 is also thought to be less stable on the shelf. For daily use, D3's real edge over D2 may be shelf stability rather than raw potency, but it remains the more common choice unless you have a specific reason to avoid animal-derived D3 (vegan strictness, supplement-availability constraints).

Food sources that actually contribute

Diet alone will not solve a Canadian winter. But food and fortified products still contribute meaningfully.

Fatty fish: the strongest natural source. A 100 g serving of cooked Atlantic salmon supplies roughly 500-700 IU. Mackerel, rainbow trout and sardines also contribute, in varying amounts. 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 and margarine must be fortified with vitamin D by law, and most plant-based beverages are fortified to match milk. Some yogurts and orange juices add vitamin D voluntarily. Check the Nutrition Facts table for the amount per serving.

UV-exposed mushrooms: the only meaningful plant source; they provide D2, compared with D3 above.

Many people's winter diets fall short of the recommended intake from food alone unless fatty fish and fortified foods are regular features. A dietitian, pharmacist or clinician can help you judge whether yours does, and whether a supplement makes sense.

When a blood test is actually worth it

For a typical healthy adult without symptoms, routine screening is not generally recommended; the US Preventive Services Task Force, for example, found insufficient evidence to recommend screening adults without symptoms. That is a population-level judgement, not a rule for you: whether testing makes sense in your case is a decision for you and your clinician, who can also advise on whether you need a supplement at all.

Testing is more likely to be worth discussing with your clinician 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, including non-vertebral fractures, whether given alone or combined with omega-3s and a home exercise program Bischoff-Ferrari 2020.

The honest summary: correcting a deficiency supports bone health, and supplementation modestly reduces respiratory infections, mainly in people who start out low. 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. Staying within the upper limit leaves a wide margin below those intakes, but more is not better, and the upper limit counts all sources combined, including multivitamins and fortified foods.

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. If that applies to you, or you take medications that affect calcium or vitamin D (a pharmacist can check), talk to your physician before supplementing. Children have lower upper limits than adults, and supplements in pregnancy are a question for your prenatal care provider.

Practical takeaways

• October through April in Canada: skin synthesis is minimal, so food and fortified products matter more. Ask your clinician or pharmacist whether you need a supplement.
• Reference intakes: the adult RDA is 600 IU/day (800 IU/day from 71), with an upper limit of 4000 IU/day from all sources.
• If you do take a supplement, taking it with a meal that contains some fat may improve absorption.
• Eat fatty fish twice a week if you can — salmon, mackerel, sardines.
• Ask your clinician about testing, particularly if you have one of the risk factors above.
• Don't exceed 4000 IU/day without a physician's reason to.
• Summer: regular time outdoors produces vitamin D between roughly May and September, but that is no reason to skip sun protection, which still matters for skin-cancer prevention.

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

References