When to start vitamin D supplementation in Ontario — the September threshold

Educational journalism, not medical advice. Every claim here is checked against its cited sources by editor Tim Bunce — a health writer, not a physician. It isn’t specific to your situation: for health decisions, talk to your own clinician. How we work →

Latitude + UVB + cholecalciferol synthesis

The mechanism is straightforward. Vitamin D3 (cholecalciferol) is synthesised in the skin when UVB photons strike 7-dehydrocholesterol in the epidermis. The photon energy required falls within a narrow window of the UVB spectrum. UVA does not produce vitamin D. UVC is filtered by the atmosphere entirely. UVB itself is filtered by atmospheric ozone, and the path length through the atmosphere depends on solar zenith angle.

Michael Holick's foundational work mapped the latitude-season interaction. Holick's 2007 review in the New England Journal of Medicine (NEJM, DOI 10.1056/NEJMra070553) reproduced the finding from earlier Webb et al. (1988) measurements: above approximately 37 degrees north latitude, no measurable vitamin D synthesis occurs in human skin from October through March. Above 43 degrees, the window shrinks further — synthesis becomes negligible from mid-September through early April. Wasaga, Barrie, Collingwood, and the GTA all fall into this "vitamin D winter" band for roughly six months annually.

The Heaney 2003 dose-response work and the 2011 Endocrine Society guideline

Holick's Holick 2007 NEJM review and Heaney 2003's extended-dosing trial (American Journal of Clinical Nutrition) established the modern dose-response relationship between oral vitamin D3 intake and serum 25-hydroxyvitamin D (25-OH-D) concentrations. The headline finding: each 100 IU of daily vitamin D3 raises serum 25-OH-D by roughly 1 to 2 nmol/L in adults, with the response varying by body weight, baseline status, and the duration of supplementation.

The Endocrine Society's 2011 clinical practice guideline (Holick 2011, Journal of Clinical Endocrinology and Metabolism) built on this with a formal treatment protocol. To reach a serum 25-OH-D of 75 nmol/L (30 ng/mL — the level Holick and others argued was the lower bound of repletion) from a deficient starting point, the guideline recommended 50,000 IU of vitamin D2 or D3 once weekly for 8 weeks (roughly equivalent to 6,000 IU per day), followed by maintenance dosing of 1,500 to 2,000 IU per day. The Institute of Medicine's more conservative 2011 recommendation of 600 IU per day to maintain serum levels in already-replete individuals reflects a different question: how much is needed to prevent deficiency-related bone disease at population level, not how much is needed to reach an optimal status for an individual already below the threshold.

Ontario seasonal-deficiency patterns (Statistics Canada CHMS)

The Canadian Health Measures Survey has measured serum 25-OH-D in nationally representative samples since 2007. Statistics Canada's published cycles (CHMS Cycle 1 through Cycle 5, covering 2007 through 2017) consistently show that 25 to 35 percent of Canadians have serum 25-OH-D below 50 nmol/L during winter months, and 5 to 10 percent fall below 30 nmol/L — the level Health Canada classifies as clinical deficiency.

The seasonal pattern is striking in Ontario data. Summer measurements (June through August) show median 25-OH-D of approximately 70 to 80 nmol/L. Winter measurements (December through February) show medians dropping to 50 to 60 nmol/L. The drop is larger in subgroups with darker skin pigmentation, in those with higher BMI, and in adults over 65 — all groups with reduced cutaneous synthesis efficiency.

The implication for an individual living in Wasaga is that summer sun exposure builds reserves, those reserves draw down through the fall and winter, and supplementation needs to start before the reserve is depleted — not after deficiency has set in.

The 25-OH-D target (75 nmol/L) and how to test

The Endocrine Society's 2011 clinical practice guideline (Holick et al., J Clin Endocrinol Metab) defined sufficiency as serum 25-OH-D ≥ 75 nmol/L (30 ng/mL), with insufficiency below that and deficiency below 50 nmol/L (20 ng/mL). Health Canada uses a more conservative 50 nmol/L threshold for sufficiency. The 75 nmol/L target is contested — recent meta-analyses (Bouillon et al. 2019, Endocrine Reviews) have argued the optimum may sit between 50 and 75 — but the lower bound for clear inadequacy is unambiguous at 50.

Testing is straightforward. A serum 25-hydroxyvitamin D blood test costs about $30 to $60 in Ontario when ordered through a private requisition (LifeLabs, Dynacare). Family physicians can order it but it is not covered by OHIP for routine screening; it is covered only when clinically indicated (osteoporosis, malabsorption syndromes, chronic kidney disease). At-home finger-prick tests (sent to a partner lab for analysis) cost $40 to $80 and produce results comparable to venous draws for the 25-OH-D assay.

Test once in early autumn (late September) to establish baseline. Re-test in late winter (February or March) to confirm that supplementation is maintaining the target.

D3 vs D2 — Tripkovic 2012

The two supplemental forms of vitamin D are D2 (ergocalciferol, derived from fungi) and D3 (cholecalciferol, derived from lanolin or lichen). The Tripkovic 2012 systematic review and meta-analysis pooled 7 randomized trials using daily, weekly, or monthly dosing schedules. Pooled across all schedules, D3 raised serum 25-OH-D more effectively than D2 overall — but broken out by regimen, that advantage was statistically significant only for infrequent (weekly or monthly) bolus dosing. For daily dosing — the regimen this article recommends throughout — the meta-analysis found no statistically significant difference between the two forms (mean difference 4.83 nmol/L; 95% CI −0.98 to 10.64; P=0.10).

The mechanism is metabolic: D3 has a longer half-life in serum and is more readily converted to 25-hydroxyvitamin D in the liver, which likely explains why its edge over D2 grows with infrequent dosing even though it doesn't reliably show up at daily doses. Prescription vitamin D in Canada is sometimes dispensed as D2 (ergocalciferol) because of historical pharmaceutical practice, but the over-the-counter market is almost entirely D3 — which is the preferred form on every modern guideline.

Lichen-derived vegan D3 is now widely available and matches the lanolin-derived form in bioavailability (Itkonen et al. 2018).

Magnesium cofactor — Uwitonze 2018

Vitamin D metabolism is magnesium-dependent. The Uwitonze 2018 review in the Journal of the American Osteopathic Association summarised the evidence that magnesium is a required cofactor for both the 25-hydroxylase and 1-alpha-hydroxylase enzymes that convert dietary vitamin D into its biologically active forms.

Practically: supplementing high-dose vitamin D in a magnesium-depleted individual produces a blunted response. The CHMS data and the Health Canada nutrition survey both indicate that 35 to 45 percent of Canadian adults have dietary magnesium intakes below the Estimated Average Requirement (310 to 420 mg per day depending on age and sex). A magnesium glycinate or magnesium citrate supplement of 200 to 400 mg per day, taken alongside vitamin D, optimises the conversion pathway.

Toxicity threshold + when to test

Vitamin D toxicity is real but requires sustained intake far above the supplementation doses discussed here. The Hathcock 2007 American Journal of Clinical Nutrition review established a no-observed-adverse-effect level (NOAEL) of 10,000 IU per day in adults. The Institute of Medicine's upper limit for daily intake is 4,000 IU, which builds in a substantial safety margin.

Toxicity manifests as hypercalcaemia — elevated blood calcium causing nausea, kidney stones, and, in severe cases, soft-tissue calcification. The cases reported in the literature almost universally involve intakes above 40,000 IU per day for weeks or months, frequently from manufacturing errors or compounding mistakes.

A daily supplemental dose of 1,000 to 4,000 IU is well within safety margins for the overwhelming majority of adults. Test serum 25-OH-D before exceeding 4,000 IU per day chronically, and re-test annually.

The mid-September supplementation start date

The practical rule for Wasaga and the surrounding region: begin daily vitamin D supplementation around the autumn equinox, roughly September 21. Continue daily through to the spring equinox, March 21. Reduce or pause during the summer months only if you spend meaningful time outdoors with skin exposed to midday sun — and even then, some evidence suggests year-round low-dose supplementation may produce more stable serum levels than the seasonal pause-and-resume pattern.

For an adult of average size with no known deficiency: 1,000 to 2,000 IU of D3 per day from September 21 through March 21, dropping to 1,000 IU or pausing in summer.

For an adult with a measured baseline below 50 nmol/L: 4,000 IU per day for 8 to 12 weeks, then re-test, then drop to a maintenance dose of 1,000 to 2,000 IU.

Take with the largest meal of the day — vitamin D is fat-soluble, and absorption is approximately 30 to 50 percent higher when taken with dietary fat (Mulligan and Licata 2010).

Practical takeaways

• Above 43 degrees north, cutaneous vitamin D synthesis is negligible from late September through mid-March.
• Target serum 25-hydroxyvitamin D between 75 and 100 nmol/L; test in early autumn and late winter.
• Start daily 1,000 to 2,000 IU of D3 around September 21; continue through to the spring equinox.
• D3 and D2 perform similarly at the daily doses recommended here; D3's edge over D2 shows up mainly with infrequent (weekly or monthly) dosing. Magnesium is required for activation.
• Toxicity requires sustained intake above roughly 10,000 IU per day; the 1,000 to 4,000 IU range is well within safety margins.

Extended takeaways

The seasonal vitamin D question is one of the few areas of nutritional science where the geography of where you live determines the answer with mathematical certainty. The 43rd parallel cutoff is not folklore — it falls out of the physics of atmospheric path length and the photochemistry of cholecalciferol synthesis. A resident of Miami at 25 degrees north does not face the same supplementation calculus as a resident of Wasaga at 44.5. The advice of generic supplement marketing — "consider vitamin D" — papers over the latitude-specific reality. In Ontario, supplementation in winter is not a consideration. It is a requirement for maintaining adequate status.

The dose-response work from Holick, Heaney, and the subsequent generation of researchers has produced unusual clarity for a nutrient question. The relationship between oral D3 intake and serum 25-OH-D is approximately linear within the relevant range, the half-life is well-characterised, and the test is cheap and reliable. The friction is logistical: getting Canadians to actually test, supplement, and re-test. The CHMS data make clear that despite decades of public-health messaging, a meaningful fraction of the Ontario population enters every winter undermined by an entirely preventable deficiency.

Vitamin D's role in immune function, mood, athletic performance, and chronic disease risk has been the subject of intense and frequently overheated debate over the past 15 years. The strongest, most consistent evidence supports its role in bone health and the prevention of osteomalacia and rickets — outcomes that are difficult to ignore once they occur. Claims about cancer prevention, cardiovascular protection, and infection resistance have been less consistent, though the VITAL trial findings (Manson 2019) suggest modest benefit in some subgroups. The conservative reading of the evidence: supplement to maintain adequate status, do not expect vitamin D to be a panacea, and re-test annually to confirm the dose is doing what you expect.

Frequently asked questions

Do I still need to supplement if I take a multivitamin?

Most standard multivitamins contain 400 to 1,000 IU of vitamin D, which is below the dose needed to maintain serum 25-OH-D at 75 nmol/L through an Ontario winter. Check the label; add a standalone D3 if the multivitamin's dose is under 2,000 IU.

Is vitamin D from food enough?

For most Canadians, no. Fatty fish (salmon, sardines, mackerel) and fortified milk are the dominant dietary sources. A typical Canadian diet provides 100 to 400 IU per day, which is well below the supplementation range needed in winter.

What about tanning beds?

Commercial tanning beds emit predominantly UVA, not UVB, and produce minimal vitamin D synthesis while substantially elevating melanoma risk. The Health Canada guidance — confirmed by Cancer Care Ontario — is to use oral supplementation rather than UV exposure for vitamin D maintenance.

Should kids supplement too?

Health Canada recommends 400 IU per day for breastfed infants and 600 IU per day for children over 1 year. Ontario pediatric guidelines support continued supplementation through childhood and adolescence at 600 to 1,000 IU per day during winter months.

Can I take a weekly mega-dose instead of daily?

Daily dosing is generally preferred for vitamin D specifically. A large randomized trial testing a single annual 500,000 IU oral dose in older women (not a monthly schedule) found the bolus increased fall risk by 15 percent (rate ratio 1.15) and fracture risk by 26 percent (rate ratio 1.26) versus placebo, with the excess risk concentrated in the first three months after each dose (Sanders 2010). That result doesn't establish that all infrequent-dosing schedules carry the same risk, but it's a reason to favour daily or weekly regimens over large, infrequent boluses.

References