Vitamin D: Who Actually Needs to Supplement, and How Much

Vitamin D is the only nutrient your skin can manufacture from sunlight — and that quirk of biology has shaped almost everything we know about it. In Canada, where the sun’s angle from October to April is too shallow to drive meaningful skin synthesis above 42° N latitude, deficiency is the rule rather than the exception. The question is what to do about it, and the answer is more nuanced than the supplement aisle suggests.

Why so many Canadians are deficient

Vitamin D is unique among nutrients because the human body synthesises it from UVB photons hitting 7-dehydrocholesterol in the skin. Above about 42° N latitude (which puts essentially all of Canada in scope), the sun’s zenith angle from October through April is too shallow for meaningful UVB to reach the skin. You cannot make vitamin D in Wasaga Beach in January. No matter how much time you spend outdoors, the photons aren’t there Holick 2007.

Holick’s 2007 NEJM review — still the canonical reference — documented this latitude effect plus several other risk factors that compound the problem in northern populations Holick 2007:

• Higher melanin (darker skin) reduces UVB conversion efficiency by 50–90%; people of African, South Asian, or Middle Eastern descent in Canada are disproportionately affected.
• Older skin contains less 7-dehydrocholesterol — adults over 70 produce roughly a quarter of the vitamin D that a 20-year-old produces from the same sun exposure.
• Sunscreen blocks UVB; even SPF 15 reduces synthesis by ~95%.
• Indoor lifestyles (most desk-working adults) leave very little summer skin exposure even when synthesis is possible.
• Obesity sequesters fat-soluble vitamin D in adipose tissue, lowering serum concentration at any given intake.
• Liver, kidney, or fat-malabsorption disorders impair conversion or absorption.

Close and colleagues’ 2013 study of 30 professional UK athletes during winter found that 62% had inadequate or deficient serum 25(OH)D concentrations — in a population that trains outdoors regularly, eats deliberately, and is generally young and lean Close 2013. If athletes in Britain are deficient at that rate, recreational adults in Canada are almost certainly worse.

"Vitamin D deficiency is now recognized as a pandemic. The major cause of vitamin D deficiency is the lack of appreciation that sun exposure has been and continues to be the major source of vitamin D for most humans." — per Holick 2007, "Vitamin D Deficiency," New England Journal of Medicine

Recommended intake

The 2011 Institute of Medicine report (still the basis for Health Canada’s position) set the following dietary reference intakes IOM 2011 Ross 2011:

The Endocrine Society’s clinical-practice guideline takes a more aggressive view, recommending 1,500–2,000 IU/day for adults at risk of deficiency — roughly the dose most Canadian winter-supplementation protocols actually use.

Practical recommendation for healthy Canadian adults:

• October–April: 1,000–2,000 IU/day vitamin D₃ (cholecalciferol). Fixed daily dose works as well as larger weekly doses.
• May–September: 600–1,000 IU/day — or omit if you spend regular time outdoors with sun exposure.
• If you’re obese, > 70, or have darker skin: consider the higher end (2,000 IU/day year-round).
• Athletes: Owens 2018 reviewed 30+ trials and recommended year-round supplementation at 1,000–4,000 IU/day for athletes training in Northern climates, with serum monitoring if practical Owens 2018.
• Always D₃ (cholecalciferol), not D₂ (ergocalciferol). D₃ raises serum 25(OH)D more efficiently and stays elevated longer.

Should you get tested?

Serum 25-hydroxyvitamin D — 25(OH)D — is the standard biomarker. Lab interpretation:

• < 30 nmol/L (< 12 ng/mL): deficient; rickets/osteomalacia risk
• 30–50 nmol/L: insufficient (IOM threshold for action)
• 50–125 nmol/L: adequate
• > 125 nmol/L: potentially excess; toxicity above 500 nmol/L

Testing is reasonable if you have a chronic condition (osteoporosis, malabsorption, chronic kidney disease), if you’re on medications that affect vitamin D metabolism, or if you have unexplained bone pain or muscle weakness. For an otherwise healthy adult planning a routine winter top-up, testing isn’t strictly necessary — 1,000–2,000 IU/day is below the upper-intake threshold and is unlikely to overshoot.

Bone and falls evidence (the strongest)

Bischoff-Ferrari and colleagues’ landmark 2009 BMJ meta-analysis pooled 8 RCTs (2,426 older adults) and found vitamin D doses ≥ 700–800 IU/day reduced falls by 19% — an effect that simply did not appear at lower doses Bischoff-Ferrari 2009. The mechanism appears to be a combination of preserved muscle strength (vitamin D receptors are expressed in skeletal muscle) and improved postural balance.

Trivedi’s 2003 BMJ trial randomised 2,686 community-dwelling older adults to 100,000 IU vitamin D₃ every 4 months (~830 IU/day equivalent) vs placebo. After 5 years: 22% lower fracture rate at hip, wrist, forearm, or vertebrae Trivedi 2003. This remains one of the cleanest fracture-prevention trials of vitamin D alone.

later larger trials (notably the VITAL trial, see below) have somewhat moderated the bone-fracture findings in non-deficient adults — but the falls-reduction signal in older adults at risk has held up consistently.

Immunity / respiratory infection

Martineau and colleagues’ 2017 individual-participant-data meta-analysis of 25 RCTs (10,933 participants) found that vitamin D supplementation reduced the risk of acute respiratory tract infection by 12% overall — with the effect concentrated in those who were deficient at baseline Martineau 2017:

• Baseline 25(OH)D < 25 nmol/L: 70% reduction in respiratory infection risk
• Baseline 25(OH)D ≥ 25 nmol/L: 25% reduction
• Daily/weekly dosing outperformed large-bolus dosing — the regular intake matters

Urashima’s 2010 trial in Japanese schoolchildren found 1,200 IU/day vitamin D₃ reduced laboratory-confirmed influenza A by 42% over the winter season Urashima 2010. The mechanism appears to be vitamin D’s role in cathelicidin production — an antimicrobial peptide central to innate immunity in the respiratory tract.

The COVID-19 era produced extensive observational data suggesting low vitamin D status correlated with worse outcomes; randomised-trial evidence in COVID specifically remains mixed and beyond the scope of this article. The non-COVID respiratory-infection evidence base, however, is now large.

VITAL: the big trial that limited expectations

The Vitamin D and Omega-3 Trial (VITAL) randomised 25,871 healthy US adults (men ≥ 50, women ≥ 55) to either 2,000 IU/day vitamin D₃ or placebo. Median follow-up: 5.3 years Manson 2019:

• Cancer incidence: no meaningful difference (HR 0.96, 95% CI 0.88–1.06)
• Major cardiovascular events: no meaningful difference (HR 0.97, 95% CI 0.85–1.12)
• Total cancer mortality: some signal of benefit (17% lower) but didn’t reach predefined statistical thresholds
• Subgroup of African-American participants: larger reduction in cancer incidence, suggesting the benefit was concentrated in people more likely to be deficient at baseline

The VITAL trial has tempered enthusiasm for vitamin D as a general-purpose preventive in already-replete populations. The earlier observational signals (e.g. Lappe’s 2017 calcium+vitamin D trial in older Nebraskan women that initially suggested cancer prevention) have not been replicated in the largest randomised data Lappe 2017. The synthesis: vitamin D supplementation has clear benefits for deficient individuals; in replete adults, the marginal benefit is small or zero Rejnmark 2017 Autier 2017.

Athletic performance

Owens 2018 reviewed the athletic-performance literature and found Owens 2018:

• Muscle strength & power: small but consistent improvements when correcting deficiency. No additional benefit at higher 25(OH)D in already-replete athletes.
• VO₂max: marginal effect, likely indirect through reduced respiratory infection.
• Bone-stress-injury risk: meaningfully elevated in deficient athletes; correcting deficiency reduces stress-fracture incidence.
• Recovery: some evidence of reduced muscle damage markers, but inconsistent.

Practical translation: don’t expect vitamin D to be a performance-enhancer if you’re not deficient. But for Canadian athletes training through winter, supplementation is a sensible default given how common winter deficiency is.

Food sources

Few foods naturally contain meaningful vitamin D. The richest sources:

• Fatty fish (sockeye salmon, mackerel, sardines, tuna): 400–1,000 IU per 100 g serving
• Cod liver oil: historically the most-concentrated traditional source
• Egg yolks: ~40 IU per yolk — modest contribution
• Mushrooms exposed to UV light (some commercially-grown brands now do this): variable, up to 400 IU per serving
• Fortified milk and orange juice in Canada: ~100 IU per cup

For most Canadian winters, you’d need to eat fatty fish 4–5 times a week to approach the recommended intake from food alone. Supplementation is more reliable.

Safety

Vitamin D toxicity is rare but possible. The IOM’s tolerable upper intake of 4,000 IU/day for adults is conservative; serum concentrations don’t reach toxicity ranges (> 500 nmol/L) until intake exceeds about 10,000 IU/day for months IOM 2011. Symptoms of toxicity:

• Hypercalcemia (high blood calcium): nausea, vomiting, weakness, frequent urination
• Kidney stones, kidney damage at chronic high intake
• Bone pain, muscle weakness

If you’re routinely taking > 4,000 IU/day or have a condition affecting calcium metabolism (sarcoidosis, primary hyperparathyroidism, granulomatous disease), have your serum 25(OH)D and calcium monitored periodically. Otherwise, supplementation in the 1,000–2,000 IU/day range carries effectively zero toxicity risk.

A practical protocol

1. Pick a vitamin D₃ (cholecalciferol) supplement. NOT D₂. Look for USP-verified or NSF-certified products for label reliability.
2. 1,000–2,000 IU/day October through April. Take with a meal containing fat (vitamin D is fat-soluble).
3. Continue 600–1,000 IU/day May through September if you’re indoors much of the time, sunscreen-conscious, dark-skinned, > 70, or obese. Skip if you’re routinely outdoors with skin exposure.
4. Pregnant/breastfeeding: 600 IU/day per IOM recommendation; consult your obstetric provider if you have specific risk factors.
5. Consider testing if you have a chronic condition that affects vitamin D, unexplained bone pain or muscle weakness, history of fracture, or recurrent infections.
6. Don’t exceed 4,000 IU/day without medical guidance.

Beachside note

For Wasaga Beach summers (May–September), regular outdoor activity at the beach — even short bouts before sunscreen application — produces meaningful skin synthesis. Mid-day, ~10–30 minutes of arm-and-leg exposure several times per week is enough for most people to reach mid-summer adequacy. Winter is a different story: supplement.

Common myths

"You can get all your vitamin D from food." Not in winter and not at northern latitudes. Most Canadians cannot reach adequacy through food alone in the Oct–Apr window.

"More is better." Not after replete. Recent large RCTs show no additional cancer or cardiovascular benefit in already-replete adults at higher doses (VITAL).

"Vitamin D prevents COVID." The evidence is mixed and does not support specific anti-COVID supplementation protocols. The respiratory-infection evidence base for non-COVID infections is much stronger and more consistent.

"D₂ works as well as D₃." No. D₃ (cholecalciferol) raises serum concentration roughly 1.7-fold more efficiently than D₂ (ergocalciferol).

"You don’t need supplementation if you take a multivitamin." Most multivitamins contain 400–800 IU vitamin D — below what most Canadian adults need in winter. Add a separate D₃ if testing or risk factors suggest you’re below adequacy.

The bottom line

• Most Canadians become functionally vitamin-D-deficient between October and April — latitude prevents skin synthesis.
• Practical winter dose: 1,000–2,000 IU/day vitamin D₃, taken with a meal containing fat.
• Strongest evidence: falls and fracture prevention in older adults; respiratory infection reduction in deficient populations; bone-stress-injury reduction in deficient athletes.
• Most disappointing evidence: in already-replete adults, no clear cancer or cardiovascular benefit at typical supplementation doses (VITAL trial).
• Don’t exceed 4,000 IU/day without medical monitoring; toxicity is rare but real.
• Test if you have a chronic condition or unexplained symptoms; for routine winter top-up in healthy adults, testing is optional.
• Top up when likely deficient. Don’t expect miracles when you’re not.

References