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, low winter levels are common. The question is what to do about it, and the answer is more nuanced than the supplement aisle suggests.

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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) means more sun exposure is needed to make the same amount of vitamin D; 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, so applied thickly it reduces synthesis. That is not a reason to skip it: sunscreen protects against sunburn and skin cancer.
• 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 unlikely to be better off.

"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 2011 clinical-practice guideline took a more aggressive view, suggesting 1,500–2,000 IU/day for adults at risk of deficiency. That guidance was written for clinicians managing patients, and the society has since revised it.

What this means for healthy Canadian adults:

• October–April: this is when low levels are most likely, because skin synthesis stops and food and fortified products have to do the work.
• May–September: regular time outdoors produces some skin synthesis, so people who are often outside are less likely to run low.
• If you live with obesity, are over 70, or have darker skin: you are more likely to run low year-round, which is worth raising with your clinician.
• Athletes: Owens 2018 reviewed 30+ trials and its authors suggested year-round supplementation at 1,000–4,000 IU/day for athletes training in northern climates, guided by blood testing Owens 2018. That is a research recommendation to discuss with a team physician or sports-medicine clinician, not a personal prescription.
• D₃ (cholecalciferol) vs D₂ (ergocalciferol): most trials used D₃, and head-to-head comparisons generally find it raises serum 25(OH)D more effectively.

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 for some people (the IOM judged 50 nmol/L enough for nearly everyone)
• 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. If you’re otherwise healthy and wondering whether to test or to supplement, ask your clinician or pharmacist: they can weigh your diet, sun exposure, medications and risk factors.

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. Proposed mechanisms include 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 moderated both the fracture and the falls findings, particularly in adults who were not deficient to begin with. Where a benefit appears, it is concentrated in people who start out low, and preventing falls in older adults involves much more than vitamin D.

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. One proposed mechanism is 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, possibly because these participants were more likely to be deficient at baseline, though subgroup findings like this can be due to chance

The VITAL trial has tempered enthusiasm for vitamin D as a general-purpose preventive in already-replete populations. Earlier encouraging signals, from observational studies and from smaller trials such as Lappe’s 2017 calcium-plus-vitamin-D trial in older Nebraskan women (which found a lower cancer rate that did not reach statistical significance), have not been borne out 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 because winter deficiency is common, Canadian athletes training through winter have good reason to ask a team physician or clinician about their vitamin D status.

Food sources

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

• Fatty fish (sockeye salmon, mackerel, sardines): the richest everyday sources, though amounts vary widely by species
• 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; check the label
• Fortified milk, margarine and some fortified plant beverages in Canada: amounts vary, so check the label

Reaching the recommended intake from food alone in a Canadian winter is difficult unless fatty fish is a regular part of your diet. A dietitian, pharmacist or clinician can help you decide whether a supplement makes sense for you.

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 have a condition affecting calcium metabolism (sarcoidosis, primary hyperparathyroidism, other granulomatous disease) or kidney disease, or you take medications that affect vitamin D or calcium, talk to your clinician or pharmacist before supplementing. Intakes above the 4,000 IU/day upper limit belong only under medical supervision, with blood monitoring. Children have lower upper limits than adults (see the table above), and supplements in pregnancy are a question for your prenatal care provider.

Practical points

1. If you and your clinician decide on a supplement: vitamin D₃ (cholecalciferol) is the form most trials used, and USP-verified or NSF-certified products offer more assurance that the label is accurate.
2. Vitamin D is fat-soluble, so a supplement may be better absorbed with a meal that contains some fat.
3. Summer is different. Regular outdoor time produces some skin synthesis; people who are mostly indoors, cover up, have darker skin, are over 70 or live with obesity are more likely to stay low year-round.
4. Pregnant/breastfeeding: the IOM RDA is 600 IU/day, the same as for other adults under 70. Ask your prenatal care provider about supplements, including how much vitamin D your prenatal vitamin already provides.
5. Ask your clinician about 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 produces skin synthesis. That is not a reason to skip sunscreen or seek out mid-day sun: the same UV that makes vitamin D causes sunburn and skin cancer, and sun-safety advice still applies. Winter is a different story: skin synthesis effectively stops, and food, fortified products and, where your clinician advises it, a supplement have to do the work.

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." Not quite. Head-to-head trials generally find D₃ (cholecalciferol) raises serum concentration more effectively than D₂ (ergocalciferol).

"You don’t need supplementation if you take a multivitamin." It depends. Many multivitamins contain some vitamin D, and the 4,000 IU/day upper limit counts all sources combined, so check the label before adding a separate product and ask a pharmacist if you’re unsure.

The bottom line

• Between October and April, latitude shuts down skin synthesis in Canada — winter levels depend on food, fortified products and stored vitamin D.
• Know the reference numbers: the RDA is 600 IU/day (800 IU/day from age 71); whether you need a supplement, and how much, is a question for your clinician or pharmacist.
• Strongest evidence: falls and fractures in older adults; respiratory infections in deficient populations; bone-stress injuries in deficient athletes. The benefits concentrate in people who start out low, and later large trials have tempered the falls and fracture findings.
• 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.
• Ask your clinician about testing, especially if you have a chronic condition, take medications that affect vitamin D, or have unexplained symptoms.
• Correcting a deficiency matters. Don’t expect miracles when you’re not deficient.

References