Vitamin D supplementation is among the most prescribed interventions in primary care, and one of the most over-claimed. Here’s where the evidence actually lands.
Vitamin D is genuinely important for bone health, and likely plays roles in immune function and several other systems. Severe deficiency causes rickets in children and osteomalacia in adults. Mild low levels are common, particularly in northern latitudes during winter.
Whether you specifically need a supplement depends on your level, your risk factors, and the season.
Where vitamin D comes from
Two sources:
- Sunlight — UVB radiation on bare skin produces vitamin D in the body. In Canada, UVB exposure is essentially zero from October to March at most latitudes.
- Diet and supplements — fatty fish, egg yolks, fortified milk and orange juice. Most people get less than they think from food alone.
The result is that most Canadians have lower vitamin D levels in winter, and a meaningful subset stay low year-round.
Who’s likely deficient
Higher-risk groups:
- Older adults (skin makes less vitamin D from sunlight with age)
- People with darker skin (more melanin reduces vitamin D production)
- People who cover their skin for cultural or religious reasons
- People who avoid sun for medical reasons
- People with malabsorption (celiac, inflammatory bowel disease, gastric bypass)
- Higher body weight (vitamin D distributes into fat tissue, leaving less circulating)
- Strict vegans without supplementation
What level is “enough”
The 25-hydroxyvitamin D blood test is the standard. The numbers (in Canadian units, nmol/L):
- Below 30 nmol/L: Deficient — needs treatment
- 30–75 nmol/L: Sufficient for most people
- 75–125 nmol/L: Considered optimal by some authorities; debated
- Above 125 nmol/L: Higher than necessary; potentially harmful at very high levels
The “optimal” zone is genuinely debated in the medical literature. The threshold for skeletal effects is around 50 nmol/L. Whether higher levels confer additional benefit for non-skeletal outcomes is not as clear as supplement marketing suggests.
What vitamin D actually does (and doesn’t)
Solid evidence
- Prevents rickets and osteomalacia
- Reduces fall risk in older adults at deficient levels
- Modestly reduces fracture risk when combined with calcium in deficient older adults
- Required for proper calcium absorption
Weaker or mixed evidence
- Cardiovascular disease prevention — large trials have not shown clear benefit
- Cancer prevention — modest signals, not strong
- Cognitive decline prevention — uncertain
- Mood and depression — weak association, supplementation effects unclear
- Respiratory infection prevention — small effect in some studies
The general pattern: correcting genuine deficiency helps for several outcomes. Pushing levels higher than the deficiency threshold doesn’t reliably add more benefit for most conditions.
Practical recommendations
If you live in Canada and don’t take a supplement
In winter (October–March), 800–2,000 IU daily is a reasonable default for most adults. The risk of mild deficiency is high enough that the supplement makes sense without testing in many cases.
If you have risk factors for deficiency
A baseline blood test is worth doing. If you’re deficient, your clinician may recommend a higher dose initially (often 50,000 IU weekly for 8–12 weeks) followed by a maintenance dose.
If you don’t fit any risk category and feel fine
You can probably skip both the test and the supplement, especially during summer months. The yield from population-level vitamin D testing is low.
Can you take too much?
Yes, but you have to work at it. Toxicity from supplements requires consistent doses well above 4,000 IU daily for prolonged periods. Standard doses (1,000–2,000 IU) are very safe.
A few interactions worth knowing: if you have hypercalcemia, sarcoidosis, or kidney stones, talk to your clinician before supplementing. Otherwise, vitamin D is one of the lowest-risk supplements available.
The takeaway
Vitamin D supplementation isn’t a miracle cure — but for most Canadians, especially in winter, modest supplementation is reasonable and low-risk. Don’t expect it to fix problems unrelated to bone and mineral metabolism. Do take it during the months when sun isn’t doing the job.
