Vitamin D Deficiency: Symptoms, Causes and Treatment

Vitamin D deficiency is one of the most common nutritional deficiencies in the world, affecting an estimated 1 billion people globally. In Canada, where sunlight is limited for much of the year, deficiency is particularly prevalent — surveys have found that roughly 32% of Canadians have insufficient vitamin D levels at some point during the year, with rates rising significantly in winter months.

Despite its name, vitamin D functions less like a vitamin and more like a hormone — it plays a role in hundreds of physiological processes far beyond bone health. Understanding deficiency, its causes, and how it is treated is important health literacy for anyone living in northern climates. For a broader look at the evidence on vitamin D and other popular supplements, read our Natural Supplements: What the Research Actually Says.

What Is Vitamin D and Why Does It Matter?

Vitamin D is a fat-soluble nutrient that the body produces in the skin when exposed to ultraviolet B (UVB) sunlight. It can also be obtained in smaller quantities through diet and supplements. After production in the skin or ingestion, vitamin D undergoes two chemical conversions — first in the liver (to 25-hydroxyvitamin D, or 25(OH)D, which is what blood tests measure) and then in the kidneys (to its active hormonal form, calcitriol).

The active form of vitamin D regulates:

• Calcium and phosphorus absorption — essential for bone mineralization and density
• Immune function — vitamin D receptors are present on virtually all immune cells; it modulates both innate and adaptive immune responses
• Muscle function — deficiency is associated with muscle weakness and increased fall risk in older adults
• Mood and brain function — vitamin D receptors are widespread in the brain; links to depression are under active research
• Cardiovascular and metabolic health — associations found in observational research, though causality remains under investigation

Blood Level Reference Ranges

Vitamin D status is measured through a blood test called the 25-hydroxyvitamin D (25(OH)D) test. Reference ranges are reported in nanomoles per litre (nmol/L) in Canada, or nanograms per millilitre (ng/mL) in the United States (multiply ng/mL by 2.5 to convert to nmol/L).

Level (nmol/L)	Level (ng/mL)	Status	Clinical Interpretation
< 30	< 12	Severe deficiency	High risk of rickets (children), osteomalacia (adults)
30–49	12–19	Deficiency	Inadequate for bone health and most physiological functions
50–74	20–29	Insufficiency	May be adequate for some, insufficient for others — clinical context matters
75–125	30–50	Sufficient	Generally considered adequate for most people
125–250	50–100	High normal	Acceptable range for some; may not provide additional benefit
> 250	> 100	Toxicity risk	Hypercalcemia risk — consult physician immediately

Symptoms of Vitamin D Deficiency

Vitamin D deficiency is often called a "silent" deficiency because mild to moderate deficiency may produce no noticeable symptoms. When symptoms do appear, they are often vague and easily attributed to other causes — which is why blood testing is the only reliable way to confirm deficiency.

Who Is at Risk for Vitamin D Deficiency?

Vitamin D deficiency does not affect everyone equally. Certain populations face substantially higher risk due to reduced sun exposure, reduced skin synthesis capacity, or increased metabolic demand:

• People living at northern latitudes — Canada, Scandinavia, the northern United States. UVB radiation is insufficient to produce vitamin D in the skin for 4–6 months of the year in most of Canada
• Older adults — skin's ability to synthesize vitamin D declines significantly with age (up to 75% reduction in adults over 70 compared to young adults); kidneys also become less efficient at converting vitamin D to its active form
• People with darker skin tones — higher melanin content reduces UVB absorption, requiring longer sun exposure to produce equivalent vitamin D; Black and South Asian Canadians have significantly higher rates of deficiency
• People who cover most of their skin — for religious, cultural, or personal reasons; sun protection (SPF 30+) also substantially reduces vitamin D synthesis
• Individuals with obesity — vitamin D is fat-soluble and can be sequestered in adipose tissue, reducing bioavailability in the bloodstream
• People with malabsorption conditions — Crohn's disease, celiac disease, gastric bypass surgery, cystic fibrosis, and liver or kidney disease all impair vitamin D absorption or conversion
• Exclusively breastfed infants — breast milk contains limited vitamin D; Health Canada recommends vitamin D supplementation (400 IU/day) for all breastfed infants
• People on certain medications — anticonvulsants, corticosteroids, cholesterol-lowering drugs, and some HIV medications can reduce vitamin D levels

Causes: Why Deficiency Occurs

Insufficient Sun Exposure

The primary cause worldwide. In Canada, meaningful UVB exposure for vitamin D synthesis is only possible from approximately April to October, and even then requires several minutes of direct midday sun exposure on exposed skin (face, arms, legs) without sunscreen. Modern indoor lifestyles, sun-protective behaviour, and long winters make sun-based vitamin D synthesis unreliable for most Canadians.

Dietary Insufficiency

Very few foods naturally contain significant amounts of vitamin D. Natural dietary sources include:

• Fatty fish (salmon, mackerel, sardines, herring) — 300–700 IU per 85g serving
• Cod liver oil — approximately 1,360 IU per tablespoon (use with caution; also high in vitamin A)
• Egg yolks — approximately 40 IU per yolk (from pasture-raised hens: up to 150 IU)
• Fortified foods — cow's milk (typically 100 IU/cup), some plant-based milk alternatives, some orange juices, certain cereals
• UV-exposed mushrooms — can provide meaningful vitamin D2 when their gills are exposed to sunlight

Even with a varied diet, reaching the 600–800 IU daily requirement through food alone is challenging without regular consumption of fatty fish or fortified foods.

Malabsorption and Medical Conditions

The small intestine absorbs dietary vitamin D alongside dietary fat — conditions that impair fat absorption (celiac disease, Crohn's disease, pancreatic insufficiency, post-bariatric surgery) reduce vitamin D absorption proportionally. Liver disease impairs the first conversion step; kidney disease impairs the second (activation) step.

Diagnosis: Testing for Vitamin D Deficiency

The standard test is the serum 25-hydroxyvitamin D [25(OH)D] blood test. In Canada, testing is generally covered by provincial health insurance when there is a clinical indication — risk factors, symptoms, or conditions associated with deficiency. Routine population-level screening of asymptomatic healthy adults without risk factors is generally not recommended by Canadian guidelines (Osteoporosis Canada, 2023), as evidence does not support universal screening.

Treatment: How Vitamin D Deficiency Is Managed

Supplementation

Vitamin D supplementation is the primary treatment for confirmed deficiency. Vitamin D3 (cholecalciferol) is generally preferred over vitamin D2 (ergocalciferol) because it is more effective at raising and maintaining serum 25(OH)D levels in most studies. Supplementation dosing depends on the severity of deficiency, individual factors, and response to treatment:

• Maintenance (no confirmed deficiency): Health Canada recommends 600 IU/day for adults under 70, 800 IU/day for adults 70+. Many Canadian physicians suggest 1,000 IU/day during fall and winter in northern climates, though this should be discussed with your healthcare provider.
• Mild insufficiency (50–74 nmol/L): Often addressed with 1,000–2,000 IU/day — re-test in 3 months
• Moderate deficiency (30–49 nmol/L): May require 2,000–4,000 IU/day for several months under medical guidance
• Severe deficiency (<30 nmol/L): High-dose loading regimens (sometimes 50,000 IU/week for several weeks) may be prescribed by physicians; requires monitoring

Dietary Optimization

While diet alone is rarely sufficient to correct established deficiency, maximizing dietary vitamin D intake supports supplementation and long-term maintenance. Regularly consuming fatty fish (2 portions/week), choosing vitamin D-fortified foods, and including egg yolks can meaningfully contribute to daily intake.

Safe Sun Exposure

A small amount of sun exposure can contribute to vitamin D levels during appropriate months. In Canada, midday sun exposure (between 10 AM and 3 PM) of arms and legs for 10–30 minutes several times a week during summer is generally considered reasonable for vitamin D production in fair-skinned individuals. However, there is no "safe" amount of UV exposure that carries no skin cancer risk — balance skin cancer prevention with vitamin D needs in consultation with your doctor, particularly for individuals with fair skin or personal/family history of skin cancer.

Monitoring and Follow-up

After starting supplementation for confirmed deficiency, blood levels should be re-checked in approximately 3 months to assess response and adjust dosing. Once levels are optimized, annual monitoring during winter months is reasonable for individuals with ongoing risk factors.

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