The Vitamin D Deficiency Nobody Catches: Cold Climates and Obesity

There’s a quiet epidemic in Canada that almost nobody screens for during a routine physical. By the end of winter — late February through April — roughly 60% of Canadian adults are vitamin D insufficient, and 30% are clinically deficient. The further north you live, the worse it gets. The longer you’ve been overweight, the worse it gets. Both factors compound.

A 2010 Statistics Canada survey found that 25% of Canadians had blood vitamin D levels below 50 nmol/L (the clinical insufficiency threshold), and the rate climbed sharply with latitude and during winter months. A 2014 follow-up showed the rate among Indigenous and South Asian populations exceeded 40% year-round.

Figure reference: Sarafin K et al. “Vitamin D status of Canadians: 2007 to 2009 Canadian Health Measures Survey.” Statistics Canada Catalogue no. 82-624-X. PMID: 24305627. See Table 2: vitamin D status by season, age, and ethnicity.

The body makes vitamin D from sunlight hitting skin. Between October and March in Coquitlam, BC, the sun’s angle is too low for the UVB spectrum that triggers synthesis to reach the skin in usable doses, even on clear days. From Halifax to Vancouver, the entire country is functionally outside the vitamin D-producing window for six months of the year.

The obesity multiplier

Here’s the part that doesn’t get explained: people with obesity are 30–50% more likely to be vitamin D deficient than people with normal body composition, even at the same latitude and sun exposure. The mechanism: vitamin D is fat-soluble. Adipose tissue acts as a reservoir that pulls vitamin D out of circulation and sequesters it inside fat cells, where it’s not available to the bloodstream.

A 2019 study by Pereira-Santos et al. in Obesity Reviews synthesized 23 cohort studies and quantified the relationship: each 10% increase in body fat percentage predicted a 4 nmol/L decrease in circulating vitamin D, independent of dietary intake or sun exposure. The fat cells were literally taking it out of circulation.

Figure reference: Pereira-Santos M et al. “Obesity and vitamin D deficiency: a systematic review and meta-analysis.” Obes Rev. 2015;16(4):341-349. PMID: 25688659. See Figure 4: dose-response curve of body fat percentage to serum 25(OH)D level.

So a person living in Vancouver with a BMI of 32 has two reasons to be deficient: insufficient UVB exposure for half the year, and adipose tissue sequestering whatever vitamin D does enter circulation.

Why it matters

Vitamin D is misnamed. It’s not really a vitamin — it acts as a steroid hormone. Vitamin D receptors are found in nearly every tissue in the body. Deficiency is associated with:

• Reduced muscle protein synthesis and grip strength
• Increased insulin resistance
• Higher rates of depression and seasonal affective disorder
• Weakened immune function (especially relevant for respiratory infections)
• Poor bone mineral density
• Higher inflammatory markers

For my clients on GLP-1 medications and for clients with PCOS, vitamin D deficiency frequently shows up as fatigue, low mood, and muscle weakness that gets attributed to the medication or the syndrome. The actual culprit is often a serum vitamin D below 50 nmol/L that nobody checked.

The protocol

Step 1 — Test before supplementing. A serum 25-hydroxyvitamin D (25(OH)D) test costs about $30–50 in Canada (often covered by insurance with a doctor’s order). It’s the only way to know your starting point.

• Below 30 nmol/L: clinically deficient. Therapeutic dose required.
• 30–50 nmol/L: insufficient. Supplementation needed.
• 50–125 nmol/L: sufficient. Maintain.
• Above 125 nmol/L: approaching the upper safe range. Reduce supplementation.

Step 2 — Supplement appropriately. The Endocrine Society’s 2011 guidelines (still the most-cited reference, updated minor revisions since) recommend:

• Adults at risk: 1,500–2,000 IU vitamin D3 daily
• Adults with documented deficiency: 6,000 IU daily for 8 weeks, then 1,500–2,000 IU maintenance
• Adults with obesity: doses may need to be 2–3× higher than normal-weight adults to reach the same blood level

For most of my clients in BC, the working dose during October–April is 2,000–4,000 IU/day of D3 (cholecalciferol, not D2). Take it with a meal containing fat — it’s fat-soluble.

Step 3 — Retest at 8–12 weeks. Adjust based on actual blood level.

Step 4 — In the summer months, scale back if you’re getting genuine sun exposure (10–15 minutes of midday sun on bare arms and legs, 3–4× per week).

What not to do

• Don’t take massive single doses without a known deficiency
• Don’t take vitamin D2 if D3 is available (D3 raises serum levels more effectively)
• Don’t rely on diet alone — even fortified milk and salmon won’t get you to sufficiency in winter
• Don’t skip the blood test — without it, you’re guessing