Vitamin D deficiency — symptoms, why it's so common, and what to do

Vitamin D deficiency is one of the most widespread nutrient deficiencies in developed countries, particularly in northern latitudes. Estimates suggest 40% or more of adults in countries like Belgium, the Netherlands, and the UK have insufficient levels. It is also consistently underdiagnosed because its symptoms overlap with so many other conditions.

What vitamin D actually does

Vitamin D is not strictly a vitamin — it functions as a hormone, synthesised in the skin from sunlight exposure and transported throughout the body where it binds to receptors in virtually every cell.

Its functions include:

• Calcium absorption from the gut (critical for bone density)
• Immune system regulation — both the innate and adaptive immune response
• Muscle function — particularly muscle protein synthesis
• Mood regulation — receptors in brain regions involved in serotonin production
• Inflammation regulation
• Insulin sensitivity

When vitamin D is deficient, all of these functions are compromised to some degree.

Symptoms of deficiency

The symptoms of vitamin D deficiency are non-specific, which is why it is so commonly missed:

Fatigue and low energy — the most common reported symptom. Often attributed to sleep problems, overwork, or other causes before vitamin D is checked.

Bone and joint pain — particularly in the back, hips, and legs. Vitamin D is essential for calcium absorption; without it, bones lose density gradually.

Muscle weakness — particularly in the legs. Affects both functional strength and exercise recovery.

Mood changes and depression — the relationship between vitamin D and mood is well-established. Seasonal affective disorder (SAD) is partly driven by the reduction in sunlight exposure (and thus vitamin D synthesis) in winter months.

Frequent illness — vitamin D plays a significant role in immune function. People with low vitamin D consistently show higher rates of respiratory infections.

Slow wound healing — vitamin D supports collagen production and immune response involved in healing.

Hair loss — vitamin D receptors are present in hair follicles and deficiency is associated with increased hair loss, particularly in the pattern of alopecia areata.

Who is most at risk

• People living in northern latitudes (above approximately 37° latitude) in winter
• People who spend most of their time indoors
• People with darker skin (more melanin reduces vitamin D synthesis from sunlight)
• Older adults (synthesis capacity declines with age)
• Perimenopausal and postmenopausal women (increased bone density risk)
• Overweight or obese individuals (vitamin D is fat-soluble and can be sequestered in adipose tissue)
• People who avoid sun exposure entirely (sunscreen, covering skin for cultural or medical reasons)

Testing

A blood test for 25-hydroxyvitamin D (25(OH)D) is the standard measure. Interpretation varies by lab and guideline, but broadly:

• Below 30 nmol/L (12 ng/mL): Deficient — clinical deficiency with health consequences
• 30–50 nmol/L (12–20 ng/mL): Insufficient — likely symptomatic in many people
• 50–125 nmol/L (20–50 ng/mL): Adequate — generally considered sufficient
• Above 125 nmol/L (50 ng/mL): Upper end of normal; above 250 nmol/L considered toxic territory

Many practitioners now consider the 75–150 nmol/L range as optimal for health outcomes rather than merely adequate.

What to do about deficiency

Sunlight — the natural route. 10–30 minutes of midday sun on significant skin area (arms and legs, not just hands and face) produces substantial vitamin D synthesis in fair-skinned individuals. This is much less effective in winter in northern latitudes, at high sun protection factor, with a lot of melanin, or in older adults.

Supplementation — the practical route for most people in northern countries, particularly in winter.

Dosing:

• Maintenance for those not severely deficient: 800–2,000 IU daily
• To correct deficiency: 3,000–5,000 IU daily (check with a GP or prescriber — some deficiency protocols use loading doses)
• Upper safe limit: 4,000 IU per day is the commonly cited safe upper limit for sustained supplementation without monitoring; up to 10,000 IU for limited periods under medical guidance

Form: Vitamin D3 (cholecalciferol) is the preferred form — it is more effective at raising blood levels than D2 (ergocalciferol). Take with a fatty meal for better absorption (it is fat-soluble).

With vitamin K2: Vitamin D3 and K2 work together in calcium metabolism. K2 directs calcium to bones and away from arteries. Supplementing D3 without K2 at higher doses is debated — many practitioners recommend combining them, particularly at doses above 2,000 IU.

Retest after 8–12 weeks of supplementation to assess the effect and adjust dose.

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For a complete guide to nutrition and supplementation specifically for perimenopause and menopause — including vitamin D, magnesium, and the evidence-based supplements — the Menopause Strength Blueprint covers the full framework.