Vitamin D Deficiency: Symptoms, Testing, and How Much to Take
Vitamin D is often described as a vitamin but functions more like a hormone — it regulates hundreds of genes and affects nearly every system in the body. Deficiency is estimated to affect over a billion people worldwide, and its symptoms are easy to attribute to everything except what's actually causing them.
What Vitamin D Does
Beyond its well-known role in calcium absorption and bone health, vitamin D affects immune function, mood regulation, muscle strength, circadian rhythm, and inflammatory response. Receptors for vitamin D are found in almost every cell in the body — which explains why deficiency shows up in so many different ways.
Common Symptoms of Deficiency
These are the symptoms most consistently linked to vitamin D deficiency in research:
- Persistent fatigue that doesn't improve with sleep
- Low mood, particularly in winter months (seasonal pattern)
- Muscle weakness and generalised aches
- Frequent infections or slow recovery from illness
- Bone pain, particularly in the lower back
- Poor sleep quality and difficulty maintaining sleep
The problem is that none of these are specific to vitamin D — they overlap with dozens of other conditions. The only way to know for certain is a blood test measuring 25-hydroxyvitamin D.
Who Is at Risk
Vitamin D is synthesised through sun exposure, so anyone who gets limited sunlight is at risk. This includes:
- People living at higher latitudes (above 35°N or below 35°S)
- Office workers who spend most daylight hours indoors
- People with darker skin (melanin reduces vitamin D synthesis)
- Older adults (synthesis efficiency declines with age)
- People who consistently use high-SPF sunscreen
- Those eating little or no fatty fish, egg yolks, or fortified foods
Testing and Optimal Levels
A 25(OH)D blood test is the standard way to assess vitamin D status. Reference ranges vary slightly, but most experts consider:
- Below 30 nmol/L (12 ng/mL) — deficient
- 30-50 nmol/L (12-20 ng/mL) — insufficient
- 50-125 nmol/L (20-50 ng/mL) — adequate
- 75-100 nmol/L (30-40 ng/mL) — optimal for most health benefits
Testing before supplementing is ideal — it tells you how much to take and allows you to confirm the dose is working.
How Much Vitamin D to Take
The official upper tolerable intake is 4,000 IU/day, but most research uses 1,000-4,000 IU/day for supplementation. Practically:
- Maintenance if not deficient: 1,000-2,000 IU/day of D3
- Correcting insufficiency: 2,000-4,000 IU/day of D3
- Correcting frank deficiency: higher doses under medical supervision
Always take D3 (cholecalciferol), not D2 (ergocalciferol) — D3 is significantly more effective at raising blood levels. Take it with a meal containing fat for best absorption.
Food Sources of Vitamin D
Dietary vitamin D is limited but worth including. Best sources:
- Salmon — 600-1,000 IU per 150g serving
- Mackerel — 400 IU per 150g
- Canned sardines — 300 IU per 100g tin
- Egg yolks — 40-50 IU per egg
- Fortified milk — 100 IU per 240ml
Food alone rarely provides enough, which is why supplementation is particularly important for those at risk of deficiency.
Vitamin D is recommended across all our meal plans as a supplement — the food-first approach helps but rarely covers the full requirement. Our Energy and Sleep plans are specifically designed around foods that support circadian rhythm and hormone balance.
Get the Energy Meal Plan →Vitamin D toxicity is possible at very high doses. Do not supplement above 4,000 IU/day without a blood test and medical supervision.