Dr. Rachel StoneVitamin D deficiency affects an estimated 1 billion people worldwide — making it one of the most common nutritional deficiencies globally. Despite this, most people don’t know whether they’re deficient, what symptoms to look for, or how much they should be taking. Here’s a complete, evidence-based guide.
What Does Vitamin D Do?
Vitamin D functions more like a hormone than a vitamin — it binds to receptors in virtually every cell in the body (over 30,000 genes have vitamin D response elements). Its roles include:
- Calcium and phosphorus absorption: Without adequate vitamin D, only 10–15% of dietary calcium is absorbed; with adequate D, absorption rises to 30–40%
- Bone mineralisation: Vitamin D deficiency causes rickets in children and osteomalacia (soft bones) in adults; contributes to osteoporosis risk
- Immune regulation: Vitamin D activates T cells and macrophages, regulates inflammatory cytokine production, and supports the antimicrobial peptide cathelicidin
- Muscle function: Vitamin D receptors are present throughout muscle tissue; deficiency causes muscle weakness and increases fall risk in older adults
- Mood and mental health: Vitamin D receptors are present in the hippocampus and prefrontal cortex; deficiency is strongly associated with depression
- Cardiovascular health: Low vitamin D is associated with higher rates of hypertension, heart failure, and cardiovascular mortality
Signs of Vitamin D Deficiency
- Fatigue and low energy — one of the most common and nonspecific symptoms
- Bone pain and muscle weakness
- Low mood, depression, or seasonal mood changes
- Frequent illness — recurrent colds, infections, or slow recovery
- Hair loss (severe deficiency)
- Back and bone pain
- Slow wound healing
These symptoms are nonspecific and many other conditions cause them — blood testing is the only reliable way to confirm vitamin D status.
Vitamin D Blood Levels: What’s Optimal?
| 25(OH)D Level | Status |
|---|---|
| Below 25 nmol/L (<10 ng/mL) | Severe deficiency — rickets, osteomalacia risk |
| 25–50 nmol/L (10–20 ng/mL) | Deficiency — significant health risk |
| 50–75 nmol/L (20–30 ng/mL) | Insufficiency — suboptimal but less severe |
| 75–125 nmol/L (30–50 ng/mL) | Sufficient — adequate for most health outcomes |
| 125–175 nmol/L (50–70 ng/mL) | Optimal — associated with best health outcomes in most research |
| Above 250 nmol/L (>100 ng/mL) | Potential toxicity risk (rare) |
How Much Vitamin D Should You Take?
Official recommendations (400–800 IU/day) are based on bone health alone and widely considered too conservative by most researchers who study vitamin D. Real-world supplementation research suggests:
- Maintenance (for those not severely deficient): 1,000–2,000 IU daily of vitamin D3
- For deficiency correction: 2,000–4,000 IU daily until levels normalise (typically 3–6 months), then maintain with 1,000–2,000 IU
- High-latitude, low-sun exposure, or darker skin: 2,000–4,000 IU daily throughout winter months
- Always test before high-dose supplementation: A 25(OH)D blood test before and after 3 months of supplementation is the rational approach
Vitamin D3 (cholecalciferol) is significantly more effective than vitamin D2 at raising blood levels. Always choose D3. Taking vitamin D with a fatty meal improves absorption. Vitamin K2 (MK-7 form) should be co-supplemented at higher D doses to direct calcium to bones rather than arteries.
Frequently Asked Questions
Vitamin D supports calcium absorption and bone mineralisation, immune regulation (activating T cells and macrophages), muscle function, mood and mental health (vitamin D receptors are present throughout the brain), and cardiovascular health. It functions more like a hormone than a vitamin, with receptor sites in virtually every cell in the body.
A 25(OH)D blood test is the only reliable way. Common symptoms of deficiency include fatigue, bone pain, muscle weakness, low mood, and frequent illness — but these are nonspecific and many conditions cause them. Testing is especially important for people who have limited sun exposure, live at high latitudes, have darker skin, are obese, or are over 65.
For most adults without confirmed deficiency: 1,000–2,000 IU of vitamin D3 daily. For confirmed deficiency: 2,000–4,000 IU daily until levels normalise, then maintain at 1,000–2,000 IU. Always choose D3 (not D2), take with a fatty meal, and consider co-supplementing vitamin K2 at higher doses to direct calcium appropriately.
Yes, in theory — but in practice, most people can’t. At latitudes above 35° North or South (most of Europe, Canada, northern USA), the sun’s angle is insufficient to trigger vitamin D synthesis for several months in winter. Cloud cover, sunscreen, indoor work, and darker skin all further reduce synthesis. Supplementation is practical and reliable for most people in northern climates.
This article is for informational purposes only. Have your vitamin D levels tested before starting high-dose supplementation, and consult a healthcare professional if you have conditions that affect calcium or vitamin D metabolism.
Sarah Mitchell
Dr. Rachel Stone
Daniel Brooks
Rachel Summers