How to Calculate Your Vitamin D Requirement

UVB sunlight converts 7-dehydrocholesterol in skin to vitamin D3. Required: UVB with solar elevation angle above 45 degrees. In the UK (latitude 51-59°N): October to March: UVB insufficient for vitamin D synthesis at most latitudes April to September: production possible during midday hours Summer production rate (approximate, light skin, arms and face exposed): 10-15 minutes at solar noon: approximately 1,000-2,000 IU produced 30 minutes: approximately 3,000-4,000 IU Darker skin: approximately 3-5x longer needed for same production Note: sunscreen SPF 15+ blocks ~95% of vitamin D synthesis. But sun protection during long exposure is important for skin cancer risk.

NHS / SACN (Scientific Advisory Committee on Nutrition) recommendations: Adults and children over 1: 400 IU (10 mcg) per day all year This is the minimum -- many experts consider this insufficient. Supplementation guidance: October to March: 400-1,000 IU/day for all UK adults High-risk groups year-round: 1,000-2,000 IU/day High-risk groups: darker skin, elderly, indoors, obese, covered clothing Vitamin D insufficiency (blood level 25-50 nmol/L): Therapeutic dose: 1,000-2,000 IU/day for 3-6 months Deficiency (below 25 nmol/L): Therapeutic dose: 3,000-4,000 IU/day for 3 months (GP may prescribe 50,000 IU weekly)

Vitamin D toxicity (hypercalcaemia) is rare but possible. UK safe upper limit: 4,000 IU/day long-term (SACN) Tolerable upper limit (EU): 4,000 IU/day Research suggests 10,000 IU/day is unlikely to cause toxicity in healthy adults but exceeding 4,000 IU/day without testing is not recommended. Toxicity risk: very low at 1,000-2,000 IU/day supplementation. Signs of overdose: nausea, weakness, frequent urination, confusion. Always check blood levels (25-OH vitamin D test) before high-dose supplementation. Target serum level (consensus): 50-125 nmol/L (20-50 ng/mL)