Let's explore vitamin D. The usual suspect "vitamin D" refers to a group of fat-soluble secosteroids. Some people consider it a hormone since it regulates calcium and phosphorous, with presence in the entire system. Low vitamin D levels are linked to various chronic diseases independently: cancer, heart disease, neurological diseases, type 2 diabetes mellitus, autoimmune diseases, depression and inflammation. Severe illnesses arise from Vitamin D deficiency. We're going to thoroughly examine this important nutrient today. Sources of Vitamin D There are two major forms of vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol)—the form that dominates in humans. About 80–90% of vitamin D is produced by the body through the skin — only 10–20% of it comes from diet. Natural dietary sources of D are limited — vegetables, grains and fruits contain little to none; animal sources (dairy products, oily fish and cod liver oil, egg yolk) provide D3 but in small amounts. Hence, sunlight fueled cutaneous production is the major. The Vitamin D Paradox You'd think: If sunlight is free, why does deficiency exist? In reality, 50% of the world's population have insufficient vitamin D (<30 ng/mL serum) and 1 billion are deficient (<10ng/mL). Deficiencies in all races, ages — male predilection. Causes of Deficiency Two main factors: environmental and behavioural. Environmental: In winter, UV-B photons at >40° latitudes are reduced by >80%. That latitude bisects Beijing in China — So the light needed to synthesize D is absent during winter months up North. Behavioral: shielding — heavy clothing, living indoors or overdosing on sunscreen. If you live in the south, maximize exposure to sun and leech off this free nutrient. Optimal Targeted Supplementation Meeting minimum requirements isn't enough for longevity. Target guidelines for family: Seniors: The priority population. Age-dependent calcium/muscle loss leads to falls; intake of 800 IU/d vitamin D reduces fractures (proven). Children: 400 IU/day to prevent rickets in infants, 600 IU/day (>1 year). Breastfeeding women need 600 IU/day for adequate breast milk levels. Everyone else: 600 IU/day. Ways to Implement Sunlight is king. 1 IU = 0.025 μg D3 - Seniors (20 μg/day): 20 min summer-sun swimsuit exposure produces 250 μg (i.e. 12.5× daily requirement). - Infants (10 μg): swimsuit for 50 sec (≈2 min clothed). - Children/Mothers (15 μg): swimsuit for 1 min (≈4 min clothed). Adolescents/adults: Outdoors 2–3 sessions per week is fine. Backup Plan For those deprived of sun: replace above targets with oral doses. Safety Limit It is toxic only at excessive doses (way beyond commercial preparations). Conclusion To maximize vitamin D, use sunlight wisely and soak up every ray possible.