Several forms (vitamers) of vitamin D have been discovered. The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol. These are known collectively as calciferol.
Adequate intake levels of vitamin D have been established by the Food and Nutrition Board at the Institute of Medicine of The National Academies. These intake levels are based only on age (i.e., they are the same regardless of weight, gender, pregnancy, or lactation) – Birth to 50 years, 5 µg (200 IU); 51–70 years, 10 µg (400 IU); 71+ years, 15 µg (600 IU). These intake levels are based on the assumption that the vitamin is not synthesized by exposure to sunlight. Vitamin D from food is absorbed from the upper part of small intestine along with dietary fat and transported to liver. In skin ultra violet radiation from Sun converts cholesterol derivative to cholecalciferol which is transported to Liver, where it is converted to calcidiol, which is convereted into Vitamin D in kidneys.