The term vitamin D is used to describe two substances from two different sources. Vitamin D-3, cholecalciferol, is formed from cholesterol and is synthesized in the body during exposure to ultraviolet light (e.g. sunlight). The other substance is vitamin D-2, ergocalciferol, which is found in plants and is used to fortify foods such as margarine, cereals, and milk. Both substances perform the same function in the body.
Vitamin D (cholecalciferol and ergocalciferol) promotes intestinal absorption of calcium and phosphate into the blood stream. It also frees calcium from bones for use in other parts of the body. It is essential for normal bone growth and development. It also works in the kidneys to prevent calcium and phosphate loss through urine. Both forms of vitamin D are converted by the body into an active form called calcitriol. The liver and kidneys are involved in this process. For this reason, kidney failure often results in calcium deficiency symptoms.
Vitamin D is available in many multivitamin formulas. The RDI is 400 IU of vitamin D daily. This recommendation does not take into account the fact that people may be producing sufficient amounts of the vitamin due to exposure to the sun. It can be toxic if taken in doses larger than 2,000 IU. Symptoms of toxicity are headache, nausea, vomiting, metal taste in the mouth, fatigue, and muscle and bone pains. In addition, excess vitamin D can lead to an accumulation of calcium in vital organs, such as the liver and kidneys, with potentially dangerous consequences.