Vitamin D toxicity
During the 1950s, rickets was more or less totally eradicated in Britain and other temperate countries. This was due to enrichment of a large number of infant foods with vitamin D. However, a small number of infants suffered from vitamin D poisoning, the most serious effect of which is an elevated plasma concentration of calcium. This can lead to contrac-tion of blood vessels, and hence dangerously high blood pressure, and calcinosis, that is the calcification of soft tissues, including the kidney, heart, lungs, and blood vessel walls.
Some infants are sensitive to intakes of vitamin D as low as 50 μg/day. To avoid the serious problem of vitamin D poisoning in these susceptible infants, the extent to which infant foods are fortified with vitamin D has been reduced considerably. Unfortunately, this means that a small proportion, who have relatively high requirements, are now at risk of developing rickets. The problem is to identify those who have higher requirements and provide them with supplements.
The toxic threshold in adults is not known, but those patients suffering from vitamin D intoxication who have been investigated were taking supplements providing more than 250 μg/day.
Although excess dietary vitamin D is toxic, exces-sive exposure to sunlight does not lead to vitamin D poisoning. There is a limited capacity to form the precursor, 7-dehydrocholesterol, in the skin, and a limited capacity to take up cholecalciferol from the skin. Furthermore, prolonged exposure of previ-tamin D to UV light results in further reactions to yield lumisterol and other biologically inactive compounds.