Sterols - Vitamin D

A number of sterols have vitamin D activity, that is to say they prevent the occurrence of rickets in higher animals. The chemistry of the sterols is very complicated, and books on physiological chemistry may be consulted for structural formulae, etc. When the common sterol of fungi, ergosterol, is irradiated with ultraviolet rays, another sterol, calciferol, is produced. This is commonly referred to as vitamin D 2. (There is no D 1, for the original use of the term was a misnomer.) D 3 is 7-dehydrocholesterol. Numerous other antirachitic (rickets-preventing) substances have also been described. Whether or not a given sterol or sterol derivative has antirachitic action can only be determined by actual test.

Bacteria do not seem to require sterols. Some species of protozoan flagellates do; also Entamoeba histolytica, the ameba that causes human dysentery. Insects, in so far as they have been studied, require sterols, but they seem to need cholesterol, a cholesterol derivative, or a plant sterol; calciferol cannot usually be substituted for these.

In mammals and birds, vitamin D causes an increase in the deposition of calcium and phosphorus in the bones. This is apparently due, in part at least, to an increased absorption through the intestine. Using radioactive isotopes, Greenberg found that vitamin D promotes the absorption of calcium and strontium from the digestive tract. Sterols may act in the same way in promoting the passage of calcium through the walls of the insect intestine. Moreover, they may be concerned with the entrance of calcium into cells. This aspect of the subject has never been investigated.