Vitamin D is a nutrient that plays a key role in bone growth and many other body processes. Unlike other vitamins, which are obtained through diet, vitamin D can be made in the body. Vitamin D is often called the “sunshine vitamin” because it forms in the skin with exposure to sunlight. Vitamin D is a fat-soluble vitamin—that is, it is absorbed into the body in fats and stored in them.
Fish-liver oils are a good dietary source of vitamin D. Food producers often add vitamin D to fortify milk and such other products as margarine, yogurt, and breakfast cereals.
Function.
Scientists classify vitamin D as a hormone rather than a vitamin. A hormone is a chemical substance made by one part of the body that has effects in other parts. Vitamin D functions to maintain normal levels of the mineral calcium in the blood by promoting its absorption in the intestines. Thus, vitamin D plays a key role in the formation and strengthening of bone. Vitamin D also supports normal cell growth, nerve and muscle function, and immune system function.
Production of vitamin D in the skin starts with a compound called 7-dehydrocholesterol. This compound reacts with ultraviolet B (UVB) radiation from sunlight to produce vitamin D. Vitamin D produced in the skin or obtained from foods is not active in the body. This inactive form is converted to active vitamin D as needed through chemical reactions in the liver and kidneys.
The amount of vitamin D produced in the skin depends on the length of exposure to sun along with the latitude, season, and time of day. Other factors that affect vitamin D production include natural skin pigmentation and the use of sunscreen. Skin production of vitamin D is greatest in fair-skinned individuals, at midday, and in latitudes closer to the equator. Less vitamin D is produced in darker skinned individuals or individuals wearing sunscreen, at higher latitudes, and during winter.
Intake.
The United States National Academy of Medicine has established a tolerable upper intake level (UL) for vitamin D of 100 micrograms (4,000 international units) per day for adults. Taking regular megadoses of vitamin D—doses many times larger than the UL—without proper medical supervision can be toxic. Vitamin D toxicity can cause high blood calcium levels and the calcification (hardening) of soft tissues.
People who lack a particular vitamin may develop a vitamin deficiency disease. Vitamin D deficiency results in the inadequate mineralization of bones, leading to soft bones, bowed legs, and pain and weakness. Vitamin D deficiency in children is called rickets and in adults is called osteomalacia. Low vitamin D intake is also a risk factor for osteoporosis , a condition of weak, porous bones that often occurs as people age.
History.
Rickets and other vitamin deficiency diseases have been known for hundreds of years. Doctors knew that cod-liver oil was effective for preventing rickets in children. In 1922, the American biochemist Elmer McCollum showed that the effective ingredient in cod-liver oil was a previously unknown vitamin. He named it vitamin D.
The British chemist Frederic Askew, the Dutch chemist Engbert H. Reerink, and the German scientist Adolf Windaus independently isolated vitamin D in 1931. At this time, Windaus also identified the chemical structure of vitamin D. Windaus further studied the active form of vitamin D, which he found to be a kind of fatty substance called a sterol. Sterols, which include cholesterol and many hormones, have key roles in many body processes.
