Vitamin D in a New Light

[Neon Beach]

Vitamin D in a New Light

by Donald W. Miller, Jr., MD

There are thirteen vitamins humans need for growth and development and to maintain good health. The human body cannot make these essential bio-molecules. They must be supplied in the diet or by bacteria in the intestine, except for vitamin D. Skin makes vitamin D when exposed to ultraviolet B (UVB) radiation from the sun. A light-skinned person will synthesize 20,000 IU(international units) of vitamin D in 20 minutes sunbathing on a Caribbean beach.

Vitamin D is also unique in another way. It is the only vitamin that is a hormone, a type of steroid hormone known as a secosteroid, with three carbon rings.

Steroid hormones such as cortisone, estrogen, and testosterone have four carbon rings. Ultraviolet B radiation in sunlight breaks open one of the rings in a steroid alcohol present in the skin, 7-dehydrocholesterol, to form vitamin D (cholecalciferol). The liver changes this molecule into its circulating form, 25-hydroxyvitamin D (calcidiol, 25[OH]D), the "vitamin D" blood tests measure. Cells throughout the body absorb 25-hydroxyvitamin D and change it into 1,25-dihydroxyvitamin D (calcitriol), the active form of vitamin D that attaches directly to receptors on the DNA of genes in the cell’s nucleus.

The vitamin D hormone system controls the expression of more than 200 genes and the proteins they produce. In addition to its well-known role in calcium metabolism, vitamin D activates genes that control cell growth and programmed cell death (apoptosis), express mediators that regulate the immune system, and release neurotransmitters (e.g., serotonin) that influence one’s mental state.

Severe deficiencies of some vitamins cause vitamin-specific diseases, such as beriberi (from a lack of vitamin B1, thiamine), pellagra (B3, niacin), pernicious anemia (B12), and scurvy, (vitamin C). A deficiency in iodine produces a goiter, mental retardation, and, when severe, cretinism.
Rickets, a softening and bending of bones in children, first described in 1651, is another nutritionally-specific disease. It reached epidemic proportions following the industrial revolution, which began in the 1750s. In the 19th century, before the importance of exposing children to sunlight was recognized, the majority of children that lived in cities with sunless, narrow alleyways and pollution developed rickets. An autopsy study done in Boston in the late 1800s showed that more than 80 percent of children had rickets.

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