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.
Early in the 20th century an investigator found that cod liver oil could prevent rickets in puppies. The nutritional factor in the oil that promotes skeletal calcium deposition was named “vitamin D,” alphabetically after already-named vitamins A, B, and C. Rickets was thought to be another vitamin-deficiency disease, and the curative agent, a steroid hormone, was mislabeled a “vitamin.”
Now, a century later, a wealth of evidence suggests that rickets, its most florid manifestation, is the tip of a vitamin D insufficiency/deficiency iceberg. A lack of Vitamin D can also trigger infections (influenza and tuberculosis), autoimmune diseases (multiple sclerosis, Type 1 diabetes, rheumatoid arthritis, and inflammatory bowel disease), cardiovascular disease, and cancer. Practitioners of conventional medicine (i.e., most MDs) are just beginning to appreciate the true impact of vitamin D deficiency. In 1990, medical journals published less than 20 reviews and editorials on vitamin D. Last year they published more than 300 reviews and editorials on this vitamin/hormone. This year, on July 19, 2007, even the New England Journal of Medicine, the bellwether of pharmaceutically-oriented conventional medicine in the U.S., published a review on vitamin D that addresses its role in autoimmune diseases, infections, cardiovascular disease, and cancer (N Engl J Med 2007;357:266–281).
Up until 1980, doctors thought that vitamin D was only involved in calcium, phosphorus, and bone metabolism. Then two investigators proposed that vitamin D and sunlight could reduce the risk of colon cancer. A growing body of evidence indicates that they were right and that vitamin D can prevent a whole host of cancers – colon, breast, lung, pancreatic, ovarian, and prostate cancer among them. Colon cancer rates are 4 to 6 times higher in North America and Europe, where solar radiation is less intense, particularly during the winter months, compared to the incidence of colon cancer near the equator. People with low blood levels of vitamin D and those who live at higher latitudes are at increased risk for acquiring various kinds of cancer. Many epidemiological, cohort, and case control studies prove, at least on a more likely than not basis, that vitamin D supplements and adequate exposure to sunlight play an important role in cancer prevention (Am J Public Health 2006;96:252–261).