Vitamin D deficiency is the most common nutritional deficiency and likely the most common medical condition in the world. The major cause of vitamin D deficiency has been the lack of appreciation that the body requires 5- to 10-fold higher intakes than is currently recommended by health agencies. There is now overwhelming and compelling scientific and epidemiologic data suggesting that the human body requires a blood level 25(OH)D above 30 ng/mL for maximum health. To increase the blood level to the minimum 30 ng/mL requires the ingestion of at least 1000 IU of vitamin D per day for adults. In general, there is no downside to increasing either a child’s or adult’s vitamin D intake.
Vitamin D is one of the oldest hormones. Early in evolution as unicellular organisms evolved and took advantage of the sun’s energy for photosynthesis of sugars, they also began to photosynthesize vitamin D. A phytoplankton species that has existed in the Sargasso sea (Atlantic Ocean) for more than 500 million years unchanged was found to have more than 1% of its total dry weight as provitamin D 2 (ergosterol). When this organism was cultured and exposed to simulated sunlight it produced vitamin D 2 . As life forms evolved in the ocean, which has a high calcium content, and ventured onto land where calcium was stored in the soil, they needed to develop a method to efficiently absorb calcium from the plants and roots that they ate. It is likely that these organisms when exposed to sunlight produced vitamin D in their skin, which was critical for them to be able to absorb their dietary calcium efficiently. Vitamin D has evolved over millions of years to play and essential role in vertebrate evolution not only for bone health but for their overall health and well being.
Sources of vitamin D
Humans have always depended on the sun for their vitamin D requirement. Thus the major source of vitamin D for children and adults is exposure of the skin to sunlight. Adults in a bathing suit exposed to an amount of sunlight that causes a slight pinkness to the skin 24 hours later (1MED) is equivalent to ingesting about 20,000 IU of vitamin D. There are few foods that naturally contain vitamin D. Because vitamin D is fat-soluble it is found in oily fish, including salmon, mackerel, and herring. Fish that have little fat in their flesh concentrate their fat in their liver, which is why cod liver oil and oil from other nonoily fish are good sources of vitamin D. Yeast and mushrooms make huge quantities of ergosterol and when exposed to sunlight or ultraviolet irradiation are excellent sources of vitamin D. In the United States and Canada, milk and several other dairy products are fortified with vitamin D. Some orange juices are also fortified with calcium and vitamin D.
Historical perspective on extraskeletal effects of vitamin D
At the turn of the twentieth century it was estimated that more than 90% of children in the industrialized cities of northern Europe and 80% of children living in the northeastern United States had skeletal evidence of rickets. Besides the obvious deformities associated with rickets, it was noted that these children had severe muscle weakness, poor tooth eruption with dental caries, and were plagued by upper respiratory tract infections. In the early 1900s Finsen observed that exposure to sunlight was effective in treating several skin disorders, including lupus vulgaris, which is caused by a tuberculosis infection of the skin. His remarkable observations resulted in him receiving the Nobel prize in 1903. In 1915 Hoffman compared cancer mortality in cities according to latitude, and demonstrated that cancer mortality increased with increasing distance from the equator ( Table 1 ). In 1941 Apperly reported that people who lived in the Northeast were more likely to die of cancer than people who lived in the South. In the 1980s it was reported that there was a latitudinal association with colorectal cancer risk.