Vitamin D, the sunshine vitamin, is actually not a vitamin; it's a hormone. The body makes vitamin D with the help of sunlight and cholesterol in the skin, and it causes other parts of the body, including intestines, kidneys and bones, to respond by making calcium available for bone growth, according to Eleanor Whitney and Sharon Rolfes in "Understanding Nutrition."
Identification
Vitamin D, also called calciferol, comes in various forms. A plant version is called vitamin D2, or ergocalciferol, and an animal version is called vitamin D3, or cholecalciferol. The liver makes the precursor of vitamin D from cholesterol. Activation of this inactive form is a closely regulated process involving the liver and kidneys.
Function
Vitamin D has an important role in maintaining the blood concentration of calcium and phosphorus, according to MayoClinic.com. The bones grow dense and strong as they are able to absorb these minerals and deposit them in bone. Other target tissues for vitamin D include the brain and nervous system, pancreas, skin, muscles, cartilage, reproductive organs and cancer cells. This suggests that vitamin D may be useful in treating many disorders, including cancer, according to "Understanding Nutrition."
Geography
Vitamin D production is linked to sunshine. Above 40 degrees north latitude and below 40 degrees south latitude, vitamin D production in human bodies essentially stops for the four to six months of winter, according to "Understanding Nutrition." Synthesis of vitamin D increases in the spring, reaches a peak in summer and declines in the fall. The 40 degrees latitude line crosses the U.S. just north of Reno, Denver, Topeka, Indianapolis and Philadelphia. People living in parts of the U.S. north of these cities have more limited vitamin D production during some parts of the year.
Considerations
The pigments of dark skin reduce vitamin D synthesis. Heavily pigmented skin makes the same amount of vitamin D in three hours as lightly pigmented skin produces in 30 minutes, according to "Understanding Nutrition." Because vitamin D synthesis in the body depends on the sun, people most at risk of vitamin D deficiency are those with limited exposure to the sun. Factors that limit sun exposure include geographic location, season of the year, time of day, air pollution, type of clothing, tall buildings, sunscreens and indoor living.
Food Sources
Two cups of vitamin D-fortified milk per day provide adequate intake. Other food sources include butter, margarine, cereals, chocolate mixes, beef, veal, egg yolk, liver, salmon, herring, sardines and fish oils. People who consume a vegan diet with no foods of animal origin may require a vitamin D supplement. Adequate intake in micrograms per day is 5 until age 50, 10 until age 70, and 15 thereafter, according to the Office of Dietary Supplements.
Warning
Vitamin D can be harmful both in deficiency and in excess. Deficiency results in the disease rickets in children and osteomalacia in adults. Osteoporosis results from failure to synthesize vitamin D or obtain enough from food and sets the stage for loss of calcium from the bones, which results in fractures.
The elderly, whose skin, liver and kidneys have diminished capacity to activate vitamin D and who seldom drink milk, rarely go outdoors or cover up and use sunscreen are at increased risk. With an excess of vitamin D, the concentration of calcium in the blood increases, kidney stones may form and calcium deposits may harden the arteries.
References
- "Understanding Nutrition (Ninth Edition)"; Eleanor Noss Whitney and Sharon Rady Rolfes; 2002
- MayoClinic.com: Vitamin D
- Office of Dietary Supplements: Vitamin D
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