Vitamin D is a fat-soluble vitamin that is synthesized by the body but can also be obtained from dietary sources. When sunlight exposure is abundant, the body can transform a cholesterol compound in the skin into a vitamin D precursor. UVB[1] exposure can provide from 80% to 100% of vitamin D needs. This means that one may meet his/her needs while sun bathing.
However, vitamin D production depends on several factors such as skin color, age, time of the day, season, and location. First, vitamin D synthesis is only effective when sun exposure occurs between 8 am and 4 pm without sunscreen over SPF 8. Experts recommend that the face, hands, and arms be exposed to sunlight for short periods of time (five to 15 minutes) at least two to three days per week. This means that people living in sunny regions may get plenty of this nutrient through sun exposure. Yet, people with darker skins may need more exposure to produce enough vitamin D. Also, as we age, the production of vitamin D in the skin decreases, placing older adults at risk of deficiency. Keep in mind that people who don’t receive adequate sunlight exposure should obtain vitamin D from dietary sources.
Vitamin D is a hormone-like[2] compound that plays an important role in maintaining bone health. It helps regulate calcium and phosphorus absorption and calcium deposition in the bones. Moreover, vitamin D seems to ensure normal development of some body cells (skin, colon, prostate, and breast), which may contribute to cancer prevention in those sites.
When vitamin D deficiency occurs during growth, it causes inadequate calcium and phosphorus deposition in the bones – a disease known as rickets. In adults, vitamin D deficiency causes osteomalacia, an ailment in which bones lose their mineral content, becoming porous, weak and susceptible to fractures.
A few foods contain significant amounts of vitamin D. Among food sources are fatty fish such as herring and salmon, and fortified milk, yogurt, orange juice, and some ready-to-eat cereals. Eggs, butter, and liver do contain some vitamin D, but large portions would have to be consumed in order to provide the adequate intake. The Adequate Intake (AI) for vitamin D is five micrograms per day. However, older adults are advised to consume two to three times this amount. The vitamin D content of processed foods is not required to appear on the Nutrition Facts panel. However, if a manufacturer makes a nutrition claim about vitamin D, it must be stated on the panel. On food labels, the Daily Value for vitamin D is set at 10 micrograms. Keep in mind that a Daily Value of 5% is considered low, while 20% is considered high.
Because vitamin D is fat-soluble, it is easily stored in the body. This means that high doses of vitamin D, even though rare, may cause toxicity. Excessive intake of vitamin D, usually through supplements, causes calcium overabsorption, which may lead to metabolic disturbances and cell death. Typical symptoms of toxicity are weakness, loss of appetite, diarrhea, vomiting, mental confusion, and increased urine output. The Upper Level of vitamin D is 50 micrograms per day.
References
Boyle, M. and Long, S. Personal Nutrition. Belmont, CA: Wadsworth Cengage Learning. 2010.
Wardlaw, G. and Smith, A. Contemporary Nutrition. New York: McGraw-Hill. 2009.