by Margie Sharpe
MUSC Dietetic Intern
As the temperature rises, we naturally begin to migrate outside to the beach, our neighborhood pool or a Sunday afternoon baseball game. In doing so, we also may be reaping some health benefits from the absorption of vitamin D, the "Sunshine Vitamin," but if so, the questions are what are these benefits and how much is enough.
Vitamin D, which is actually a hormone produced in the body, is used to aid in the body's absorption of calcium. People who get too little vitamin D may develop soft, thin and brittle bones. In addition, muscles need vitamin D to move and the immune system needs vitamin D to fight off invading bacteria and viruses.
Some researchers have found that vitamin D deficiency is associated with increased inflammation in otherwise healthy people and that inflammation was lowered by simple vitamin D. Increased inflammation in the body can increase the risk of chronic inflammatory conditions, including coronary heart disease (CHD) and diabetes. These studies, however, are controversial and have been subject to debate in the medical community.
The Institute of Medicine recently increased the Dietary Reference Intake (DRI) to 600 international units (IUs) per day (up from 400 IU) to maintain health for adults and 800 IUs for those 71 and older. But evidence shows that this may still not be enough to reap the benefits of vitamin D. Studies support intakes of 2,000 – 5,000 IUs daily in adults to replenish deficient stores without causing toxicity.
Very few foods in nature actually contain vitamin D. The flesh of fatty fish (salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts are also found in beef liver, cheese and egg yolks. Fortified foods such as milk, most ready-to-eat breakfast cereals, some brands of orange juice, yogurt and margarine provide most of the vitamin D in the American diet. For example, almost all of the United States milk supply is fortified with 100 IUs per 8 ounce cup.
In addition, the body makes vitamin D when skin is directly exposed to the sun. The National Institute of Health estimates that 80 to 90 percent of the vitamin D in our bodies comes from sun exposure. It has been suggested by some vitamin D researchers, including MUSC's Bruce Hollis, Ph.D., professor of pediatrics, biochemistry and molecular biology, that just five to 30 minutes of sun exposure between 10 a.m. and 3 p.m. twice a week to the face, arms, legs or back without sunscreen usually leads to sufficient vitamin D synthesis. Cloudy days, shade, and dark-colored skin cut down on the amount of this vitamin the body can convert into usable form.
Those especially at risk for not getting enough vitamin D include older adults, as their skin doesn't make the vitamin when exposed to sunlight as efficiently as when they were younger and their kidneys are less able to convert vitamin D to its active form; people with dark skin, because their skin has less ability to produce vitamin D from the sun; and obese people, because their body fat binds to some vitamin D and prevents it from getting into the blood.
Obtaining sufficient vitamin D from natural food sources alone is difficult. For most people, consuming vitamin D-fortified foods, taking a supplement or being exposed to some sunlight are essential for maintaining a healthy status.