Many of us are short on Vitamin D, which has plenty of health benefits.
By Kathleen Zelman, MPH, RD/LD
WebMD Weight Loss Clinic - Feature
Reviewed by Brunilda Nazario, MD
Scientists have known for some time about vitamin D's role in helping the body absorb calcium, in maintaining bone density, and in preventing osteoporosis. But new research suggests it may also help protect against chronic diseases such as cancer, type 1 diabetes, rheumatoid arthritis, multiple sclerosis, and autoimmune diseases.
Yet many adults have low blood levels of vitamin D. Vitamin D is not abundant in our usual food choices, so we get most of the vitamin from sun exposure and multivitamins. The problem is that the sun is not a reliable source for everyone.
The season, time of day, geography, latitude, level of air pollution, color of your skin, and your age all affect your skin's ability to produce vitamin D. Further, the form of Vitamin D found in most multivitamins is vitamin D2, which does not deliver the same amount of the vitamin to the body as the more desirable D3 form.
Deficient in D?
Vitamin D is the only vitamin that is also a hormone. After Vitamin D is made by the skin or eaten, the kidney and liver help to convert it into an active hormone form. As a hormone, it controls calcium absorption to help the body build strong bones and teeth, and it helps maintain muscle strength. When you are deficient in calcium and vitamin D, your bones break down to supply calcium to the rest of your body. But being deficient in vitamin D can take a toll on more than just your skeleton.
"There have been concerns about vitamin D status in the U.S. because of increasing reports of deficiencies, with an estimated 10 million Americans over age 50 diagnosed with osteoporosis," says Atlanta-based rheumatologist Eduardo Baetti, MD.
"Vitamin D deficiency is associated with low bone mass and osteoporosis because vitamin D is needed to absorb calcium from the diet. Low levels of vitamin D have also been linked with poor muscle strength and other chronic conditions, such as autoimmune disease and some forms of cancer."
The Sunshine Vitamin
D is also the only vitamin that does not need to be consumed in food or supplements because our bodies are efficient at making it when our skin is exposed to direct sunlight (not through a window). But not all sun exposure is the same, and many factors help determine how much we absorb. In general, the further away you are from the equator, the more efficient the vitamin D production, but cloud cover and air pollution can hinder the sun's ultraviolet (UV) rays.
Many people living in the Southern United States can get enough vitamin D by getting about 10-15 minutes of sun exposure on their arms and face a few times a week -- as long as they don't use sunscreen, which blocks some of the UV rays necessary to make the vitamin.
Along with geography, age and skin pigmentation are also factors.
Even in Atlanta, where the sunshine is adequate all year long, Baetti says, many of his patients -- especially elderly and dark-skinned people -- have low levels of vitamin D.
Dark pigment in the skin reduces the skin's ability to synthesize vitamin D from sunlight by 95%.
"Darker skinned people need 5-10 times as much exposure to synthesize the same amount of vitamin D as lighter skinned people," says vitamin D expert Michael Holick, PhD, MD.
But what about the dangers of sun exposure, which dermatologists often warn us about?
How Much Do We Need?
The current recommended intake of vitamin D is 200 IUs (international units) for those up to age 50; 400 IUs for people 51-70; and 600 IUs for those older than 70. Requirements increase with age because older skin produces less vitamin D.
But these recommendations date back to 1997. "Additional studies have been published since that time documenting the effectiveness of higher levels of vitamin D," says Holick, who was a member of the Institute of Medicine (IOM) committee that issued the recommendations.
Evidence is mounting that we may need even more -- especially older, dark-skinned, or housebound people.
According to the IOM Dietary Reference Intakes, the safe upper limit for vitamin D is 2,000 IUs for children, adults, and pregnant and lactating women. Some experts have suggested increasing the recommended amount to more than 2,000 IUs daily. But since vitamin D is a fat-soluble vitamin that is stored in the body, there's some concern it can be harmful in large doses.
"The current recommendations are not adequate to protect against chronic diseases or prevent osteoporosis," says Holick. "All evidence suggests that infants and adults can tolerate 1,000 IUs a day as safe, without risk of toxicity."
"Take a daily vitamin D3 supplement of 1,000 IUs or [get] safe sun exposure to maintain proper blood levels of vitamin D and reduce the risk of common cancers, type 1 diabetes, multiple sclerosis, rheumatoid arthritis, and inflammation associated with cardiovascular diseases," he suggests.
You're unlikely to get too much vitamin D in your diet unless you overdose on cod liver oil: "Read the label, and do not take cod liver oil if it contains vitamin A, because too much vitamin A can be toxic and weaken bones and cause birth defects in young women," Holick advises.
Sunlight will only produce as much vitamin D as you need, so there is no fear of overdosing from the sun, Holick says.
Boosting Vitamin D
Indeed, sunshine and supplements -- not food -- are the best sources of vitamin D, providing you choose a supplement with D3.
"Sunlight is the easiest, it's free, and your body is very efficient at making vitamin D from the sun, and it lasts twice as long as other sources," says Holick.
Good dietary sources are fortified foods such as milk, yogurt, margarines and cereals, catfish, sardines, salmon, tuna and egg yolks. But "it is hard to get enough vitamin D from your diet unless you enjoy dairy and fish, so it makes sense to try to get limited exposure to sunlight if you can and take a vitamin supplement," says Elisa Zeid, MS, RD, a spokesperson for the American Dietetic Association
Food for Thought
Chances are, you are not getting enough vitamin D for good health. So boost your D with safe sun exposure, or supplement your diet with 1,000 IUs of vitamin D3 a day. And be sure to eat a variety of foods rich in vitamin D. Also, check with your dermatologist about guidelines for safe sun exposure.
Originally published December 22, 2006.
Medically updated April 23, 2008.
SOURCES: Institute of Medicine Food and Nutrition Board, Dietary Reference Intakes: Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride. National Academy Press, Washington, D.C.,1999. Journal of Nutrition, Oct. 2005. Michael Holick, MD, PhD, professor of medicine, physiology, and biophysics, Boston University Medical Center. Elisa Zied, MS, RD, spokeswoman for American Dietetic Association; author, So What Can I Eat?! (Wiley, 2006). Eduardo Baetti, MD, rheumatologist, Kaiser Permanente, Atlanta.
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