Vitamin D is a star nutrient these days, as research links it to numerous health benefits. Studies suggest vitamin D may go beyond its well-established role in bone health and reduce the risk of cancer, heart disease, stroke, diabetes, autoimmune diseases, and more.
What makes vitamin D unique is that it is a vitamin and also a hormone your body can make from the sun. Despite the ability to get vitamin D from food and the sun, an estimated 40%-75% of people are deficient.
Why? Vitamin D is not abundant in our food choices and the sun is not a reliable source for everyone.
Many factors affect the skin's ability to produce vitamin D, including season, time of day, latitude, air pollution, cloud cover, sunscreen, body parts exposed, color, and age. Dermatologists recommend using sunscreen and getting vitamin D from food and supplements rather than risk the harmful rays of the sun.
Role of Vitamin D
Vitamin D is naturally present in few foods. Since 1930, virtually all cow's milk in the U.S. has been voluntarily fortified with 100 IU of vitamin D per cup. Food manufacturers are fortifying other foods, such as yogurt, cereal, and orange juice, to help consumers fill the nutrient gap in their diets.
Together with calcium, vitamin D can help prevent osteoporosis in older adults. Without enough vitamin D, bones can become brittle and prone to fracture. It is estimated that more than 40 million adults in the U.S. have or are at risk of developing osteoporosis.
"Vitamin D deficiency is associated with low bone mass and osteoporosis, which is estimated to affect 10 million adults over the age of 50 in the U.S.," says Atlanta rheumatologist Eduardo Baetti, MD. 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 because the sun is not a reliable source.
How Much Vitamin D Do You Need?
Bone health was the single focus of the Institute of Medicine's recommendations on how much vitamin D and calcium people should get.
The recommendations for adults up to age 69 rose to 600 IU/day, and to 800 IU/day for adults starting at age 70. Older adults need more vitamin D because as they age, their skin does not produce vitamin D efficiently, they spend less time outdoors, and they tend to not get enough vitamin D.
The committee did not consider the emerging research on any other conditions. Patsy Brannon, PhD, RD, a Cornell University professor of nutritional sciences and a member of the IOM committee, spoke about that at the American Dietetic Association's 2011 annual meeting in San Diego. "The committee of 14 scientists reviewed more than 1,000 publications and determined that the evidence was inconsistent and inconclusive to include any other health benefits in the new recommendations," Brannon said. "The committee is not dismissing the role of vitamin D in other areas, we need more clinical trials, consistent evidence, and evidence that supports causality."
Best Sources of Vitamin D
The sun is an excellent source of vitamin D, but it is hard to quantify how much vitamin D you get from time in the sun and the risk of skin cancer may outweigh the benefits. Food first, says Baylor College of Medicine dietitian Keli Hawthorne. "Supplements can fill in the gaps but it is always better to try to meet your nutritional needs with foods that contain fiber, phytonutrients, and so much more," Hawthorne says.
Unless you enjoy a diet that includes fatty fish or fish liver oils, it may be hard to get enough vitamin D naturally without eating fortified foods or taking a supplement. "The major dietary source of vitamin D comes from fortified diary, along with some yogurts and cereals," Hawthorne says. Mushrooms, eggs, cheese, and beef liver contain small amounts.
Reading Food Labels
Daily Values (DV) are on nutrition fact panels to help consumers compare nutrients in products and to choose a healthy diet. The DV for vitamin D is currently set at 400 IU by the FDA, which is less than the recommended 600 IU.
Hawthorne's advice: "Do the math: When one serving says it meets 100% DV, you still need an additional 200 IU to satisfy your requirement."
Amount of vitamin D in sample food sources:
- 1 Tbsp cod liver oil: 1,360 IU
- 3 oz. salmon: 800 IU
- 8 oz. fortified milk:100 IU
- 8 oz. fortified orange juice: 100 IU
- 3 oz. irradiated mushrooms: 400 IU
How Much Is Too Much?
Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins can build up in the body and are not as easily excreted as water-soluble vitamins. The IOM committee set a level of 4,000 IU as the ‘tolerable upper limit' or the maximum amount that is safe to consume daily.
Vitamin D researcher and Creighton University professor Robert Heaney, MD, agrees with the new level but would like to see it even higher.
"I am delighted the upper limit for vitamin D has been doubled to 4,000 IUs per day, although this is a conservative level, considering the body of scientific evidence indicating it should be 10,000 IU," Heaney tells WebMD. "However, few people need more than 4,000 IUs, which will meet the needs of most healthy people, give physicians confidence to recommend supplementation, and allow research at higher vitamin D levels."
In July 2011, the Endocrine Society Practice Guidelines published recommendations for the evaluation, treatment, and prevention of vitamin D recommending an upper limit of 10,000 IU/day.
"There is a potential to cause harm if you overdose on supplements above 4,000 IU/day but there is no fear of overdosing from the sun because your skin acts like a regulatory system, only allowing production of the amount of vitamin D you need," Brannon says.
Acceptable Vitamin D Blood Levels
Part of the confusion about whether or not you are getting enough vitamin D may be the definition of the acceptable blood level of vitamin D, clinically measured as 25-hydroxyvitamin D [25(OH)D].
Using vitamin D blood levels is the best estimate of adequacy that accounts for dietary intake and sunshine, yet experts differ on what that level should be.
"A 25(OH)D blood level of at least 20 nanograms/ml was used by the IOM committee to set the recommendations for vitamin D because this level showed adequacy for a wide variety of bone health indicators" says Brannon.
The Endocrine Society Practice Guidelines, as well as many laboratories and experts (including Baetti), recommend a minimum vitamin D blood level of 30 nanograms/ml as an acceptable level.