Key gaps remain in understanding health effects of vitamin D
Despite considerable progress in research to understand the health effects of vitamin D, experts convened by the NIH to review the available data found major gaps in the evidence. The data are strongest in the area of bone health among elderly men and post-menopausal women, suggesting that increased vitamin D intake can improve bone health and prevent falls. For other age groups and health issues, though, it is too early to say conclusively whether more vitamin D might be beneficial.
An in-depth review of current research on the health effects of vitamin D is being published as the proceedings of the NIH conference, "Vitamin D and Health in the 21st Century: An Update", which will appear in an August 2008 supplement to the American Journal of Clinical Nutrition.
Intriguing findings from research conducted in recent years have led to increased interest in vitamin D among health care providers, researchers, and the general public, including concern about possibly widespread deficiency, calls for supplementation, and even use of large doses of vitamin D as treatments for a variety of conditions.
"Given recent findings, it’s easy to see why people are so enthusiastic about the potential power of vitamin D, but we must recognise the limitations of any study and exercise caution when making broad public health recommendations," said Mary Frances Picciano, Ph.D., a senior nutrition research scientist in the NIH Office of Dietary Supplements, who co-authored an overview of the conference included in the journal supplement. "This is a very complex set of issues and there is still a lot we don’t know about how vitamin D levels affect health, especially across different age groups and ethnic populations."
"It’s tempting to think that an essential nutrient is safe at any level — that if some is good, more is better," said Paul M. Coates, Ph.D., director of the Office of Dietary Supplements. "We’ve learned that this isn’t always true, and there are potential harms associated with high levels of many nutrients."
Participants in the NIH conference identified a number of limitations of the existing evidence on vitamin D, including:
- Many studies have failed to control for factors that could confuse study findings, such as diet, baseline vitamin D status, age, disease, season (as relevant to sun exposure), and physical activity.
- Few studies have examined the effects of vitamin D independent of calcium or other nutrients.
- Reliable data on the vitamin D content of foods is not available.
- Existing laboratory tests used to measure vitamin D levels in blood vary widely.
- Preliminary research findings suggest a role for vitamin D in preventing chronic diseases such as diabetes, immune function, and cancer, but further study is needed.
- Research has not identified the vitamin D levels needed to achieve desired health outcomes in people at various life and reproductive stages and in dark-skinned individuals.
Vitamin D is an essential component in bone health that helps ensure that the body absorbs calcium, which is critical for building strong, healthy bones. People get this nutrient from three sources: sunlight, dietary supplements, and foods. Most people meet their vitamin D needs through exposure to sunlight, but questions remain about what amount of sun exposure would yield beneficial levels of vitamin D without unacceptably elevating skin cancer risk. Very few foods naturally contain vitamin D, so much of the vitamin D in Americans’ diets comes from fortified foods such as milk and cereal. The flesh of certain fish such as salmon, tuna, and mackerel and fish liver oils are among the best naturally-occurring sources. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks.
It is possible to get the currently recommended amounts of vitamin D from diet. Two glasses of vitamin D-fortified milk per day, for example, provides enough vitamin D for a healthy person under age 50. But individuals who are not consuming vitamin D-rich or fortified foods, or getting regular sun exposure may want to consult a health care provider about taking supplements to ensure adequate intake. To learn about vitamin D intake recommendations for different age groups, read the Office of Dietary Supplements’ vitamin D fact sheet at http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp.
Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D deficiency can lead to rickets in children and osteomalacia (softening of the bones) in adults. Together with calcium, vitamin D also helps protect older adults from developing osteoporosis. However, excess vitamin D intake can also cause harmful side effects, including nausea, vomiting, diarrhoea, constipation and development of kidney stones. Healthcare providers may check vitamin D blood levels in individuals at increased risk for deficiency such as breastfed infants, older adults, people with limited sun exposure, people with dark skin, people with fat malabsorption, and people who are obese.
Investigations of vitamin D’s health effects are expanding and areas of promising research include its role in type 1 diabetes, some cancers, autoimmune diseases such as multiple sclerosis, and infectious diseases such as tuberculosis.
In light of recent research, some advocates and researchers have called for a review of the U.S. Dietary Reference Intakes for vitamin D. Current recommendations for daily vitamin D intake were developed in 1997 by the Food and Nutrition Board of the Institute of Medicine. The U.S. Department of Health and Human Services, in collaboration with the U.S. Departments of Agriculture and Defence, and Health Canada are currently in discussions with the Institute of Medicine to revisit the recommendations.
(Source: American Journal of Clinical Nutrition: National Institute of Health USA: August 2008)