Key Gaps Remain in Understanding Health
Effects of Vitamin D
Special Journal Issue Summarizes What We Know and What
We Need to Learn
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
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, itís easy to see why people are so
enthusiastic about the potential power of vitamin D, but we must
recognize 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 donít know
about how vitamin D levels affect health, especially across different
age groups and ethnic populations."
"Itís 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. "Weíve
learned that this isnít 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
- 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 Americansí 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
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 Supplementsí 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, diarrhea, 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 Dís 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 Defense, and Health
Canada are currently in discussions with the Institute of Medicine
to revisit the recommendations.
The American Journal of Clinical Nutrition supplement is available
to subscribers at http://www.ajcn.org/,
and contains an overview of the conference, invited papers from
many of the conference speakers, and a summary of the roundtable
discussion held following the conference. The supplement may be
accessed via the ODS website, at http://ods.od.nih.gov/news/AJCN2008.aspx.
The American Journal of Clinical Nutrition is published by the
American Society for Nutrition (http://www.nutrition.org/).
To learn more about vitamin D or other dietary supplements through
fact sheets, databases, and other research resources, please visit
the ODS website (http://dietary-supplements.info.nih.gov/index.aspx).
The Office of Dietary Supplements was established in 1995 as a
result of the Dietary Supplement Health and Education Act. The
mission of ODS is to strengthen knowledge and understanding of
dietary supplements by evaluating scientific information, stimulating
and supporting research, disseminating research results, and educating
the public to foster an enhanced quality of life and health for
the U.S. population.
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and Centers
and is a component of the U.S. Department of Health and Human Services.
It is the primary federal agency for conducting and supporting basic,
clinical and translational medical research, and it investigates
the causes, treatments, and cures for both common and rare diseases.
For more information about NIH and its programs, visit www.nih.gov.