Vitamin D and Bone Health
Vitamin D promotes absorption of dietary calcium from the intestine and plays an important role in regulating calcium deposition in our bones. Humans can derive vitamin D from dietary sources or from exposure to UV. Historically, humans have derived most of their vitamin D as a result of exposure to sunlight. Cells in our skin have a precursor for vitamin D synthesis, 7-dehydrocholesterol. Exposure to UV-B radiation (290–315 nm) causes a chemical change in 7-hydroxycholesterol that converts it to vitamin D3. Vitamin D from the skin or from dietary sources is then converted to 25-hydroxyvitamin D in the liver. This, in turn, is then converted into the active form of vitamin D is (1,25-dihydroxyvitamin D) primarily in the kidney.
Vitamin D promotes calcium absorption from the intestine and is essential to bone mineralization and maintenance of calcium and phosphate concentrations in blood. Without sufficient vitamin D and calcium bone mineralization is impaired, resulting in rickets in children and osteoporosis in adults. Dr. Michael Holick, an authority on vitamin D, published a succinct review of "Vitamin D Deficiency" in 2007 in the New England Journal of Medicine.
Dietary Sources of Vitamin D:
Relatively few foods have significant levels of vitamin D in nature. The best sources are fish, such as salmon, tuna, and mackerel and fish liver oils. Smaller amounts are found in beef liver, cheese, and egg yolks. Most of the dietary vitamin D in US comes from foods that have been fortified. For example, milk in the US is fortified with 100 IU per cup, and milk products and calcium-fortified fruit juices may also be fortified.An authoritative summary of the effects of vitamin D, the consequences of vitamin D deficiency and the human requirements for vitamin D can be obtained from the Dietary Supplement Fact Sheet on Vitamin D, published by the Office of Dietary Supplements from the National Institutes of Health.
Vitamin D from Sun Exposure
The amount of vitamin D generated by exposure to sunlight is affected by many factors including the season of the year, latitude, time of day, cloud cover, smog, number of hours spent outdoors, and usage of sunscreen. Latitudes greater then 42 degrees of latitude (e.g., north of California and Boston) do not provide enough UVB radiation to sustain vitamin D levels from November through February. UVB intensity that is sufficient to sustain vitamin D levels throughout the entire year only occurs below 34 degrees latitude (e.g., south of Los Angeles and Columbia, South Carolina). UVB is blocked by cloud cover, air pollution, glass, and sun screen lotions with SPF 8 or greater if applied in sufficient quantity.
Some vitamin D researchers have recommended sunlight or tanning bed exposure for 5-30 minutes several times a week as an alternative to dietary supplementation in northern latitudes during the winter. However, given the well-established causal role that UV radiation plays in skin cancer, this recommendation is controversial to say the least.
Vitamin D Requirements in Humans
In November 2010 the Institute of Medicine conducted an exhaustive review of the scientific findings regarding the effects of calcium and vitamin D on health. The IOM concluded:
|
"Scientific evidence indicates that calcium and vitamin D play key roles in bone health. The current evidence, however, does not support other benefits for vitamin D or calcium intake."
"Vitamin D levels in the body may come from not only vitamin D in the diet but also from synthesis in the skin through sunlight exposure. The amount of sun exposure one receives varies greatly from person to person, and people are advised against sun exposure to reduce the risk of skin cancer. Therefore, the committee assumed minimal sun exposure when establishing the DRIs for vitamin D, and it determined that North Americans need on average 400 International Units (IUs) of vitamin D per day (see table for the Recommended Dietary Allowances— values sufficient to meet the needs of virtually all persons). People age 71 and older may require as much as 800 IUs per day because of potential changes in people's bodies as they age." "At this time, the scientific data available indicate a key role for calcium and vitamin D in skeletal health and provide a sound basis for DRIs. The data do not, however, provide compelling evidence that either nutrient is causally related to extra-skeletal health outcomes or that intakes greater than those established in the DRI process have benefits for health." |
The IOM's specific recommendations for vitamin D intake are summarized in this table:
For a one-minute summary of the IOM committee's findings, see the video below.
.
Other Benefits of Vitamin D
Ecological studies have noted that populations living at higher latitudes have an increased risk of colon, breast, prostate and certain other cancers. Since people living at higher latitudes have less exposure to sunlight and lower production of vitamin D3, it was suggested that there may be an association between vitamin D deficiency and cancer. Prospective studies also found that adults with low blood levels of vitamin D had a 30-50% increased risk of breast, colon, pancreatic and other cancers. Moreover, additional studies indicate that dietary supplementation with vitamin D is associated with a decreased risk of cancer. Less convincing studies suggest that vitamin D may decrease the risk of depression, eclampsia in pregnancy, and type I diabetes. Michael Holick, PhD, MD at Boston University Medical Center, an endocrinologist who has published many studies on vitamin D. Two of his review articles summarize evidence for the potential benefits of vitamin D. See:
- Holick MF: Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. American Journal of Clinical Nutrition, Vol. 80, No. 6, 1678S-1688S, December 2004
- Holick MF and Chen TC: Assessment of Vitamin D in Population-Based Studies: Vitamin D deficiency: a worldwide problem with health consequences. American Journal of Clinical Nutrition, Vol. 87, No. 4, 1080S-1086S, April 2008
|
Click below the question to view the answer. |
|
This content requires JavaScript enabled.
|
|
Summary
There is strong evidence for the benefits of vitamin D to ensure bone health, but as noted in the IOM report, there is insufficient evidence to support these other claims at the present time.
Medical Uses of Ultraviolet Radiation (UV)
UV may be used in medical applications, but general exposure to sunlight or tanning devices should not be used as a replacement for actual medical care. Examples where phototherapy can be useful in treating several conditions, include
Is There a Controversy?
The following is an excerpt from Dr. Barbara Gilchrest's article entitled "Sun exposure and vitamin D sufficiency".Am J Clin Nutr 2008;88(suppl):570S–7S.
The Indoor Tanning Industry
"In the United States alone, the indoor tanning business earns $5 billion per year (51, 52) and has >50 000 tanning facilities, 28 million customers annually (53), and >1 million visits per day. In some regions of the United States, more than half of all teenage girls have visited a tanning facility at least 3 times in the previous year. Because professional groups such as the American Academy of Dermatology have requested stricter guidelines and better enforcement of existing regulations governing indoor tanning, over the past decade, at least 29 states have enacted legislation restricting access to tanning parlors for teenagers,and at least 3 additional states are considering similar legislation .... The indoor tanning industry vigorously opposes such legislation through paid lobbyists and a well-orchestrated media campaign. The cornerstone of the industry's argument to curtail proposed restrictions on teenage use and general overuse is that more UV exposure is healthy, indoor tanning is safer than natural tanning, and UV radiation exposure reduces the risk of multiple diseases. The message that the tanning industry has shared with state legislatures and the media and that the UV Foundation's web site publicizes is: 'Vitamin D from UV exposure is free and easy to get—why pass up the simplest way to improve your odds of preventing cancer?' "
