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Vitamin D is a fat-soluble steroid compound that the body needs to remain healthy. In some ways, vitamin D is not a true vitamin because the skin can make vitamin D when exposed to sunlight. However, if the body does not make enough vitamin D, additional amounts must be acquired through diet.
The main role of vitamin D is to regulate amount of calcium circulating in the blood. Calcium is a mineral acquired through diet that is involved in building bones, muscle contraction, and nerve impulse transtransmission.
(Illustration by GGS Information Services/Thomson Gale.)
mission. Vitamin D helps regulate the absorption of calcium from the small intestine. Too little vitamin D can cause weak, brittle, deformed bones. There is also evidence that vitamin D plays a role in controlling cell differentiation and may help to protect the body from developing some types of cancer.
Vitamin D exists in several forms, two of which are important to humans. Vitamin D2, called ergocalciferol, is made by plants. Vitamin D2 can be manufactured synthetically by irradiating yeast. This type of vitamin D is most often found in dietary supplements and foods fortified with vitamin D. Vitamin D3, called cholecalci-ferol, is made naturally by the skin when it is exposed to ultraviolet rays in sunlight. Neither vitamin D2 nor D3 is active in the body. Both must be converted, first in the liver and then in the kidney, into an active form of vitamin D (1alpha, 25-dihydroxyvitamin D). Vitamin D in this topic means the active form of vitamin D.
Vitamin D's role in health
Although Vitamin D has been known to play a role in bone health for many years, only recently have researchers begun to explore its effects on cell differentiation and the immune system.
BONE HEALTH. The role of vitamin D and calcium are closely connected. The body needs calcium to build bones and teeth, contract muscles, transmit nerve
impulses, and help blood to clot. Vitamin D helps the body get the calcium it needs by increasing the amount of calcium absorbed in the small intestine. Vitamin D is an active part of the feedback loop that maintains a normal level of calcium in the blood.
To maintain health, the amount of calcium in the blood must stay within a very narrow range. When the amount of calcium in the blood falls below normal, the drop is sensed by the parathyroid glands. The parathyroid glands are four separate clusters of specialized cells in the neck. Low blood calcium levels stimulate the parathyroid glands to secrete parathyroid hormone (PTH). PTH travels through the bloodstream and stimulates the kidney to increase the conversion of vitamin D2 and D3 into its active form. Active vitamin D is released into blood and stimulates the cells lining the small intestine to increase the amount of calcium that they absorbed from digesting food. Vitamin D also causes the kidney to conserve calcium so that less is lost in urine. If these actions do not return the level of calcium in the blood to normal, vitamin D activates cells called osteoclasts that break down bone and return calcium from the bone to the bloodstream. People who do not have enough vitamin D absorb less calcium from the food they eat. To make up for this, calcium is taken from their bones and the bones weaken and break more easily.
CANCER PREVENTION AND TREATMENT. Vitamin D also helps regulate cell differentiation. During development, cells divide over and over again. At some point, they are triggered to specialize (differentiate) into different types of cells, for example, skin, muscle, blood, or nerve cells. Vitamin D joins with other compounds to turn on and off more than 50 different genes that stop cell growth and start cell differentiation.
One characteristic of cancer cells is that they grow wildly, dividing many times more than normal cells without differentiating. Since vitamin D can stimulate cells to stop dividing and begin differentiating, researchers are investigating whether vitamin D can protect people from getting certain cancers, especially colon, prostate, skin, and breast cancer. The research has produced mixed results. Some studies found that vitamin D protected against colon cancer, while other found it offered no protection. The official position of the American Cancer Society described in their 2006 Nutrition and Physical Activity Guidelines states,“There is a growing body of evidence from population studies (not yet tested in clinical trials) that vitamin D may have helpful effects on some types of cancer, including cancers of the colon, prostate, and breast.” However, the American Cancer Society makes no recommendations on the amount of vitamin D needed to have a beneficial effect. Clinical trials are underway to determine safety and effectiveness of vitamin D in a variety of situations. Individuals interested in participating in a clinical trial at no charge can find a list of open trials at <http://www.clinicaltrials.gov>
Normal vitamin D requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97-98 % of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
The IOM has not set RDA values for vitamin D because of incomplete scientific information and variability in the amount of vitamin D the body makes when the skin is exposed to sunshine. Instead, it has set AI and UL levels. Recently the UL level has become somewhat controversial and has been challenged by some researchers as being set too low. AI and UL levels are measured in both weight (micrograms or mcg) and international units (IU). The IU measurement is the measurement used on dietary supplement labels. For vitamin D, 1.0 mcg equals 40 IU.
The following are the AIs and ULs for vitamin D for healthy individuals:
Exposing the face, arms, and legs to sunshine for 15 minutes three or four times a week meets the dietary requirements for vitamin D for people with fair skin much of the time. However, people who live north of 40° latitude (approximately a line that extends from Philadelphia to San Francisco) may not get enough sun exposure to meet their dietary needs during winter months. Dark-skinned people may need to spend triple the amount of time in the sun as fair-skinned people to synthesize adequate amounts of vitamin D, since the increased amount melanin pigment in dark skin slows vitamin D production. Using sunscreen with an SPF of 8 or higher also slows the production of vitamin D in the skin.
Vitamin D is not found in large amounts in many foods. However, since the 1930s vitamin D has been added to about 99%: of all milk, and to some breakfast cereals, bread, orange juice, and infant formula. In addition, the Food and Drug Administration requires all foods containing olestra, a compound that reduces fat absorption, to be fortified with the fat-soluble vitamins A, D, E, and K.
The following list gives the approximate vitamin D content for some common foods:
Vitamin D deficiency
Vitamin D deficiency results in rickets in children and osteomalacia in adults. Rickets is a condition in which the bones do not harder because of a lack of calcium deposited in them. Instead they remain soft and become deformed. Osteomalacia is a weakening of bones in adults that occurs when they are broken down (demineralized) and calcium in the bones is returned to the blood. Vitamin D deficiency also can cause joint and muscle pain, and muscle spasm. Less severe cases can result in osteoporosis in older adults.
The vitamin D fortification program, along with the popularity of daily multivitamins, has greatly reduced the number of people in the United States who are vitamin D deficient. However some groups remain at risk of vitamin D deficiency. These include:
Vitamin D excess
Vitamin D excess in healthy individuals occurs only when large quantities of vitamin D are taken as a dietary supplement over several months. This can result in high calcium levels in the blood (hypercalcemia). Symptoms of vitamin D excess include nausea, vomiting, excessive thirst, weakness, and high blood pressure. Calcium deposits may develop in the kidneys, blood vessels, heart, and lungs. The kidneys may be permanently damaged and eventually fail completely.
Research suggests that the following types of medications may increase the available amount of vitamin D in the body. People taking these drugs should not take a vitamin D supplement without consulting their healthcare provider.
Research suggests that the following types of medications may decrease the available amount of vitamin D in the body. People taking these drugs should discuss with their healthcare provider whether a vitamin D supplement is right for them.
No complications are expected when vitamin D is used in the recommended amounts. The complications resulting from insufficient or excess use are discussed above.
Parents should be aware that the RDA and UL for vitamins and minerals are much lower for children than for adults. Accidental overdose may occur if children are give adult vitamins or dietary supplements.
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Tish Davidson, A.M.