Although vitamin D is typically classified as a fat-soluble vitamin, it actually functions as a hormone in the body. Because it can be manufactured by the body (formed in the skin following exposure to the ultraviolet rays of the sun), vitamin D is not technically classified as an essential nutrient at all. In the skin, exposure to ultraviolet rays converts vitamin D precursors (compounds with structures similar to cholesterol) into an inactive form of vitamin D. This inactive form is then converted to the active form by enzymes located in the liver and kidneys. Regular sunlight exposure is the primary way that most of us get our vitamin D. Food sources of vitamin D include only a few, such as milk fortified with vitamin D (100 ILI/cup), cod liver oil, and fatty fish such as salmon. Small amounts of vitamin D are also found in egg yolks and liver. Because of the role of vitamin D in stimulating calcium absorption, dietary supplements for bone health are the main supplement category.

The primary effect of vitamin D is to maintain calcium levels in the blood. To do this, vitamin D promotes both the absorption of calcium from the intestines into the blood and the removal of calcium from the bones into the blood. Vitamin D also reduces calcium loss in the urine. In most cases, the increased calcium absorption results in an increase in bone density and bone strength, which can help reduce the risk of osteoporosis.

For most people, there is no reason to take more than the recommended AI level (200-600 IU) of vitamin D. During the winter months, however, synthesis of vitamin D in the skin is severely reduced because of reduced exposure to sunlight. In some parts of the country {northern latitudes such as Boston and Seattle), virtually no vitamin synthesis occurs in skin during the winter months (November through February). Therefore, vitamin D supplementation should be considered for people living in northern cities as well as for those who are not exposed to sunlight on a regular basis (Margiloff et al., 2001; Patel et al., 2001). In addition, older people should consider vitamin D supplements because with aging, skin loses its ability to adequately synthesize vitamin D and kidneys lose some of their ability to convert vitamin D precursors to their most active form. People who use sun block frequently might also consider a vitamin D supplement because a sun block with a sun protection factor (SPF) of 8 or greater can reduce the skin’s ability to produce vitamin D.

It is well accepted that adequate vitamin D levels are crucial for healthy bone development, maintenance of bone density and bone strength, and prevention of osteoporosis. Vitamin D exists in several forms within die body, with both the liver and kidney helping to convert vitamin D into its most active hormone form (1,25OH-D3). Vitamin D deficiency results in rickets (in children) and osteomalacia (in adults), both of which are characterized by a reduced level of calcium being deposited in bones and a weakening of bone strength.

Several studies have clearly demonstrated that supplemental vitamin D intake (200-1,000 day), usually combined with calcium, increases bone density and helps prevent osteoporosis (Feskanich et al., 2003). In one study, 240 healthy postmenopausal women consumed calcium (900 mg/day and vitamin D (200 day) for 2 years. Results showed a reduced loss of calcium in the urine and a highly significant increase of almost 2% in lumbar spine bone mineral density (Recker et al., 1999). Another study, also in postmenopausal women, gave supplements containing 1,000 mg of calcium (as calcium carbonate) and 500 IU of vitamin D and showed a positive effect on bone density (Baeksgaard et al., 1998), even though initial calcium and vitamin D status was adequate. It is interesting to note that although supplementation with calcium and vitamin D reduces bone loss and prevents fractures in elderly people, after discontinuing the supplements, bone turnover (loss) rates to their original high levels, and the supplement-induced increases n bone mineral density are lost within 2 years (Dawson-Hughes et i., 2000), suggesting that supplementation needs to be continued for prolonged benefits.

As widi calcium supplementation and with milk consumption, some controversy exists concerning the value of vitamin D supplementation n preventing osteoporotic fractures. In shorter supplementation trials, ngher calcium intake, higher milk consumption, and higher vitamin D were associated with reduced bone loss in postmenopausal women Dawson-Hughes, 1998). Longer observation trials, however, do not find a lower risk of fractures associated with calcium intake or milk consumption, but a mediating factor may be vitamin D intake, “eskanich et al. (2003) assessed the relationship between postmenopausal hip fracture risk and intake of milk, calcium, and vitamin D, finding significant relationship only between a higher vitamin D intake and over risk of hip fractures. Supporting this finding are data from Deroisy al. (2002) showing that a dietary supplement combining calcium 500 mg) plus -vitamin D (200 IU) for 90 days was more effective than calcium alone in reducing hyperparathyroidism in a population of postmenopausal women. Other researchers (Reginster et ai., 2002} have shown related effects on serum PTH with similar combinations of calcium (500-1,000 mg/day) and vitamin D (400-880 Ill/day).

In the United States, milk is fortified with 10mg of vitamin D per quart, meaning that each 1-cup serving of milk provides about one fourth of the estimated daily need for vitamin D. Because there is not sufficient evidence to establish an RDA for vitamin D, an AI was established in 1998 to ensure maintenance of healthy blood levels of active vitamin D. Men and women have the same daily Al level of vitamin D within the following age ranges: 19-50 years, 5 ug or 200 ILI; 51-69 years, 10 ug or 400 HI; and 70+ years, 15 ug or 600 1U.

Because vitamin D is a fat-soluble vitamin, it is stored in the body and has the potential to reach toxic levels if taken in high doses for prolonged periods. Getting too much vitamin D from food sources is unlikely unless high consumption of cod liver oil is routine. Prolonged sunlight exposure does not cause buildup of vitamin D as the body down regulates its production when levels are adequate. The Pood and Nutrition Board considers an intake of 50mg (200) to be the UL for vitamin D in adults. Intakes over that amount can cause nausea, diarrhea, skin rash, headaches, muscle weakness, calcium deposits, and kidney stones.

The DV for vitamin D is 400, and supplements at this level have been shown to be safe and effective in reducing calcium loss and maintaining bone density in postmenopausal women. Dietary supplements are not necessary in healthy, young individuals who are frequently exposed to moderate amounts of sunlight (15 minutes or so per day) in dietary supplements, vitamin D and calcium do not have to be taken together to be effective, but many calcium/vitamin D combinations are available and may be more convenient than taking separate tablets.

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