Much of the nutrition research on osteoporosis has focused on calcium, with less emphasis on the role of vitamin D. Unlike calcium intakes, which tend to be significantly below recommended levels, the majority of Americans probably consume adequate amounts of vitamin D–even those who obtain all of their vitamin D from their diet. However, certain populations are at greater risk for deficiencies in this vitamin which can result in serious consequences in the skeleton. This issue of NIH ORBD~NRC NEWS will focus on vitamin D and its role in bone health.
Understanding Vitamin D
Vitamin D is a precursor to calcitriol, the active form of vitamin D. Calcitriol is a hormone that stimulates the intestinal absorption of calcium. Without adequate vitamin D, dietary absorption of calcium is impaired and skeletal stores of calcium are activated, potentially weakening existing bone and preventing the formation of strong, new bone.
Vitamin D is obtained in two ways: through the skin and from the diet. The sun’s ultraviolet rays trigger vitamin D synthesis in the skin. Experts suggest that 15 to 30 minutes of daily direct sunlight exposure is generally believed to be sufficient for most individuals to manufacture and store the vitamin D they need. In the United States, fortified foods such as milk, margarine and cereals, are the primary sources of vitamin D. Natural food sources include egg yolks, liver and fish. Vitamin D is fat soluble and is stored in different forms in the body. The vitamin D produced in the skin is known as vitamin D3, while vitamin D2 and D3 are found in foods. Vitamin D2 and D3 are equally effective precursors of calcitriol and are stored in the body until it needs the hormone. The liver and the kidneys help convert vitamin D2 and D3 to calcitriol.
While there is not an RDA (Recommended Dietary Allowance) for vitamin D, the Food and Nutrition Board of the National Academy of Sciences established in 1997 a daily Adequate Intake (AI) level of 400 International Units (IU) in men and women ages 51-69 and 600 IU in those over the age of 70. This level was believed to be sufficient to maintain healthy blood levels of cacitriol in the body. However, recently many researchers in the field are considering that higher intakes may be necessary for optimal health.
Triggers for Vitamin D Deficiency
Certain factors can increase the likelihood of vitamin D deficiency. With age, the skin becomes less efficient at converting vitamin D to calcitriol, and the gastrointestinal tract is less able to absorb the vitamin. For these and other reasons, the elderly often have depleted stores of vitamin D. Certain medications, such as anticonvulsants, can increase the breakdown and excretion of vitamin D. Individuals with limited exposure to the sun and those living in northern latitudes are also more likely to have low vitamin D levels. Disorders associated with fat malabsorption, such as Crohn’s disease, are problematic since vitamin D is fat soluble. Also, kidney and liver diseases can impair the ability of these organs to convert vitamin D to calcitriol.
Since ultraviolet rays from sunlight stimulate vitamin D synthesis in the skin, factors that limit ultraviolet ray exposure can result in low vitamin D levels. According to the NIH Office of Dietary Supplements (NIH ODS), sun screens with a sun protection factor of 8 or greater will block those rays that produce vitamin D. The NIH ODS does recommend that sunscreen be used when sun exposure is longer than 10 to 15 minutes. Other factors that affect ultraviolet ray exposure include season, time of day, cloud cover and smog.
Studies of elderly populations throughout the world have found marginal to low vitamin D stores, especially in those elderly who are housebound, who live in northern latitudes, and who do not consume dietary sources of vitamin D or supplements. Studies have found vitamin D deficiencies in up to half of all elderly women who experience a hip fracture.
Skeletal Effects of Vitamin D
Vitamin D plays an important role in the normal mineralization of bone. Prior to milk fortification in the 1930s, rickets and osteomalacia were prevalent public health problems in the United States. Due to low levels of vitamin D, both rickets, which occurs in children, and osteomalacia, which occurs in adults, cause softening of the bones and skeletal deformities. However, vitamin D fortification has reduced the occurrence of these skeletal diseases significantly.
Still, decreased vitamin D levels negatively impact bone density. Low levels impair calcium absorption and trigger increases in parathyroid hormone, resulting in an increase in bone resorption and bone loss. Low levels have also been linked to an increase in hip fractures in elderly women, whereas daily supplementation has exhibited a bone sparing effect, particularly in those who are vitamin D and/or calcium deficient.
As far as fracture prevention is concerned, studies suggest that vitamin D supplementation can decrease the incidence of vertebral fractures, while the effects of supplementation on non-vertebral fractures is less clear. A stronger fracture reduction benefit has been found when both vitamin D and calcium are supplemented together.
Whether calcitriol itself might be beneficial in the prevention or treatment of osteoporosis is the subject of much debate. Due to its potency and the fact that it can reach toxic levels in the body fairly quickly, calcitriol is available only by prescription. Experts recommend that the use of the hormone be carefully monitored by experienced physicians.
British researchers recently reported on the skeletal effects of high doses of vitamin D in elderly men and women. 100,000 IU of oral vitamin D3 (cholecalciferol) administered every four months for five years reduced the incidence of fractures at major osteoporosis sites by about one third. In this study of more than 2600 men and women, a high percentage of whom were physicians, no significant side-effects were reported. Further research is needed before high-dose vitamin D can be considered as a therapeutic approach for fracture prevention.
Vitamin D, Muscle Function and Falls
Vitamin D deficiency has been associated with decreased muscle strength and increased body sway, factors that can increase the likelihood of falls. (Up to 95% of hip fractures can be attributed to falls.) In fact, some evidence suggests that serum vitamin D levels are independently and inversely associated with falls in elderly women. Short-term supplementation with vitamin D and calcium has been shown to reduce body sway, secondary hyperparathyroidism and fall risk in elderly women who are vitamin D deficient. Short-term supplementation with vitamin D and calcium has been shown to reduce body sway, secondary hyperparathyroidism and fall risk in elderly women who are vitamin D deficient