Vitamin D is critical to bone health. A deficiency is associated with rickets in children and osteomalacia in adults. Rickets was once widespread in industrialized cities of North American and Europe at the turn of the 20th century, due mainly to decreased sunlight exposure as a result of industrial smog and the northern latitude. Food fortification and cod liver oil were found to be effective measures against rickets. Osteomalacia is somewhat the adult equivalent of rickets, however, since it occurs after bone elongation has stopped, it is more subtle. The defective bone mineralization of this condition causes bone pain, muscle weakness and more fragile bones.
Calcium Absorption
It is believed that the main function of vitamin D with regard to bone health is via its role in the maintenance of optimal levels of calcium and phosphorus in the blood. As vitamin D levels in the body fall and are in the range of insufficiency, calcium absorption from the intestines is decreased by 30 to 40 percent.
Vitamin D and PTH
A drop in blood calcium concentrations causes the body to respond by secreting parathyroid hormone, PTH, from the parathyroid gland. PTH acts in the body to raise calcium levels in the blood; it does this not only by increasing absorption of calcium from the kidneys but also by causing cells in the bone called osteoclasts to break down the calcium in the bone. This causes bones to soften and weaken.
Vitamin D Deficiency and Osteoporosis
Vitamin D deficiency has also been shown to cause osteoporosis and increased fracture risk in adults. A study by Steven Boonen and colleagues at Leuven University Center for Metabolic Bone Diseases in Belgium, has estimated that 33 percent of women aged 60 to 70 years and 66 percent of those 80 years or older have osteoporosis. The researchers calculated that about 47 percent of women and 22 percent of men aged 50 or older will suffer an osteoporotic fracture in their remaining lifetime.
Vitamin D Supplementation
Supplementation with vitamin D and calcium has been shown to inhibit excess parathyroid hormone secretion and thus improve bone mineral density. The same supplementation regimen has been shown to improve body sway and limb strength, thus reducing the risk of falls. Supplementation with calcium and vitamin D together has been shown to reduce parathyroid hormone levels more than when either is administered alone, particularly in patients with vitamin D deficiencies or a low calcium intake.
Falls and Fractures
Four large-scale, randomized, placebo-controlled multicenter studies have shown that supplementation with vitamin D and calcium reduces fracture rates. One of these, conducted by Marie C. Chapuy, Ph.D., and colleagues at the National Institute of Health and Medical Research in Lyon, France, and reported in the "New England Journal of Medicine," involved 3,270 elderly institutionalized women, all of whom had inadequate calcium intakes and 44 percent had vitamin D insufficiency. After 18 months supplementation with 1,200 mg of calcium per day together with 800 IU vitamin D daily, hip fracture incidence was reduced by 43 percent while nonvertebral fractures were reduced by 32 percent.
References
- "Journal of Internal Medicine"; Calcium and vitamin D in the prevention and treatment of osteoporosis - a clinical update; S. Boonen, D. Vanderschueren, P. Haentjens, and P. Lips; June 2006.
- "New England Journal of Medicine"; Vitamin D3 and calcium to prevent hip fractures in the elderly women; M.C. Chapuy, M.E. Arlot, F. Duboeuf, J. Brun, B. Crouzet, and S. Arnaud; December 1992.
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