Vitamin D is an important nutrient in the maintenance of bone health. Decreased synthesis of endogenous vitamin D results from aging of the skin and from lower sunlight exposure. Vitamin D is necessary for the regulation of intestinal calcium absorption and the stimulation of bone resorption. Sources of vitamin D include sunlight, diet, and supplements. Deficiency in vitamin D leads to decreased calcium absorption from the intestines, which can result in increased osteoclast production and subsequent mobilization of calcium from the bone.
In vitamin D deficiency state, the active form of vitamin D (1,25(OH)2D) interacts with receptors in osteoblasts, and thereby increases the formation of osteoclasts. The mature osteoclast then releases enzymes that are necessary to breakdown bone matrix. If the serum free calcium level remains low, the parathyroid gland is stimulated to release parathyroid hormone (PTH), which increases renal reabsorption of calcium as well as stimulates the production of osteoclast, which leads to an increase in serum levels of calcium. If vitamin D deficiency is not corrected, calcium will be continuously mobilized from the bone thus leading to osteoporosis.
The National Osteoporosis Foundation (NOF) recommends an intake of 800 to 1000 international units (IU) of vitamin D3 per day for adults aged >50 years. Prevalence of vitamin D deficiency increases with age (see Fig. 12). Higher intakes of vitamin D can reduce bone resorption and subsequent bone loss.
Vitamin D supplementation improves musculoskeletal function and reduces the risk of falling by 49% in elderly wome. 48 Pachalis reported that a 3-year supplementation of vitamin D and calcium significantly alters bone mineral and organic matrix quality in postmenopausal women with osteoporosis. A metaanalysis of randomized control trials supports the use of calcium plus vitamin D supplements as an intervention for fracture risk reduction.
With calcium plus vitamin D supplementation there was a statistically significant 15% reduced risk of total fractures and a 30 % reduced risk of hip fractures. Deficiency of vitamin D levels is widely prevalent among women with osteoporosis. A daily median supplemental dose of 2000 IU vitamin D was effective in repleting 88% of postmenopausal women with osteoporosis within 11 to 48 days to a serum vitamin D level of 50 nmol/L. The mean time to successful repletion was 31 days (see Fig. 13).