What Happened When Inadequate vitamin D intake ?
February 26, 2009 by rainier
Inadequate vitamin D intake can also lead to bone loss and increased risk of osteoporosis. It has long been known in the Northern Hemisphere, for example, that serum levels of 25-hydroxyvitamin D—the best clinical index of vitamin D status—are higher in summer and autumn than in winter and spring. In addition to the general finding of seasonal variation in 25-hydroxyvitamin D, levels of this metabolite decline with age. Reduced levels of 25-hydroxyvitamin D in older persons result from declining intake, decreased sun exposure and, perhaps most importantly, less efficient skin synthesis of vitamin D.
Dawson-Hughes et al. (54) reported that 25-hydroxyvitamin D levels were lower in winter only among healthy postmenopausal women with daily vitamin D intakes under 220 IU. Those women with low vitamin D intakes had intact parathyroid hormone levels that were higher, although still within the normal range. Treatment of this population with a 400 IU vitamin D supplement prevented significant seasonal variation in either 25-hydroxyvitamin D or parathyroid hormone and, more importantly, reduced wintertime bone loss from the spine. During this vitamin D trial, both the placebo and supplemented groups had similar significant gains in bone density of the spine and whole body in the summer/autumn. Overall, there was a significant net benefit at the spine from vitamin D supplementation. Those subjects exhibiting seasonal bone changes (increases in summer/autumn, decreases in winter/spring) also experience seasonal changes of similar magnitude in lean tissue mass. As indicated earlier, moderate changes in body composition can have a significant impact on level of function in older persons. Much is still to be learned about why bone, and lean and fat tissues, fluctuate with the seasons.
Potential contributors to these circannual changes include seasonal differences in exercise, nutrition and blood levels of hormones that are known to affect the metabolism of these three tissue compartments.
