Vitamins D is not a vitamin but a steroid hormone. In this regard it has broad regulatory roles that include the maintenance of proper calcium homeostasis. Increasingly vitamin D deficiencies are being identified in populations living at high latitudes who do not have access to year round sunlight exposure. In addition, the modern living environment often precludes access to the sun even when it is at its strongest in the summer, and the use of sunblock during periods of sunlight exposure further limits vitamin D levels. The elderly are at a particularly high risk of developing vitamin D deficiencies because they spend long periods in doors. One problem associated with such chronic low levels of vitamin D may be a reduction in calcium absorption, a process that requires vitamin D. Vitamin D is converted to 25-hydroxyvitamin D in the liver, and as levels fall, calcium absorption is also reduced. Administration of vitamin D supplements raises levels of 25-hydroxyvitamin D and this is turn increases calcium absorption.
Evidence suggests that long term, low vitamin D status negatively affects calcium metabolism and that this may have knock on effects on bone metabolism. In particular, chronic low vitamin D plasma levels may cause skeletal dysfunction as balance starts to favour bone resorption. For example, in a study published in the American Journal of Clinical Nutrition in 19851, researchers administered 15000 IU vitamin D in the form of ergocalciferol or a placebo to 109 women aged between 65 and 74 years. After two years of treatment the plasma 25-hydroxyvitamin D concentrations of the subjects receiving the vitamin D had increased significantly compared to those receiving the placebo. In addition, the vitamin D group showed a significantly reduced rate of cortical bone loss compared to the placebo group. The subjects in this study originated in the North of England, and all had low plasma levels of 25-hydroxyvitamin D. However, these levels were normalised by 2 years of treatment with vitamin D.
In this study, the participants had ergocalciferol (vitamin D2) which is the prefered medical form of vitamin D derived from plants. However, cholecalciferol (vitamin D3) is known to be a superior supplements for raising plasma levels of 25-hydroxyvitamin D. Therefore had the subjects been administered cholecalciferol, it might have been expected that the plasma levels of 25-hydroxyvitamin D would have been raised even higher. Current recommendations are to consume around 2000 IU of vitamin D during times of low sunlight exposure. The subjects in this study received 2143 IU per day, which is therefore in line with current guidelines. Back when this study was published (1985) these recommendations were not in place and as a result the 15000 IU administered per week was considered a high dose. It is now known that higher doses are safe for long term consumption in those with initially low levels of vitamin D, that but further improvements in health are not see once plasma levels reach the normal range.
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