Vitamin D has traditionally been associated solely with the classic deficiency diseases of osteomalacia and rickets in adults and children, respectively. However, this viewpoint has been challenged with recent researcher suggesting that vitamin D plays a much wider role in metabolism than was previously thought. Recent research shows that plasma levels of the vitamin D metabolite 25-hydroxyvitamin D [25(OH)D], are inversely associated with a number of conditions including insulin resistance, diabetes, metabolic syndrome, cancer and multiple sclerosis. Because plasma levels of 25(OH)D are dependent on ultraviolet light or adequate vitamin D intake, those who live in high latitudes and eat vitamin D deficient diets are at risk of vitamin D deficiency. Research suggests that the vitamin D intakes of many individuals in high latitude Western countries is low, and as a result the number of individuals with sub-optimal levels of 25(OH)D is high.
Because of the association of 25(OH)D with serious disease, researchers have used a meta-analysis design study to investigate the associations between the risk of mortality and the plasma levels of 25(OH)D. Studies that were included in the analysis were taken from a PubMed search and contained data for the relative risk for mortality with different 25(OH)D concentrations. These included 14 cohort studies with a total of 62,548 individuals and 5562 deaths. For non-parametric highest compared with lowest categories of 25(OH)D, the relative risk of mortality was 0.71. In the parametric model of greater than two categories of plasma 25(OH)D, the relative risk of mortality was 0.86, 0.77 and 0.69 for plasma increases of 12.5, 25 and 50nmol/L, respectively (assuming a media plasma concentration of 27.5nmol/L). Increasing the plasma levels by 87.5nmol/L above the media inferred no addition reduction in risk of mortality.
The authors concluded, based on these results, that the optimal concentration of 25(OH)D was between 75 and 87.5nmol/L with regard total mortality risk. Increasing 25(OH)D concentrations from 50 to 77.5nmol/L corresponds to a 31% reduction in total mortality. These results support other studies that suggest that circulating levels of 25(OH)D show an inverse association with total mortality. Previous research has shown based on these figures, that large numbers of individuals have sub-optimal levels of vitamin D. For example, supplementation of elderly people with 10-20µg of vitamin D was associated with a 8% mortality reduction during the 5.7 year follow up period. The exact optimal concentration of 25(OH)D is not known and will vary for each individual. However, these results suggest that benefits of increasing plasma levels above 100nmol/L do not show additional benefits. This suggests a non-linear decrease in mortality risk, as would be expected.
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