Research indicates that vitamin D requirements in human has been vastly underestimated until recently. Vitamin D insufficiency is a term used to describe individuals who do not develop the typical vitamin D deficiency diseases such as osteomalacia or rickets, but have plasma levels of vitamin D below the level necessary for optimal metabolic function. Researchers are interested in identifying surrogate markers for vitamin D status, such as exposure to sunlight or oral intake of vitamin D supplements, in order to be able to predict the plasma levels of vitamin D in individuals or populations. This could be beneficial because it would allow estimation of vitamin D status without the need for the drawing of a blood sample. A study published in the American Journal of Clinical Nutrition in 20101, attempted to create a predictive model of vitamin D status using surrogate markers of vitamin D.
The study used data from the Women’s Initiative Calcium plus Vitamin D Clinical Trial, and measure the 25-hydroxyvitamin D [25(OH)D] concentration as a marker for vitamin D status. More than half of the 3055 postmenopausal women sampled in the study were deficient in vitamin D, with 57.1% showing plasma levels of 25(OH)D of less than 50 nmol/L. Only 21% of the variation in vitamin D concentrations could be explained by the predictive model based on known surrogate marker. Distributions of vitamin D serum levels based on latitude, sun exposure and intake of vitamin D showed no predictive pattern and did not allow correlations to be made between the predicted data and the real world data. Adjustments were made for possible confounding variables, but even then, only total vitamin intake from foods and supplement, waist circumference, physical activity, ethnicity, age and sun exposure were weakly correlated with serum 25(OH)D levels.
These data suggest that surrogate markers for vitamin D do not correlate well with serum levels of 25(OH)D. However, the associations were in the directions expected, but were weak and not statistically significant. This suggests that vitamin D status is more complex that previously thought. The best way of determining the vitamin D status of individuals and populations therefore remains a blood test using 25(OH)D as a marker for vitamin D. Interestingly, this study did support previous findings that have shown large proportions of the population to be deficient in vitamin D (57.1% in this case). More concerning was that 13% of the subjects were severely deficient in the vitamin, having blood levels below 25nmol/L. Another possible point worth highlighting from this study was that the subjects were all post-menopausal women. If the study was repeated on a more varied cross section of the general population stronger associations may be observed.
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