Studies show that hospital patients have poorer vitamin status that non-hospitalised controls. From this is it tempting to assume that poor vitamin status can cause ill health. And it can. However, the poor vitamin status may be an effect of the illness, rather than the cause, particularly for selected vitamins such as vitamin C. Research shows for example that illness depletes the body of certain vitamins and so it would be expected that patients who have current illness would have poor vitamin status (here). Further, research published on the vitamin A levels of children following chickenpox supports this viewpoint1. In this study, researchers administered high doses (200,000 IU) of vitamin A to subjects and measured their response to the dose (relative dose response: RDR (here)) at baseline, 30, 60 and 120 days. Of those children who subsequently became infected with chickenpox, 74 % showed a positive response to the RDR in comparison to only 10 % of the controls who did not become infected.
The principle of measuring the retinol, rather than the retinol binding protein (RBP) in the relative dose response test has been questioned2. However, the method is considered an accurate measure of vitamin A status. The principle of the test centres on the fact that the RBP that normally binds plasma retinol accumulates in the liver during vitamin A deficiency. When vitamin A is administered, the retinol binds to the RBP in the liver and is released to circulation. Small increases in vitamin A therefore lead to large increases in plasma levels of retinol in vitamin A deficient patients when a RDR is untaken. In the case of the children with chicken pox in the above study, this increase in vitamin A occurred around 90 days after infection, when clear statistical differences could be seen in comparison to the plasma levels in healthy children. Taken as a whole these results with others suggest that the cause and effect of vitamin status and illness is not straightforward and care should be taken when interpreting data.
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