Nearly all tissues in humans contains the vitamin D receptor and are able to synthesise 1, 25-(OH)2D from 25(OH)D because they contain the correct hydroxylase enzyme. This suggests that vitamin D has a role in human biology beyond its traditional role in calcium metabolism. Evidence now suggests that vitamin D is protective of cardiovascular disease, cancer, multiple sclerosis and diabetes. Recommendations for vitamin D intakes are controversial because many are based on the incorrect use of scientific data (here) and concerns continue to arise regarding vitamin D intakes at anything above the highest dose available on prescription (25 µg/d). However, suggests that the previous recommendations for vitamin D need revising upwards and that the intake necessary to cause hypercalcaemia is higher than was previously thought. Studies investigating the toxicity of vitamin D suggest that doses higher than are currently recommended are quite safe and actually beneficial.
For example, researchers1 investigated the efficacy and safety of prolonged vitamin D3 intakes at 25 µg/d (1000 iu/d) and 100 µg/d (4000 iu/d) in healthy subjects. The study lasted for 5 months and was begun between January and February so as to minimise the effects of the sun. Serum 25(OH)D at baseline was 40.7 nmol/L and peaked at 68.7 nmol/L and 96.4 nmol/L at 3 months for those subjects taking the 25 µg/d and 100 µg/d dose, respectively. The researchers also measured the serum 25(OH)D levels of subjects not taking vitamin D supplements in the summer months and the mean value was 46.7 nmol/L. The highest and lowest serum concentration of vitamin D in the 25 µg/d and 100 µg/d group were 40 and 100 nmol/L and 69 and 125 nmol/L respectively. Toxicity was assessed by measuring calcium excretion, but it was unchanged during the study.
These results confirm that vitamin D is safe at a level of 100 µg/d (4000 iu/d) for at least 5 months. This would be long enough to allow supplementation throughout the winter months in temperate climates where winter sun exposure can cause vitamin D levels to drop. Increases in urinary calcium levels are the first sign of vitamin D toxicity, but this was not detected in this study. Serum levels of 25(OH)D above 40 nmol/L are thought to be desirable to maintain optimum vitamin D status, and it would appear from this study that sun exposure in the summer months without supplements are able to achieve this. In the winter months 25 µg/d of vitamin D3 was necessary to ensure that all subjects attained serum levels above 40 nmol/L. This intake would appears to be quite safe and maintains serum levels attained during the summer months.
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