The most effective internal concentration of vitamin D hormone is 100 to 125 nmol/l. To achieve this serum concentration a daily intake of 4,000 IU/d of supplements or the equivlent in UV exposure is recommended. A combination of both methods is also as effective.
The duration of UV exposure required is highly variable on account of season, ski pigmentation, time of day, altitude, etc. This paper
provides a nice summary of the vit D and MS concept, including doseages.
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