Bonjour Christine
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
paperprovides a nice summary of the vit D and MS concept, including doseages.
Bon chance et au revoir
Nick