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|As most vitamin
D is derived through production in the skin after exposure to sunlight, it is logical to
look at seasonal and geographic variations in the occurrence of the disease.
There is some evidence for higher incidence rates for Norther regions vs. more Southern areas, but this North-South gradient is broken by exceptions such as Sardinia. Differences in genetic susceptibility are likely to explain some of the geographical variation, and may distort any clear North-South gradients that could be due to sunlight availability.
Seasonal trends in the time of birth or time of onset of the disease can be used to indicate a link between vitamin D exposure and a disease. Studies investigating these associations have failed to show any generally persistent trends. I believe this may be due to the multifactorial nature of type 1 diabetes, seasonal differences in the supplementation recommendations and the often long latency period from the actual initiation of the process until manifestation of identifiable symptoms.