An introduction to Cardiovascular diseases Exposure to sunlight

   Sunlight is the primary source of vitamin D for most people.

There are latitudinal and seasonal variations in solar ultraviolet B (UVB) doses, highest in summer and lowest in winter, with no vitamin D production possible at sea level for part of the year for latitudes above about 25º - 30º1.  

While there are no solid indications that latitudinal variations of CVD incidence or mortality rates exist, there is ample evidence that seasonal variations do.  

Seasonal amplitudes with higher rates of CVD mortality rates in winter and lowest rates in summer have been observed in tropical, temperate and polar regions2 3 4 5 6 with greatest seasonality near 35º, the latitude region with the greatest annual variation in solar UVB doses3.  

While temperature was often given as the explanation for the seasonality, that explanation is undermined by finding seasonal amplitudes in countries with mild winters such as Australia2 7 5 and Kuwait.  

Also, the seasonality of influenza incidence was hypothesized by John Cannell et al8 to be due largely to seasonal variations in vitamin D production, which has been supported in randomized controlled trials such as one in Japan9.

Page last edited: 17 May 2011


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  9. Urashima, M. Segawa, T. Okazaki, M. Kurihara, M. Wada, Y. Ida, H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May; 91 (5): 1255-60.