Winter may not officially begin for another month, but much of the United States and Canada had snow and below freezing temperatures this week. Whether or not you are snowed in, the shorter days mean less sunshine. Sun exposure is an important source of vitamin D, especially since few foods naturally contain Vitamin D. As a result, at northern latitudes, serum concentrations of serum 25-hydroxyvitamin D concentrations are significantly lower in the winter than the summer (McKenna MJ, 1992).
Regular readers of Talking Nutrition will be familiar with the wide range of health problems associated with lower serum vitamin D, which has been the topic of many posts. However, with observational studies, where there is no intervention, it can be unclear whether low serum Vitamin D is the cause or the result of poor health. Afzal and colleagues assessed low serum 25-hydroxyvitamin D concentrations in 95,766 Danish adults in relation to risk of death. The twist was that the risk of death was compared between two groups: people with genetic variants known to decrease serum vitamin D concentrations (Wang et al, Ahn et al) and people without these variants. Genetically low serum Vitamin D was associated with 30% increased risk of mortality and 40% increased mortality from cancer. This provides compelling evidence that lifelong low vitamin D has a causal relationship with mortality.
As researchers continue to discover links between vitamin D and health, the importance of meeting the dietary reference intakes (DRIs) for adequacy is apparent. When sun exposure is low, both food and supplemental sources should be considered.
Afzal S et al. Genetically low vitamin D concentrations and increased mortality: mendelian randomisation analysis in three large cohorts. BMJ 2014; epub ahead of print
MJ McKenna. Differences in vitamin D status between countries in young adults and the elderly. The Am J of Med. 1992;93,69-77
Wang TJ, et al. Common genetic determinants of vitamin D insufficiency: a genome-wide association study. Lancet. 2010;376:180-8
Ahn J, et al. Genome-wide association study of circulating vitamin D levels. Hum Mol Genet. 2010;19:2739-45