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U of T Mississauga research indicates seasonal fluctuations in vitamin D levels

Tuesday, March 15, 2011 - 11:22am
Carla DeMarco

The results of a Vitamin-D study led by U of T Mississauga professor Esteban Parra were recently published in The Journal of Nutrition. This research was part of the Ph.D. thesis of Parra's student, Agnes Gozdzik.

Parra, an associate professor in the Department of Anthropology, says the study supports earlier research that Vitamin-D concentrations in persons residing in higher latitudes, such as Canada, fluctuate with the change in seasons. The research also indicated that there were differences in vitamin-D concentrations between ancestral groups, with participants of South Asian and East Asian background having lower levels of the vitamin than their European counterparts.

A large proportion of the 351 participants, however, had Vitamin-D levels that were inadequate for optimal health (less than 50 nmol/L of serum 25-hydroxyvitamin D, which is the main indicator of vitamin-D status). The study also found that levels decreased significantly during the winter months, and contributing factors that influence concentrations include Vitamin-D intake, sun exposure and skin pigmentation.

Parra says that Vitamin-D insufficiency among Canadians was already a concern, but these results suggest that levels may be even lower than previously reported; past research focused primarily on those of European ancestry, while individuals with darker skin have an increased risk of Vitamin-D insufficiency. The findings from this study are of particular relevance to the health of Canadians since Vitamin D plays an important role in bone metabolism, and deficiencies of this nutrient have been linked to several chronic illnesses such as cancer, cardiovascular disease, diabetes, microbial infections, multiple sclerosis, osteoporosis, and rheumatoid arthritis.

Recently, the Institute of Medicine (IOM) updated its vitamin-D recommendations. The Recommended Dietary Allowance (RDA) has been established as 600 International Units/day (IU/d) for persons 19-70 years old, and 800 IU/d for persons over 70 years of age (see the IOM website for detailed recommendations).

In 2007, the Canadian Cancer Society announced their new vitamin-D guidelines (see the Society's website for details), recommending that adults living in Canada should consider taking vitamin-D supplementation of 1000 IU/d during the fall and winter, and that adults at higher risk of having lower vitamin-D levels should consider taking vitamin-D supplementation of 1000 IU/d all year round. This includes people who are older, with dark skin, who don't go outside often, and who wear clothing that covers most of the skin.