Vitamin D's genetic effect seasonal

ATLANTA, Nov. 25 (UPI) -- U.S. and Brazilian researchers say genetic factors affect vitamin D levels only in winter.

Study leader Christina Karohl of Emory University in Atlanta and at Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, linked 70 percent of the variation in concentrations of vitamin D levels during the winter to genetic factors.

However in the summer, 53 percent of the variation in vitamin D concentrations was due to shared environmental factors and 47 percent were due to unique environmental factors.

The study, scheduled to be published in December issue of The American Journal of Clinical Nutrition, suggests summer environmental conditions such as sun exposure trumped genetic factors in determining blood levels of vitamin D.

Karohl and colleagues looked at 510 middle-age men -- 310 identical twins and 200 fraternal twins from the Vietnam Era Twin Registry. Generalized estimating equations and structural equation models were used to test the association between levels of vitamin D and other study factors.