e200921c_sity_of_Michigan_Health_System__Why
[Supplement] 2 Stars Vitamin D 300 IU per day for three months in winter [Why] 
Research suggests that supplementing with vitamin D may prevent upper respiratory tract infections in people who are deficient in the vitamin, but not in those who have normal vitamin D status. In a double-blind trial, supplementation with 300 IU per day of vitamin D for three months during the winter decreased the frequency of upper respiratory tract infections in Mongolian children with vitamin D deficiency.31 However, in a double-blind study of healthy adults in New Zealand, vitamin D supplementation did not reduce the incidence or severity of upper respiratory tract infections. The participants in that trial had either normal vitamin D levels or mild vitamin D deficiency (depending on which definition of vitamin D deficiency is used).32 These results suggest that vitamin D supplementation can prevent upper respiratory tract infections in people who are deficient in the vitamin, but not in those who have normal vitamin D status.
Common Cold/Sore Throat (Holistic) | University of Michigan Health System
Why