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As daylight hours grow shorter, incidence of the common cold, flu, and respiratory infections spikes upwards. Scientists have identified reduced Vitamin D levels in winter months as a prime suspect for this increase in infectious disease cases. Vitamin D from all sources lowers the incidence of respiratory infections. For example, Dutch children with the least sun exposure are twice as likely to develop a cough and three times more likely to develop a runny nose compared with children with the most sun exposure.

Numerous studies have shown vitamin D levels may lower the risk several conditions ranging from depression to various cancers. The connection between vitamin D deficiency and cancer was first made by doctors Frank and Cedric Garland of the University of California, San Diego. After finding that evidence of colon cancer was nearly 3 times higher in New York than it was in New Mexico, the Garland Brothers hypothesized lack of sun exposure (resulting in lack of vitamin D) played a role. Observational studies found risk of breast cancer and rectal cancer falls as vitamin D blood levels rise.