Numerous peer-reviewed published studies have linked an increase in Vitamin D uptake with a decrease in influenza infection during the flu seasons. The logic is easy to understand: winter brings shorter days with less sunlight, when influenza is most prevalent, while influenza outbreaks are rare during the summer months with increased exposure to sunlight, the most common source of Vitamin D.
One of those studies, a meta-study that looked at 25 separate studies involving over 11,000 people, was published in the British Medical Journal last year (2017), and published by Dr. Mercola. (See: Study: Vitamin D Is More Effective Than Flu Vaccine.)
A study published from researchers in Japan may have unraveled this seeming paradox, which is exactly what they set out to study:
In their conclusion, they admit that overall the statistics did not significantly lower the risk of influenza through Vitamin D supplementation, except for one sub-group – those who had NOT received a flu vaccine.
In a subgroup of participants without vaccination, vitamin D sufficiency (≥30 ng/mL) was associated with a significantly lower risk of influenza (odds ratio 0.14; 95% confidence interval 0.03-0.74).