Many epidemiologists now believe that flu is seasonal because vitamin D levels fluctuate seasonally. Vitamin D is a potent immune modulator; individuals tend to have higher levels during summer months with higher exposure to UV radiation and, therefore, are less likely to contract the flu while sun-kissed.
Researchers followed the seasonal health adventures of 208 post-menopausal women over three years as correlated with their monitored intake of D.(1) The results, shown in the bar graph above, were as follows:
- Those ladies in the placebo group (lightest bars) got cold and flu symptoms all year long, but especially in the winter.
- The test subjects (intermediate shaded bars) who received 800 units/day of supplemental D were as likely to get sick in the summer as winter, but were far less likely to take to their beds than their D-ficient colleagues.
- In the final year of the study, the test subjects were plied with 2000 units of D each day. Only one lady (as represented by the itty-bitty dark bar in summer) got sick while D-eeply dosed with the vitamin.
1)Aloia JF, Li-Ng M: Epidemic influenza and vitamin D. Epidemiol Infect 2007; 135: 1095–1096.