A growing body of research suggests that certain dietary factors may help support immune function during cold and flu season.
Probiotics and Upper Respiratory Infections
Several studies have examined the role of probiotics in immune health. Research published in Vaccine (Volume 24, Issues 44–46; November 10, 2006; pages 6670–6674) evaluated probiotic supplementation and its effect on upper respiratory tract infections, including colds and influenza.
This double-blind, placebo-controlled study was conducted over two winter/spring seasons and included 479 healthy adults. Participants received either a vitamin-mineral supplement containing probiotics (lactobacilli and bifidobacteria) or a placebo containing the same vitamin-mineral supplement without probiotics.
Probiotic supplementation did not reduce the total number of upper respiratory infections. However, it significantly shortened the duration of illness by nearly two days compared with placebo, and symptoms were less severe. The probiotic group also showed increases in several immune cell populations, including cytotoxic T cells, suppressor T cells, and helper T cells.
Additional evidence comes from a study published in the International Journal of Sports Nutrition, Exercise and Metabolism (February 2011; 21(1):55–64), which examined probiotic use in 58 athletes. Participants were randomly assigned to receive either a probiotic supplement containing Lactobacillus casei Shirota or a placebo for 16 weeks.
The placebo group experienced a 36% higher incidence of upper respiratory tract infections compared with the probiotic group. The authors concluded that regular ingestion of Lactobacillus casei Shirota appeared to reduce the frequency of respiratory infections, possibly by supporting salivary IgA levels during periods of intense training.
Green Tea and Influenza Risk
Another dietary factor that has received attention for immune support is green tea. Research suggests that green tea consumption may be associated with a reduced risk of influenza.
A study published in the Journal of Nutrition (October 2011; 141(10):1862–1870) evaluated green tea intake during flu season using questionnaires administered twice during the study period. Influenza infection was confirmed using antigen testing.
The researchers found that individuals who consumed between one and five cups of green tea per day had a lower incidence of influenza. A clear inverse relationship was observed between green tea consumption and laboratory-confirmed influenza infection.
