Background: Heavy exercise induces marked immunodepression that is multifactorial in origin. Nutrition can modulate normal immune function.
Objective: To assess the efficacy of nutritional supplements in exercise-induced immunodepression in athletes.
Design: Systematic review.
Review methods: Randomised and/or controlled trials of athletes undertaking nutritional supplements to minimise the immunodepression after exercise were retrieved. The primary outcome measure was incidence of upper respiratory tract (URT) illness symptoms after exercise, and secondary outcomes included cortisol, cell counts, plasma cytokine concentration, cell proliferative response, oxidative burst, natural killer cell activity and immunoglobulins. When data were available for a pooled estimate of the effect of intervention, meta-analyses were conducted for direct comparisons.
Results: Forty-five studies were included (1603 subjects). The studies were heterogeneous in terms of exercise interventions, selection of athletes, settings and outcomes. The overall methodological quality of most of the trials was poor. Twenty studies addressed carbohydrate supplementation, eight glutamine, 13 vitamin C and four others interventions. Three trials assessed the effect of intervention on prevention of URT infections. The pooled rate ratio for URT infections after vitamin C supplementation against placebo was 0.49 (0.34-0.71). Carbohydrate supplementation attenuated the increase in cortisol and neutrophils after exercise; vitamin C attenuated the decrease in lymphocytes after exercise. No other interventions had significant or consistent effect on any of the studied outcomes.
Conclusions: Although the prevention of URT infections by vitamin C was supported by two trials, further studies are needed. The available evidence failed to support a role for other nutritional supplements in preventing exercise-induced immune suppression. Larger trials with clinically relevant and uniform end points are necessary to clarify the role of these nutritional interventions.