Does Caffeine Consumption before High-Intensity Intermittent Exercise Enhance Immunity?
Regular participation in moderately intense physical activity decreases the risk of picking up common colds below that of a sedentary individual. However, performing prolonged, high-intensity, exercise, or sustained periods of strenuous training, is associated with an above average risk of getting infections. However, less is known about how the immune system responds to brief (less than 30 min) bouts of high-intensity intermittent exercise (HIIE), which has become popular due to its efficacy in enhancing health and fitness in general and clinical populations. Furthermore, nutrition and exercise have powerful influences on the body’s immune system and therefore dietary factors and exercise could be coupled to help improve immune function. One potential dietary substance is caffeine, which is now a common element in most people’s diet, due to its alertness-enhancing effects. Though research is limited, caffeine has been shown to enhance the activation of both natural state and antigen-stimulated T (CD4⁺ and CD8⁺) and NK cells following strenuous endurance exercise. However, there is no research investigating the interaction between HIIE and caffeine ingestion on the lymphocytes of innate and adaptive immune functions. Therefore, the aim of this study was to determine the effects of ingesting caffeine before HIIE on innate and adaptive immune functions following brief (20 min) HIIE. A double-blind cross-over design was adopted, during which 10 healthy active men participated in two exercise trials following acute (60 min pre-exercise) consumption of 6 mg.kg⁻¹ caffeine or placebo. Each trial required participants to perform a 20 min HIIE protocol (10 x 1 min at ~90% HRmax; 1 min active recovery, 50W) in the laboratory on a cycle ergometer. Venous blood samples were collected pre-supplement, pre-exercise, immediately post-exercise and 1 h post-exercise. Samples were analysed for numbers of natural stage and antigen-stimulated T (CD4⁺ and CD8⁺) and NK cells expressing CD69 markers, as well as the GMFI of the expressed CD69. Serum caffeine and, plasma cortisol and adrenaline concentration were also determined. Consuming caffeine one hour before HIIE increased the number of circulating NK cells by 56% at the pre-exercise (P<0.01) stage. Although not significant, caffeine also increased the circulating NK cells number by 13% measured immediately post-exercise. However, caffeine had minimal effect on the number of circulating CD4⁺ and CD8⁺ T cells. Caffeine also increased the number of unstimulated and antigen-stimulated NK cells expressing CD69 (unstimulated: 71%, P<0.01; stimulated: 51%, P<0.05) at pre-exercise stage, but had little effect on T cells. Although not statistically significant, caffeine increased the number of NK cells expressing CD69 by 7% for unstimulated and 13% for stimulated cells at the post-exercise stage. However, caffeine had minimal effect on GMFI expression of CD69 on T and NK cells. Compared with pre-supplement, HIIE induced the main time effect of CD69 GMFI expression of antigen-stimulated NK cells (P<0.01) at 1 h post-exercise. Overall, the thesis findings suggest that caffeine ingestion one hour before HIIE may increase the innate immune function, as NK cell numbers and activation were increased. However, caffeine prior to HIIE does not appear to alter the circulating number and activation of adaptive immune cells (CD4⁺ and CD8⁺ T cells). While the observed acute innate immune response to caffeine consumption appears desirable, it remains to be determined if acutely improved innate immune function will actually result in reduction of an individual’s susceptibility to infection following HIIE.