The effects of caffeine ingestion on immunoendocrine responses to prolonged and intensive exercise in humans
Athletes commonly consume caffeine as an ergogenic aid. The effect of caffeine ingestion on the immune response in humans, and in particular following exercise, has received little scientific attention. Caffeine may affect immune responses via adenosine receptor antagonism or adrenaline-mediated mechanisms. Following the relatively recent removal of caffeine from the WADA list of prohibited substances, its use is expected to increase further, therefore any influence of caffeine on immune function is of particular relevance. The aim of this thesis was to investigate the influence of caffeine ingestion on immunoendocrine responses following prolonged and intensive cycling in humans. Initially, caffeine (6 mg. kg4 body mass) did not affect neutrophil functional responses when participants rested for 3.5 h following ingestion (Chapter 4). Caffeine ingestion attenuated the post-exercise decline in neutrophil oxidative burst response when stimulated by f-MLP (Chapter 6) but not PMA (Chapter 5). An in vitro study suggested that this most likely occurs as a result of adenosine-receptor antagonism by caffeine (Chapter 8). Although caffeine ingestion improved pre-loaded TT performance by 4%, there was no attenuating effect of caffeine on post-exercise f- MLP-stimulated neutrophil oxidative burst responses (Chapter 7) nor was there any benefit of co-ingesting caffeine with CHO on oxidative responses, compared with either supplement alone (Chapter 9). Caffeine supplementation did not affect total circulating leukocyte or neutrophil count at rest or following exercise, but increased circulating lymphocyte count both at rest and during exercise following ingestion. Caffeine ingestion was consistently associated with an increased plasma adrenaline concentration but not noradrenaline concentration at rest and following exercise. Caffeine generally had no effect on plasma cortisol concentration, though it was consistently associated with an increased plasma IL-6 concentration following exercise of various duration and intensity. In conclusion, the attenuating/positive effects of caffeine ingestion on post-exercise neutrophil oxidative burst responses appear to be stimulant-dependent. None of the experimentals tudies-,h, owever, indicated that caffeine ingestion was more detrimental than placebo on the f-MLP-stimulated neutrophil oxidative burst response to exercise.