Growth Hormone Responses to Consecutive Exercise Bouts with Ingestion of Carbohydrate plus Protein

Endocrine responses to repeated exercise have hardly been investigated and no data are available regarding the mediating influence of nutrition. On three occasions, participants ran for 90 min at 70% VO2max (R1) before a second exhaustive treadmill run at the same intensity (R2; 91.6 ± 17.9 min). During the intervening 4 h recovery, participants ingested either: (i) 0.8 g sucrose·kg-1·h-1 with 0.3 g·kg-1·h-1 whey protein isolate (CHO-PRO); (ii) 0.8 g sucrose·kg-1·h-1 (CHO) or; (iii) 1.1 g sucrose·kg-1·h-1 (CHO-CHO). The latter two solutions therefore matched the former for carbohydrate or for available energy, respectively. Serum growth hormone concentrations increased from 1.7±0.9 μg·l-1 to 16.7±7.8 μg·l-1 during R1 considered across all treatments (means±standard deviations; P≤0.01). Concentrations were similar immediately after R2 irrespective of whether CHO or CHO-CHO was ingested (19±4 μg·l-1 and 19±5μg·l-1, respectively), whereas ingestion of CHO-PRO produced an augmented response (31±4μg·l-1; P≤0.05). Growth hormone binding protein concentrations were unaffected by R1 but increased similarly across all treatments during R2 (414±202 pmol·l-1 to 577±167 pmol·l-1; P≤0.01), as was the case for plasma total testosterone (9.3±3.3 nmol·l-1 to 14.7±4.6 nmol·l-1; P≤0.01). There was an overall treatment effect for serum cortisol (P≤0.05), with no specific differences at any given time-point but lower concentrations immediately after R2 with CHO-PRO (608±133 nmol·l-1) than CHO (796±278 nmol·l-1) or CHO-CHO (838±134 nmol·l-1). Ingesting carbohydrate with added whey protein isolate during short-term recovery from 90 minutes of treadmill running increases the growth hormone response to a second exhaustive exercise bout of similar duration.