Growth-hormone responses to consecutive exercise bouts with ingestion of carbohydrate plus protein

• Published in: International journal of sport nutrition and exercise metabolism 2013-06, Vol.23 (3), p.259-270
• Main Authors: Betts, James A, Stokes, Keith A, Toone, Rebecca J, Williams, Clyde
• Format: Article
• Language: English
• Subjects: Adolescent; Body Mass Index; Cross-Over Studies; Diet Records; Dietary Carbohydrates - administration & dosage; Dietary Proteins - administration & dosage; Double-Blind Method; Energy Intake; Exercise Test; Human Growth Hormone - blood; Humans; Hydrocortisone - blood; Male; Oxygen Consumption - physiology; Physical Endurance - physiology; Running - physiology; Young Adult
• ISSN: 1526-484X; 1543-2742
• DOI: 10.1123/ijsnem.23.3.259

Endocrine responses to repeated exercise have barely been investigated, and no data are available regarding the mediating influence of nutrition. On 3 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-hr recovery, participants ingested either 0.8 g sucrose · kg-1 · hr-1 with 0.3 g · kg-1 · hr-1 whey-protein isolate (CHO-PRO), 0.8 g sucrose · kg-1 · hr-1 (CHO), or 1.1 g sucrose · kg-1 · hr-1 (CHO-CHO). The latter 2 solutions therefore matched the former for carbohydrate or for available energy, respectively. Serum growth-hormone concentrations increased from 2 ± 1 μg/L to 17 ± 8 μg/L during R1 considered across all treatments (M ± SD; p ≤ .01). Concentrations were similar immediately after R2 irrespective of whether CHO or CHO-CHO was ingested (19 ± 4 μg/L and 19 ± 5 μg/L, respectively), whereas ingestion of CHO-PRO produced an augmented response (31 ± 4 μg/L; p ≤ .05). Growth-hormone-binding protein concentrations were unaffected by R1 but increased similarly across all treatments during R2 (from 414 ± 202 pmol/L to 577 ± 167 pmol/L; p ≤ .01), as was the case for plasma total testosterone (from 9.3 ± 3.3 nmol/L to 14.7 ± 4.6 nmol/L; p ≤ .01). There was an overall treatment effect for serum cortisol (p ≤ .05), with no specific differences at any given time point but lower concentrations immediately after R2 with CHO-PRO (608 ± 133 nmol/L) than with CHO (796 ± 278 nmol/L) or CHO-CHO (838 ± 134 nmol/L). Ingesting carbohydrate with added whey-protein isolate during short-term recovery from 90 min of treadmill running increases the growth-hormone response to a second exhaustive exercise bout of similar duration.