Original articles   

The Journal of Sports Medicine and Physical Fitness 2000 March;40(1):41-50

Copyright © 2002 EDIZIONI MINERVA MEDICA

language: English

Pre-exercise carbohydrate and fluid ingestion: Influence of glycemic response on 10-km treadmill running performance in the heat

Mitchell J. B., Braun W. A., Pizza F. X., Forrest M.

From the Exercise Physiology Laboratory Department of Kinesiology Texas Christian University For Worth, Texas, USA

Background. The pur­pose of ­this ­study was to deter­mine the influ­ence of ingest­ing solu­tions con­tain­ing mix­tures of car­bo­hy­drate(CHO) ­types on pre-exer­cise gly­cem­ic ­response, exer­cise-­induced hypo­gly­ce­mia, meta­bol­ic respons­es, and 10-km tread­mill run­ning per­for­mance in a ­warm envi­ron­ment.
Methods. Ten ­trained run­ners com­plet­ed 6, ­self-­paced 10-km tread­mill ­runs one ­hour ­after ingest­ing 900 ml of one of the fol­low­ing ­test solu­tions: a ­water pla­ce­bo (WP), an 8 g 100 ml-1 ­high fruc­tose ­corn syr­up solu­tion (HFG; 72 g CHO), a 6 g 100 ml-1 glu­cose solu­tion (GLU; 54 g CHO), a 6 g.100 ml-1 ­sucrose/glu­cose mix­ture (SUG; 54 g CHO), or bana­na ­with ­water to ­equal 900 ml (BAN; ­approx. 54 g CHO). The ­sixth con­di­tion was 675 ml of an 8 g.100 ml-1 ­HFCS solu­tion (LFG; 54 g CHO). Blood sam­ples ­were tak­en ­prior to inges­tion and eve­ry 15 min dur­ing ­rest and at 15 and 30 min, and at the end of the 10-km run. Blood was ana­lyzed for glu­cose (BG) insu­lin (IN), glyce­rol, lac­tate, and per­cent ­change in plas­ma vol­ume. Urine vol­ume dur­ing the 1 hour of ­rest and ­change in ­body ­mass dur­ing exer­cise ­were ­also deter­mined.
Results. A sig­nif­i­cant (p<0.05) cor­re­la­tion (r=-0.684) was ­seen ­between the pre-exer­cise gly­cem­ic ­response (­PEGR=­area ­under the rest­ing BG ­curve) and the ­change in BG ­from pre-EX to 15 min of exer­cise. BG at 15 min of exer­cise was sig­nif­i­cant­ly high­er in the WP (5.22 mM) ver­sus the oth­er con­di­tions (HFG=3.32, LFG=3.91, GLU=3.38, BAN=3.74 & SUG=3.63 mM). Pre-exer­cise IN was low­er in the WP (6.54 U ml-1) con­di­tion ver­sus the oth­er con­di­tions (HFG=22.1, LFG=16.2, GLU=23.3, BAN=18.8 & SUG=12.8 U·ml-1). Ten km per­for­mance ­times ­were not dif­fer­ent (WP=41.87, HFG=41.66, LFG=41.79, GLU=41.65, BAN=41.53, and SUG=41.75 min). A sig­nif­i­cant­ly great­er ­body ­mass ­loss ­occurred due to ­urine pro­duc­tion dur­ing the 60 min of ­rest in the WP com­pared to the oth­er con­di­tions. The ­degree of exer­cise-­induced ­decline in ­blood glu­cose was relat­ed to the ­PEGR; how­ev­er, the ­decline in BG did not ­affect 10-km run­ning per­for­mance. In addi­tion, ­there ­were no dif­fer­enc­es in the meta­bol­ic respons­es dur­ing exer­cise ­between the dif­fer­ent CHO ­types, nor did the ­type of CHO influ­ence run­ning per­for­mance. Finally, the pres­ence of CHO and/or elec­tro­lytes in the hydra­tion solu­tions pro­duced a bet­ter ­fluid reten­tion dur­ing the 60- min pre-exer­cise ­rest peri­od com­pared to ­water.
Conclusions. The ­results con­fermed ­that if a com­pet­i­tive ath­lete con­sumed a break­fast ­prior to ingest­ing a CHO-elec­tro­lyte bev­er­age, a prac­tice ­that is com­mon, the gly­cem­ic respons­es may be dif­fer­ent.