Carbohydrate drinks

Carbohydrates and perceived exertion

Does carbohydrate supplementation exert an ergogenic effect during marathon running? That is the question US researchers set out to answer in a study of 98 male and female entrants to the 1999 Charlotte Marathon and the 2000 Grandfather Mountain Marathon in Boone, both in North Carolina.

The highly experienced (but non-élite) participants, ranging in age from 21 to 72, underwent a series of blood and anthropometric tests on the morning of the race and were then randomly assigned to one of two conditions:

  • supplementation with a 6% carbohydrate drink, with each runner ingesting 650ml about 30 minutes before the start of the race and approximately 1,000ml at hourly intervals during the event;
  • the same amounts of an inactive placebo drink, identical in appearance and taste to the carbohydrate solution.

A chest heart rate monitor was attached to each runner, and research assistants, positioned every 3.2km along the racecourse to deliver the drinks, recorded heart rates and ratings of perceived exertion (RPE) at the same time. After runners crossed the finish line, blood samples were collected from each within five minutes.

Key findings for the two races combined were as follows:

  • Race times for both the carbohydrate and the placebo group were slower than their personal bests of the previous year due to the hilly terrain of both these marathons. Although race times did not differ significantly between the groups, the placebo group was about 15 minutes slower by comparison with these earlier PBs than the carb group;
  • RPEs during running did not differ significantly between the two conditions, although there was a non-significant trend towards a higher RPE during the later portion of the race with placebo;
  • Runners in the carbohydrate group were able to run at a higher intensity – ie at a higher percentage of their maximum heart rate – particularly during the final 10km;
  • Despite the similarity in RPE between the two conditions, there was a significant decrease in plasma glucose and insulin, concomitant with an increase in plasma cortisol and growth hormone, with placebo compared with the carbohydrate condition.

Based on the evidence of their previous laboratory-based studies, the researchers had hypothesised that RPE would be lower – ie running would feel easier – with carbohydrate supplementation. A possible explanation for their failure to replicate this finding ‘in the field’ is that experimental outcomes during an actual race can be easily affected by many extraneous variables, including weather, terrain and motivation as well as variations in the intensities at which the runners were performing from point to point.

‘These findings suggest,’ they conclude, ‘that the attainment of a greater percentage of maximum heart rate at a given RPE can be attributable to a sustained supply of carbohydrate energy substrates to the exercising muscle.’

But they add: ‘During prolonged strenuous exercsie, where intensity varies from point-to-point as in marathon running, it appears that factors other than carbohydrate energy substrate availability play an important role in mediating the strength of perceived exertion.

Med Sci Sports Exerc 2002 Nov; 34(11): pp1779-84

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