33

Influence of Carbohydrate Ingestion on Counterregulatory Hormones During Prolonged Exercise J. B. Mitchell, D. L. Costill, J. A. Houmard, M. G. Flynn, W. J. Fink, andJ. D. Beltz Human Performance Laboratory, Ball state University, Muncie, Indiana 47306

Abstract J. B. Mitchell, D. L. Costill, .1. A. Houmard, M.

G. Flynn, W. J. Fink, and J. D. Beltz, Influence of Carbohy-

drate Ingestion on Counterregulatory Hormones During Prolonged Exercise. hit J Sports Med, Vol 11, No 1, pp 33— 36, 1990. Accepted after revision: May 1, 1989

This study was undertaken to determine the effects of ingesting 5.0 (CHO-5), 6.0 (CHO-6), and 7.5 g/100 ml (CHO-7.5) carbohydrate (CHO) solutions on blood glucose and counterregulatory hormonal responses during prolonged intermittent exercise. Eight well-trained cyclists performed four trials consisting of seven 1 2-mm cycling bouts at 70% of VOzmax with 3 mm rest between each ride. A final 12 mm ride was an all-out self-paced perform-

ance ride. During the rest interval the subjects ingested either a water placebo (WP) or one of the CHO solutions at a

rate of 8.5 mg/kg/h (approx. 150 ml). Blood samples were taken at 0, 25, 55, 85, and 115 mm of exercise and were assayed for glucose, glucagon (GO), cortisol (CT), insulin (IN), epinephrine (EP), and norepinephrine (NE). Blood glucose levels were significantly lower in the WP trial compared to the CHO trials at 25 (4.6 0.2 vs 5.7 0.5 mmol/l) and 55 mm (4.4 0.3 vs 5.0 0.8 mmol/l). At 85 mm blood glucose was significantly lower in the WP compared to the CHO-6 and CHO-7.5 trials. GG and IN levels were not significantly different between trials; however, the GO : IN molar ratio was significantly higher in the WP than in the CHO-7.5 trial. CT was significantly elevated in the WP trial compared to the CHO-7.5 trial. EP and NE levels were not affected by CHO ingestion. These data suggest that

CHO feedings prevent the typical hormonal responses which are responsible for hepatic glucose release, thus eliciting a possible hepatic glycogen sparing. Key words

Blood glucose, glucagon, insulin, cortisol, catecholamines

The ingestion of carbohydrate (CHO) during exercise provides an exogenous source of CHO, which has been shown to enhance endurance performance (4, 13). Although glycogen sparing has been demonstrated in some CHO-feeding studies (7, 9), there is also evidence suggesting that the beneficial effects of CHO ingestion are due to the maintenance of blood glucose (4, 6, 13). In the absence of muscle glycogen sparing, the maintenance of blood glucose may, therefore, be due to the sparing of other endogenous CHO; in particular, hepatic stores.

The counterregulatory hormones, cortisol (CT), glucagon (GO), insulin (IN), norepinephrine (NE) and epinephrine (EP) (1, 5,7), are responsible for the regulation of CHO metabolism. The maintenance of blood glucose by CHO ingestion can cause the normal counterregulatory hormone response to be altered such that decreases in blood glucose levels are attenuated (1,3, 5,7, 12). It has been suggested that GO is the primary mediator of the counterregulatory effort to maintain euglycemia (11, 15). In addition, it has been proposed that an elevation in the 00 to IN molar ratio (GIR) is indicative of hepatic glucose release (15). The effects of small serial CHO feedings on the hormonal responses during prolonged exercise have not been

completely characterized. In addition, the relationship between the hormonal responses and improvements in exercise performance has not been elucidated. If the improved performance with CHO ingestion is due to the maintenance of blood glucose, with a concomitant reduction in the release of glucose by the liver, the GIR should be lowered when CHO are administered. The gastric emptying characteristics and the performance results associated with CHO ingestion were investigated and reported elsewhere (13). Therefore, the purpose of this segment of the investigation was to determine the effects of the ingestion of various concentrations of CHO on the glucose counterregulatory hormone levels during submaximal intermittent exercise. This was done in an attempt to provide a possible mechanism for the performance benefits frequently observed with CHO ingestion. Methods

mt. J. Sports Med. 11(1990)33—36

GeorgmiemeVerlagStuttgart NewYork

Eight highly trained male cyclists served as subjects in this study. Subject characteristics are as follows: age = 21.5 0.67 yrs, height = 181.9 2.1 cm, weight

Downloaded by: NYU. Copyrighted material.

Introduction

mt. J. Sports Med. 11(1990) Table 1

J. B. Mitchell, D. L. Costill, J.A. Houmard, M. G. Flynn, W. J. Fink, andJ. D. Beltz

CHO Drink Characteristics • WATOR

Drink CHO-5 CHO-6 CHO-7.5

%CHO

Maltrin

Glucose

5.0 6.0

270 0

2.30 2.14

75

5.5

0

Fructose

Sucrose

0

0

1.88 2.0

0

• CR05 w

CR06

Cl)

• CR07.5

5.5

+

1.95 50

Values are reported as g/100 ml solution.

4.5

1.4 kg, % fat = 10.2 0.71, and maximal oxygen uptake (VO2max) = 4.7 0.13 1/mm. Each subject was tested for VO2max prior to the experimental trials. Percent fat was estimated from skin-folds. The test drinks were 5.0, 6.0, and 7.5 g/100 ml CHO solutions and an artificially flavored water placebo (WP) (Table 1). Prior to each 1 2-mm bout, the test solutions were consumed at a rate of 8.5 mi/kg/h. This rate of consumption corresponded to 33.5, 39.4, and 50.1 g CHO/h for the above drinks, respectively. Each subject was tested in a random order under each of the four conditions,

4.0

= 78.6

0

55

25

85

115

TIME (mm)

Fig. 1 Blood glucose levels. The * indicates all the CHO trials are significantly different from the WP. The + indicates that the CHO-6 and CHO-7.5 are significantly different from the WP (P

Influence of carbohydrate ingestion on counterregulatory hormones during prolonged exercise.

This study was undertaken to determine the effects of ingesting 5.0 (CHO-5), 6.0 (CHO-6), and 7.5 g/100 ml (CHO-7.5) carbohydrate (CHO) solutions on b...
91KB Sizes 0 Downloads 0 Views