Effect of caffeinated drinks on substrate metabolism,

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Dear All,

Below is the abstract and part of the discussion which was passed on to me

by a friend (so thank him not me!)and I think some may find it interesting

with regards the recent discussion on this subject. Enjoy,

,

Melbourne,

Australia

EVA M. R. KOVACS,1 JOS H. C. H. STEGEN,1 AND FRED BROUNS2

1Department of Human Biology and 2Novartis Nutrition Research Unit,

Maastricht University, The Netherlands

Kovacs, Eva M. R., Jos H. C. H. Stegen, and Fred

Brouns.

Effect of caffeinated drinks on substrate metabolism,

caffeine excretion, and performance. J. Appl. Physiol.

85(2): 709-715, 1998.

-The effect of addition of different

dosages of caffeine (Caf) to a carbohydrate-electrolyte solution

(CES) on metabolism, Caf excretion, and performance

was examined. Subjects (n 5 15) ingested 8 ml/kg of water

placebo (Pla-W), 7% CES (Pla-CES), or 7% CES with 150, 225,

and 320 mg/l Caf (CES-150, CES-225, and CES-320, respectively)

during a warm-up protocol (20 min) and 3 ml/kg at

one-third and two-thirds of a 1-h time trial. Performance was

improved with Caf supplementation: 62.5 6 1.3, 61.5 6 1.1,

60.4 6 1.0, 58.9 6 1.0, and 58.9 6 1.2 min for Pla-W, Pla-CES,

CES-150, CES-225, and CES-320, respectively. The postexercise

urinary Caf concentration (range 1.3-2.5 µg/ml) was dose

dependent and always far below the doping level of the

International Olympic Committee (12 µg/ml) in all subjects.

Sweat Caf excretion during exercise exceeded postexercise

early-void urinary Caf excretion. Caffeinated CES did not

enhance free fatty acid availability, ruling out the fact that

performance improvement resulted from enhanced fat oxidation.

It is concluded that addition of relatively low amounts of

Caf to CES improves performance and that postexercise

urinary Caf concentration remained low.

Discussion.

...........Despite the low level, the interindividual variations

in the postexercise urinary Caf concentration were

large (Fig. 4). Several subjects always had relatively

high urinary Caf levels, whereas others always had

relatively low urinary Caf levels. This was the case for

all dosages given. These differences among subjects

may be explained by individual variations in the rate at

which Caf is metabolized by the liver.

Although it has been shown that Caf is a diuretic

under resting conditions (18, 40), no Caf-induced diuresis

has been observed during exercise (40). Also, in the

present study no significant increase in urine volume

was found as a result of Caf ingestion. It is possible that

during exercise the diuretic effect of Caf may be counteracted

by action of catecholamines, which induce constriction

of renal arterioles and reduce glomerular filtration

rate (6). Additionally, catecholamines may increase

Na1 and Cl2 reabsorption in the proximal and distal

tubules by affecting aldosterone and/or antidiuretic

hormone, resulting in water conservation (2, 10). Additionally,

Caf intake during exercise appears to have no

effect on sweat loss, body temperature, and plasma

volume (15). For this reason, the suggestion that Caf

intake may result in a poor hydration status and

thereby affect performance negatively is unfounded.

From the present data it is concluded that a relatively

low dose of Caf added to CES supports 1-h time

trial cycling performance. Furthermore, an ingestion of

4.5 mg Caf/kg BW did not result in further performance

improvement compared with 3.2 mg Caf/kg BW. These

ergogenic effects cannot be explained by differences in

fat metabolism. No effects on diuresis were observed,

and urinary Caf levels remained widely below the IOC

doping limit in all individuals.

2. Bello-Reuss, E. Effect of catecholamines on fluid reabsorption

by the isolated proximal convoluted tubule. Am. J. Physiol. 238

(Renal Fluid Electrolyte Physiol. 7): F347-F352, 1980.

6. Clausen, J. P., and J. Trap-Jensen. Arteriohepatic venous

oxygen difference and heart rate during initial phases of exercise.

J. Appl. Physiol. 37: 716-719, 1974.

10. Di Bona, G. F. Neural regulation of renal tubular sodium

reabsorption and renin secretion. Federation Proc. 44: 2816-

2822, 1985.

15. Falk, B., R. Burstein, I. Ashkenazi, O. Spilberg, J. Alter, E.

Zylber-Katz, A. Rubinstein, N. Bashan, and Y. Shapiro. The

effect of caffeine ingestion on physical performance after prolonged

exercise. Eur. J. Appl. Physiol. 59: 168-173, 1989.

18. Gonza´ lez-Alonso, J., C. L. Heaps, and E. F. Coyle. Rehydration

after exercise with common beverages and water. Int. J.

Sports Med. 13: 399-406.

40. Wemple, R. D., D. R. Lamb, and K. H. McKeever. Caffeine vs.

caffeine-free sports drinks: effect on urine production at rest and

during prolonged exercise. Int. J. Sports Med. 18: 40-46, 1997.

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