Carbohydrate supplementation, glycogen depletion, and amino acid metabolism during exercise

Abstract

Eight highly trained cyclists were studied during exercise after glycogen depletion (test A) and during carbohydrate (CHO) loading (test B). In test B subjects were able to complete 2 h of exercise at 70-75% maximal workload (Wmax), whereas the initial intensity of 70% Wmax had to be reduced to 50% in test A. Plasma ammonia increased more rapidly, and plasma alanine, glutamate, and glutamine were lower in test A. Exercise caused a 3.6-fold increase in the proportion of active branched-chain 2-oxoacid dehydrogenase (BC) complex in muscle in test A. No activation occurred in test B. There was an inverse correlation between the activity of the BC complex and the glycogen content of the postexercise biopsies. Exercise did not cause changes in the muscle content of ATP, ADP, AMP, IMP, hypoxanthine, and lactate. It is concluded that CHO loading abolishes increases in branched-chain amino acid (BCAA) oxidation during exercise and that part of the ammonia production during prolonged exercise originates from deamination of amino acids. The data appear to confirm the hypothesis (A.J. M. Wagenmakers, J.H. Coakley, and R.H.T. Edwards. Int. J. Sports Med. 11: S101-S113, 1990) that acceleration of the BCAA aminotransferase reaction may drain the tricarboxylic acid cycle and that glycogen is a carbon chain precursor of tricarboxylic acid cycle intermediates and glutamine.

Effect of carbohydrate availability on time to exhaustion in exercise performed at two different intensities

“…However, fat oxidation may not be sufficient to maintain the power output required during the cycling time-to-exhaustion test. In fact, a number of studies have reported that exogenous carbohydrate supplementation, especially when muscle glyco-Carbohydrate availability and time to exhaustion www.bjournal.com.br gen concentrations are low, can maintain blood glucose and total carbohydrate oxidation and delay the onset of fatigue (4)(5)(6)(7)(8)(9). Therefore, while carbohydrate-loading diets are associated with improved endurance capacity, ascribed mainly to maintenance of a high rate of carbohydrate oxidation throughout exercise, low carbohydrate diets have been shown to impair endurance exercise capacity, which has been ascribed to anticipated reduction of endogenous carbohydrate availability and to a decreased rate of carbohydrate oxidation (4)(5)(6)(7)(8)(9).…”

Training in the fasted state facilitates re-activation of eEF2 activity during recovery from endurance exercise

“…It is well documented that exercise in the fasted state, compared with exercise in the fed state, generates a number of specific metabolic responses, most typically enhanced contribution of fat oxidation in energy provision, but also higher rate of muscle protein degradation (Gibala 2007;Koopman et al 2004). Furthermore, net muscle protein breakdown is markedly enhanced during exercise when muscle glycogen stores are depleted (Wagenmakers et al 1991;Jackman et al 1997). In this regard, we have previously shown that net glycogen degradation is enhanced during exercise in the fasted state (De Bock et al 2007a), which conceivably could result in premature glycogen depletion.…”

“…Thus, if no training adaptations were present one would rather expect a blunted eEF2 response after an exercise bout with high carbohydrate availability following a period of consistent fasted exercise. Second, given the greater rate of muscle protein degradation during exercise in the carbohydrate-depleted state than during exercise with high carbohydrate availability (Gibala 2007;Wagenmakers et al 1991;Jackman et al 1997;Koopman et al 2004), rapid re-activation of eEF2 conceivably could contribute to facilitating post exercise protein repair during a period of consistently repeated exercise in the fasted state. Such mechanism is probably redundant during consistent exercise training with ample carbohydrate intake.…”

“…Similarly, both FAT/CD36 and membrane-bound fatty acid binding protein (FABP pm ) contents were elevated after short-term training in the fasted state, but this effect was negated by carbohydrate intake before and during the training sessions (De Bock et al 2008). Furthermore, it also has been demonstrated that net muscle protein degradation is increased during prolonged exercise in a carbohydrate-deficient state (Wagenmakers et al 1991;Jackman et al 1997), which might in turn stimulate adaptation of signalling pathways implicated in post-exercise protein repair. Support for such assumption contention comes from our recent observation that p70 s6k phosphorylation is elevated during recovery from a resistance exercise bout in the fasted state, indicating higher degree of activation of muscle anabolism than after an identical exercise bout in the fed state (Deldique et al 2010).…”

Physical exercise as a modulator of adaptation to low and high carbohydrate and low and high fat intakes

“…Decreased glycogen stores are consistently demonstrated in subjects consuming low carbohydrate diets (Coggan & Mendenhall, 1992). Several investigators reported indices of accelerated protein breakdown in response to endurance exercise in athletes who were glycogen depleted vs glycogen repleted (Bazarre et al, 1992;Lemon and Mullin 1980;Wagenmakers et al, 1991). If the endurance athlete continues training in a chronic glycogen-depleted state, negative changes in body composition could result over time.…”