Liquid carbohydrate/essential amino acid ingestion during a short-term bout of resistance exercise suppresses myofibrillar protein degradation
Introduction
Resistance exercise stimulates immediate changes in the rate of muscle protein turnover, resulting in an increase in both protein synthesis and protein degradation [1], [2], [3]. Any imbalance between the rate of protein synthesis and rate of protein degradation will lead to a change in the size of the protein tissue pool [4]. Thus, for protein accretion to occur, the rate of synthesis must exceed the rate of degradation. However, in the absence of nutritional intake, net muscle protein balance (ie, the difference between protein synthesis and protein degradation) remains negative in the early stages of recovery [2]. A number of studies support the notion that hormonal events play critical roles in controlling protein turnover [5], [6], [7], with the response of insulin and cortisol receiving much attention, as they are intimately involved in this cyclical process. Although the ability to alter the anabolic hormonal milieu and the influence that such adjustments have on modifying protein synthesis are well documented [8], [9], [10], the ability to alter the catabolic hormonal environment and the impact that such a change might have on affecting protein degradation have received considerably less attention.
Urinary 3-methylhistidine (3-MH) excretion has been used as an index of myofibrillar protein degradation [11], [12], [13], [14]. 3-Methylhistidine is known to be a constituent amino acid of actin and myosin, and upon catabolism, the only fate of 3-MH is that of excretion in urine [14]. Estimates indicate that ~90% of total human 3-MH is located in skeletal muscle [15], validating its use as an index of myofibrillar protein degradation [14], [16]. Yet, the clinical use of 3-MH has been met with criticism. A substantial limitation appears to be the effect of protein intake on 3-MH excretion [17]. Lukaski et al [18] determined that meat consumption increases 3-MH excretion and that 3 days of a meat-free diet are required to return urinary 3-MH to baseline levels. However, with careful dietary and exercise controls, qualitative inferences can be made concerning skeletal muscle metabolism [14], [16], [19].
A variety of resistance exercise protocols result in an immediate increase in cortisol [20], [21], [22]. However, Goldberg and Goodman [23] reported that elevated cortisol levels are the primary factor stimulating exercise-induced increases in protein degradation. Therefore, long-term elevations in cortisol associated with resistance training may have a negative impact on skeletal muscle hypertrophic adaptations [21]. Such a contention implicates the glucoregulatory action of cortisol. Therefore, if an individual ingested a carbohydrate (CHO) solution during the exercise bout, the exogenous glucose load would raise blood glucose levels. This response may modify biochemical signals and attenuate the stimulus for the adrenal cortex to secrete cortisol to catabolize cellular protein for gluconeogenic purposes. Moreover, addition of essential amino acids (EAAs) may augment this response, as recent reports demonstrate that protein feeding [24] and leucine ingestion [25] rapidly reduce the release of 3-MH. These findings indicate that, at least to some extent, myofibrillar protein degradation may be controlled by extracellular amino acid availability.
Previous work has demonstrated that liquid CHO ingestion during a short-term bout of resistance exercise can decrease exercise-induced cortisol release [21]. Resistance exercise with CHO ingestion blunted exercise-induced cortisol release both during and after the exercise bout. This response is in contrast to significantly elevated cortisol levels occurring with resistance exercise alone (99%) [21]. Thus, modification of exercise-induced cortisol release could influence protein turnover by altering the balance between hormone-mediated anabolic and catabolic activities [26]. A reduction in the cortisol response and hormone-induced protein degradation are potential mechanisms by which protein accretion occurs, that of enhancing skeletal muscle growth by suppressing myofibrillar protein degradation [21]. Furthermore, such responses may be synergistically potentiated by the addition of EAA (CHO + EAA). Therefore, the purpose of the present investigation was to examine the influence of liquid CHO and EAA ingestion during resistance exercise and modification of the immediate hormonal response on myofibrillar protein degradation as assessed by 3-MH excretion.
Section snippets
Subjects and study design
After a full explanation of all procedures and possible risks of the investigation, we obtained written informed consent from 32 untrained young men (mean [±SD]; age, 21.0 ± 2.4 years; height, 182.7 ± 6.9 cm; body mass, 79.6 ± 12.1 kg) who volunteered to participated in this investigation. The subjects were physically active but considered untrained as none had been involved in any regular exercise or resistance training for at least 6 months before the start of the study. After a full
Physical characteristics, 1-RM performance data, and daily dietary intake
The physical characteristics, 1-RM performance data, and daily dietary intake of the research subjects are shown in Table 1. The 4 groups did not significantly differ with respect to their physical characteristics and 1-RM performance data. Thus, the groups were matched for these variables. Nutritional analysis of daily dietary intake revealed that diets were consistent between groups. The macronutrient composition of the dietary intake was also similar between groups.
Glucose
Both the PLA and EAA
Discussion
The primary findings from this investigation were that resistance exercise performed in conjunction with liquid CHO ingestion significantly affected the immediate hormonal response to exercise, resulting in significantly elevated insulin levels and a significant decrease in cortisol concentration during and after the exercise bout. Moreover, the addition of EAA (CHO + EAA) potentiated these responses. This altered hormonal response was associated with attenuated myofibrillar protein degradation
Acknowledgment
The authors thank the dedicated group of subjects that participated in this project. We also thank Brian Heffernan and Matthew O'Neal at the Central West Pathology Service, Bathurst Base Hospital NSW, Australia, and Dr Gary Ma, Trudi Jones, and Vicki Pitsiavas at the Institute of Clinical Pathology and Medical Research, Laboratory of Endocrinology, Westmead Hospital NSW, Australia, for technical assistance with the immunoassay procedures. We would like to thank Jason Poposki for
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