The results from the present study indicated that there were no differences in CK, LDH, DOMS, and squat repetitions to fatigue in subjects consuming pre and postexercise carbohydrate beverages with or without the addition of leucine. This is the first study that has attempted to isolate the effects of adding leucine to a carbohydrate beverage on recovery from lower-body resistance exercise. This experiment is also unique in that we monitored dietary intake for up to 72 hours postexercise. Within the context of our study design, the addition of 45 mg·kg−1 leucine split into pre and postexercise carbohydrate beverages did not reduce blood markers of exercise-induced muscle damage or reduce DOMS.
In the present investigation, the sole intervention was the addition of leucine to pre and postexercise carbohydrate beverages. It is interesting to note that of the nutrient intervention studies that have found improvements in performance or selected markers of muscle damage with amino acid supplementation, positive results have been found when the supplementation period lasts several days before or after exercise (7,8,18,23). For example, Nosaka et al. (23) demonstrated that when subjects consumed a supplement containing 12 amino acids or placebo 30 minutes before and immediately after exercise, no significant differences were found between groups. However, when supplementation continued over a 4-day period, CK, aldolase, myoglobin, and muscle soreness were significantly lower for the amino acid condition. Furthermore, the optimal dose of leucine needed to improve exercise performance or recovery in athletes is unclear. In the present investigation, a dose of 45 mg·kg−1 was used and split into pre and postexercise feedings based upon the positive findings reported by Crowe et al. (8). However, other leucine studies have used doses ranging from 50 mg·kg−1 (21) to 200 mg·kg−1 (24).
A secondary purpose of the present investigation was to determine the effects of adding leucine to pre and postexercise carbohydrate beverages on squat repetitions to fatigue both acutely and 72 hours later. As hypothesized, there was no difference between groups during the initial workout when subjects consumed their respective beverage 30 minutes before exercise, and to our knowledge, no study has demonstrated acute performance improvements from BCAA or leucine supplementation. Kraemer et al. (18) reported that subjects consuming amino acids during an overreaching program were able to maintain strength and displayed reduced CK, indicating a potential relationship between muscle damage and performance in resistance trained subjects. Based on the potential relationship between reduced CK and force production, it was our hypothesis that the subjects consuming a leucine-carbohydrate beverage before and after resistance training would display reduced serum levels of CK and LDH compared to subjects consuming a carbohydrate beverage, which would allow for subjects to perform more repetitions of the squat exercise during a subsequent training bout. Our results indicated that the ability to perform repeated repetitions of the squat exercise declined independent of the beverages consumed. Additionally, before performing the second bout of squats to fatigue, both groups reported significant DOMS, suggesting that 72 hours may not be a sufficient time period to recover from 6 sets of squats to fatigue even in resistance trained individuals.
In the present investigation, the subjects were instructed to maintain their normal ad libitum diet but avoid protein powders, drinks, and bars. Interestingly, the results from the present study indicated that the leucine content of the diet accounted for approximately 13% of the variance in CK (Figure 6), whereas the protein intake was not significantly associated with CK levels postexercise (Figure 5). It is important to note that although there were no significant between-group differences for any of the nutrients analyzed, the average daily protein intakes for the LCHO and CHO groups were 1.62 and 1.65 g·kg−1, respectively, which evidence has shown is sufficient to preserve positive nitrogen balance in strength athletes (32). Differing from our study design, Coombes and McNaughton (7) tested the hypothesis that BCAA supplementation would improve muscle recovery even in conjunction with a high protein diet. To do so, the authors performed daily food recalls. In the event that a subject's protein and BCAA intake for that specific day were too low, the subject was instructed to drink additional milk. The authors reported that BCAA supplementation decreased serum CK and LDH after endurance exercise even when the subjects consumed a diet rich in BCAA. With the contrasting results of the present investigation and those of Coombes and McNaughton (7), future studies should investigate the amounts of leucine and BCAA in the diet, and meal timing and frequency, needed to optimize recovery from resistance training.
As with all research studies, these findings are context specific. It is important to acknowledge the potential limitations of the present investigation. Although CK is frequently used as an indirect measure of muscle damage after exercise, a large amount of variability existed between subjects postexercise in the present study. As recently noted by Green et al. (12), a significant factor in comparing the results of exercise-induced muscle damage studies with an intervention as the independent variable is the authors' decision to use a crossover design or a between-subjects design. Using a crossover design has the advantage of allowing subjects to serve as their own controls and, therefore, significantly reduces variability. However, the use of a crossover design allows subjects to perform an activity multiple times, and it is therefore unknown to what extent the results are because of a true treatment effect vs. a consequence of the repeated bout effect. Although the subjects in the present investigation were resistance trained, the high volume and intensity of the squat protocol resulted in significantly increased CK levels and DOMS in both groups. Additionally, it should be noted that of the 3 women recruited for the present investigation, 2 were in the LCHO group and 1 was in the CHO group, and exclusion of these subjects from data analysis does not significantly change the results of any of the dependent variables. In regards to changes in CK after exercise, 2 studies have reported no sex differences (9,30), whereas 2 other studies (29,31) have reported that women demonstrate lower CK responses before and after eccentric exercise. Studies investigating DOMS after exercise have not found sex differences (9,25,29). Thus, when designing the study, it was our decision to allow both men and women to volunteer for the present investigation under the contention that all subjects were healthy, resistance trained, and familiar with the squat exercise. However, the high degree of variability in CK found in the present study suggests that these results must be interpreted with caution.
In summary, these results do not provide support for adding leucine to carbohydrate beverages before and after resistance training within the context of this study. Although leucine has been shown to play an important role in augmenting muscle protein synthesis after exercise (1,2), the addition of 45 mg·kg−1 leucine divided into pre and postexercise carbohydrate beverages was insufficient at reducing acute exercise-induced muscle damage and DOMS in the current study. When combined with other studies (7,8,18,23), the results of the present investigation suggested that future studies should investigate higher doses of leucine on muscle recovery after exercise, or do so using a longer supplementation period (i.e., days or weeks).
Allowing proper recovery between workouts is an important consideration when designing resistance training programs. It is in the best interest of coaches to consider methods of enhancing muscle recovery after resistance training. Although past studies have shown improvement in performance (8) and muscle recovery (3,6,7) after exercise with amino acid supplementation, the results of the present study suggested that blood CK and LDH, DOMS, and squat performance were not improved after resistance training by adding leucine to pre and postexercise carbohydrate beverages. It should be noted, however, that these findings are context specific, and leucine's effects on muscle protein synthesis and cell signaling (1,2) suggest further studies are needed to investigate the optimal dose of leucine, and meal timing and frequency, needed to optimize recovery from resistance training.
We would like to thank Dr. Deborah Keil, Janis Glatzel, and Missi Brown for their assistance with data collection. Financial support for this study was awarded through a UNLV Graduate and Professional Student Association Grant. The results of the present study do not constitute endorsement by the authors or the National Strength and Conditioning Association.
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