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Effect of a Pre-Exercise Energy Supplement on the Acute Hormonal Response to Resistance Exercise

Hoffman, Jay R; Ratamess, Nicholas A; Ross, Ryan; Shanklin, Miles; Kang, Jie; Faigenbaum, Avery D

Journal of Strength and Conditioning Research: May 2008 - Volume 22 - Issue 3 - p 874-882
doi: 10.1519/JSC.0b013e31816d5db6
Original Research
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The effect of a pre-exercise energy sport drink on the acute hormonal response to resistance exercise was examined in eight experienced resistance trained men. Subjects were randomly provided either a placebo (P: maltodextrin) or the supplement (S: combination of branched chain amino acids, creatine, taurine, caffeine, and glucuronolactone). Subjects performed 6 sets of no more than 10 repetitions of the squat exercise at 75% of their 1 repetition maximum (1RM) with 2 minutes of rest between sets. Blood draws occurred at baseline pre-exercise, immediately post- (IP), 15 minutes post- (15P), and 30-minutes post (30P) exercise for measurement of serum growth hormone, total and free testosterone, cortisol, and insulin concentrations. Although significant differences were seen only at set 5, the total number of repetitions and training volume tended (p = 0.08) to be higher with S compared to P. Serum growth hormone and insulin concentrations were significantly higher at 15P and IP, respectively, in S compared to P. Results suggest that a pre-exercise energy S consumed 10 minutes before resistance exercise can enhance acute exercise performance by increasing the number of repetitions performed and the total volume of exercise. The enhanced exercise performance resulted in a significantly greater increase in both growth hormone and insulin concentrations, indicating an augmented anabolic hormone response to this pre-exercise S.

Department of Health and Exercise Science, College of New Jersey, Ewing, New Jersey

Address correspondence to Jay R. Hoffman, hoffmanj@tcnj.edu.

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Introduction

The important role that essential amino acids have in increasing protein synthesis is well acknowledged. It has been shown that resistance exercise can sensitize skeletal muscle to amino acid ingestion by enhancing the magnitude of protein synthesis after exercise sessions (34). To maximize the effect of the resistance training session, the timing of amino acid or protein intake appears to be of importance. Greater stimulation of net muscle protein balance is seen when essential amino acids are provided immediately before or within 1 hour of resistance exercise (6). Branched-chain amino acids (BCAAs), composed of leucine, isoleucine, and valine, account for 35-40% of the essential amino acids in the diet (29). In addition to enhancing protein synthesis, the BCAAs are reported to reduce muscle soreness and muscle fatigue after resistance training exercise protocols (30).

Creatine is an amino acid derivative synthesized from arginine, glycine, and methionine in the liver, kidneys, and pancreas. It has been shown to be effective in enhancing strength and power performance and increasing lean body mass (25,31,33). Most studies examining creatine supplementation generally employed a week-long loading phase (20 g·d−1) followed by daily maintenance doses (5 g·d−1). However, dosing protocols incorporating only a loading dose have also been shown to be efficacious in eliciting strength and power performance improvements during short-duration (1 week) training studies (9,11).

Though the efficacy of these 2 supplements has been well established, the combination of creatine and BCAAs has not been extensively examined. Recently, the sport supplement industry has begun to market these nutritional supplements in combination as high energy drinks with the addition of taurine, caffeine, and glucuronolactone to enhance endurance, anaerobic exercise performance, and alertness (1,27). They have also been shown to be the most frequently used supplements by American collegiate athletes (12). The combination of these ingredients consumed before a resistance exercise session may provide an enhanced stimulus to the acute bout of exercise. To date, studies examining the effect of a stimulant (primarily caffeine) during anaerobic exercise performance have been unable to demonstrate any consistent efficacy (20). However, a recent study has suggested that when stimulants or energy compounds are provided in combination, the ergogenic effect may be enhanced (18). Thus, it was the purpose of this study to examine the effect of the supplement combination (BCAAs, creatine, caffeine, taurine, and glucuronolactone) on the quality of a resistance exercise session as reflected by training volume and its subsequent effect on the acute hormonal response to this exercise and supplement stimulus.

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Methods

Experimental Approach to the Problem

This study was conducted using a double-blind, crossover design and involved 8 experienced resistance-trained college-age males. Subjects reported to the Human Performance Laboratory (HPL) on 5 separate occasions. During the initial visit, subjects performed a maximal strength test (1 repetition maximum (1RM)) with the squat exercise. In the second and third visits (72 hours between each visit), subjects performed familiarization sessions with the exercise protocol (6 sets of up to 10 repetitions with 75% of their 1RM with and a 2-minute rest between sets). Subjects then began a 1-week creatine-loading phase (20 g·d−1) in which they were permitted to maintain their normal training routine. Subjects then returned to the HPL for their final 2 exercise sessions. Upon arrival to the laboratory, a cannula was placed in superficial forearm vein. Subjects were then provided either a supplement (S) or a placebo (P) in randomized order. Ten minutes after consumption of the pre-exercise drink, subjects began the experimental protocol. Blood draws were obtained pre-exercise (PRE), immediately post-exercise (IP), and 15 (15P) and 30 (30P) minutes post-exercise

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Subjects

Eight experienced resistance-trained college-age males (age, 20.3 ± 1.6 years; height, 183.4 ± 5.5 cm; weight, 91.8 ± 5.7 kg; ≥3 years of resistance training experience) volunteered for this study. Following an explanation of all procedures, risks, and benefits, each subject gave his informed consent before participation in this study. The Institutional Review Board of the college approved the research protocol. Subjects were not permitted to use any additional nutritional supplementation and did not consume anabolic steroids or any other anabolic agents known to increase performance. Screening for steroid use and additional supplementation was accomplished via a health questionnaire completed during subject recruitment.

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Testing Protocol

The investigation was performed as a double-blind, randomized, crossover design. Subjects initially reported to the HPL for maximal strength testing (a 1RM) on the free weight squat exercise. Following 1RM testing, subjects reported back to the HPL to begin 3 familiarization sessions with the exercise protocol. At least 3 days of rest was provided between each familiarization session. Subjects then began a 1-week creatine-loading phase (20 g·d−1) in which they were permitted to maintain their normal training routine, after which they returned to the HPL for the 2 experimental sessions. Creatine consumption using this loading dose continued during the experimental sessions. Because creatine is ergogenic, the loading phase was employed to negate the acute effects of creatine supplementation.

On the days of the experimental testing sessions, the subjects arrived at the HPL after an overnight fast. After the initial baseline blood draw, after a 15-minute equilibrium period, subjects were randomly provided either P (maltodextrin) or S drink. A 10-minute rest period was provided from the time the subjects consumed the drink until they began warming up for the resistance exercise protocol. The warm-up consisted of 5 minutes of light stationary cycling at a self-selected cadence and 5 minutes of light stretching and 2-3 light to moderate sets of the squat exercise. Thus, subjects initiated the resistance exercise protocol 20 minutes after consumption of the S or P. During each experimental session, subjects performed 6 sets of the squat exercise with a load that was equivalent to 75% of their measured 1RM. A 2-minute rest period was provided between each set. Subjects were encouraged to perform up to 10 repetitions per set. Subjects returned 3 days later and were provided the other condition and repeated the exercise protocol. Repetitions not completed in the full range of motion were not counted. The volume of each set was calculated as the number of complete repetitions completed × resistance used. All experimental testing sessions occurred at the same time of day. The experimental design is depicted in Figure 1.

Figure 1

Figure 1

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Maximal Strength Testing

The 1RM squat test was performed using methods previously described by Hoffman (16). Each subject performed a warm-up set using a resistance that was approximately 40-60% of his perceived maximum and then performed 3-4 subsequent trials to determine the 1RM. A 3- to 5-minute rest period was provided between each trial. The squat exercise required the subject to place an Olympic bar across the trapezius muscle at a self-selected location. Each subject descended to the parallel position, which was attained when the greater trochanter of the femur reached the same level as the knee. The subject then ascended until full knee extension. Trials not meeting the range of motion criteria were discarded.

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Blood Measurements

During each experimental session, blood samples were obtained PRE, IP, 15P, and 30P. All blood samples were obtained using a 20-gauge Teflon cannula placed in a superficial forearm vein. The cannula was maintained patent using an isotonic saline solution (with 10% heparin) placed in a 3-way stopcock with a male luer lock adapter. PRE blood samples were drawn after a 15-minute equilibration period before exercise. IP blood samples were taken within 15 seconds of exercise cessation. All blood samples were drawn with a plastic syringe while the subject was in a seated position. Following the resistance exercise protocol, subjects were seated and remained seated for the full 30-minute recovery phase.

Blood samples were collected into 2 Vacutainer tubes, one containing SST gel and clot activator and the second containing ethylenediamine tetraacetic acid. A small aliquot of whole blood was removed from the second tube and used for microcapillary determination of hematocrit. The remaining blood in that tube was used for hemoglobin analysis. The blood in the first tube was allowed to clot at room temperature and subsequently centrifuged at 1500g for 15 minutes. The resulting serum was placed into separate 1.8-mL microcentrifuge tubes and frozen at −80°C for later analysis.

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Biochemical and Hormonal Analyses

Serum testosterone, growth hormone, cortisol, free testosterone, and insulin concentrations were determined using enzyme immunoassays and enzyme-linked immunosorbent assays (Diagnostic Systems Laboratories, Webster, TX and Alpco Diagnostics, Salem, NH, respectively). Determinations of serum immunoreactivity values were made using a SpectraMax340 Spectrophotometer (Molecular Devices, Sunnyvale, CA). To eliminate interassay variance, all samples for a particular assay were thawed once and analyzed in the same assay run. All samples were run in duplicate with a mean intra-assay variance of <10%. The detection limits of the testosterone, growth hormone, cortisol, free testosterone, and insulin assays were 0.14 nmol·L−1, 0.03 ng·mL−1, 2.76 nmol·L−1, 0.66 pmol·L−1, and 1.87 pmol·L−1, respectively.

Hemoglobin was analyzed in triplicate from whole blood using the cyanmethemoglobin method (Sigma Diagnostics, St. Louis, MO). Hematocrit was analyzed in triplicate from whole blood via microcentrifugation (Micro-MB centrifuge; IEC, Needham, MA) and microcapillary technique. Plasma volume shifts after the workout were calculated using the formula of Dill and Costill (10). Serum lactate and glucose concentrations were determined with an Analox GM7 enzymatic metabolite analyzer (Analox Instruments USA, Lunenburg, MA).

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Supplement Composition

S is commercially marketed as Amino Shooters (Champion Nutrition Inc., Concord, CA) and consisted of 19 g of a powder containing essential BCAAs (3000 mg L-leucine, 1100 mg L-isoleucine, and 1100 mg valine), essential amino acids (1100 mg L-lysine, 300 mg L-methionine, 1100 mg L-phenylalanine, 700 mg histidine, 1100 mg L-threonine), 5000 mg creatine monohydrate, 1500 mg L-taurine, 350 mg glucuronolactone, and 110 mg of caffeine) and mixing it with 500 mL of water. The nutritional composition per serving of the supplement was 45 calories with 0 g of fat and 0 g of carbohydrate. P consisted of an equivalent amount of maltodextrin mixed with water. The nutritional composition per serving of P was 60 calories with 14.9 g of carbohydrates and 0 g of both fat and protein. Both drinks were fruit punch flavor and indistinguishable in appearance and taste.

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Statistical Analyses

Statistical evaluation of the data was accomplished by a repeated-measures analysis of variance. In the event of a significant F ratio, least significant difference post hoc tests were used for pairwise comparisons. In addition, dependent t-tests were used to analyze the area under curve (AUC) for hormonal measurements, which were calculated by a standard trapezoidal technique. Dependent t-tests were also used to analyze total repetitions and total training volume performed during the exercise protocols. A criterion α level of p ≤ 0.05 was used to determine statistical significance. All data are reported as mean ± SD.

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Results

The number of repetitions completed per set and the volume of exercise per set are shown in Figures 2 and 3, respectively. A significant increase in the number of repetitions and volume of exercise was seen with S compared to P in set 5 of the exercise protocol. No other significant differences between sets were observed. The total number of repetitions performed during the exercise protocol tended to be higher (p = 0.08) with S (51.0 ± 5.0 repetitions) than with P (47.9 ± 8.2 repetitions). Similarly, the total volume of exercise also tended to be higher (p = 0.09) with S (15,939 ± 1524 kg) than with P (14,927 ± 2415 kg) as well.

Figure 2

Figure 2

Figure 3

Figure 3

No significant increases from PRE were seen in glucose concentrations for both S (5.30 ± 0.40 mmol·L−1) and P (5.50 ± 1.12 mmol·L−1). Lactate concentrations were significantly increased IP for both S (17.8 ± 5.7 mmol·L−1) and P (16.5 ± 4.7 mmol·L−1), and remained increased from PRE throughout the 30-minute recovery period, but no significant group differences were observed.

The acute growth hormone response to the exercise protocol can be seen in Figure 4A. Significant increases from PRE were seen at both 15P and 30P for S, while a significant increase from PRE was seen at 30P for P. The increase in growth hormone at 15P was significantly greater for S compared to P. No other significant between group differences were noted. AUC analysis for growth hormone concentrations (Figure 4B) revealed a significant difference between the groups.

Figure 4

Figure 4

Significant 25% and 37% increases from PRE were seen in total testosterone concentration at both IP and 15P, respectively for S. A 10% increase was seen at these same time points with P, but these increases were not significantly different from PRE. In addition, no significant differences were seen between S and P at any time point. AUC analysis for total testosterone revealed no significant between-group differences. No significant differences from PRE were seen in free testosterone in either experimental group. In addition, no between-group differences were seen at any time point. Figure 5A shows the response of free testosterone to the exercise protocol. AUC analysis also failed to demonstrate any significant difference in free testosterone between exercise sessions (Figure 5B).

Figure 5

Figure 5

Figure 6A shows the response of cortisol to the exercise protocol under both experimental conditions. A significant in free testosterone at 15P from PRE was seen with S, but no other changes from PRE were seen with S and no significant changes at any time point from PRE were seen with P. AUC analysis (Figure 6B) showed no difference between the groups.

Figure 6

Figure 6

The response of insulin to the experimental protocols can be seen in Figure 7A. No significant differences from PRE were seen with either S or P. However, insulin concentrations with S were significantly higher IP than with P. Figure 7B depicts the AUC analysis for insulin concentration. No significant difference was noted between S and P.

Figure 7

Figure 7

Plasma volumes decreased −29.1 ± 11.0% and −22.1 ± 9.8% IP with S and P, respectively. However, the difference between these groups was not significant. Average plasma volume shifts at 15P and 30P ranged from −0.3 ± 8.1% to −11.9 ± 3.7% with S and P, but no significant differences between the groups were observed. However, blood variables were not corrected for plasma volume shifts due to the importance of molar exposure at the tissue level.

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Discussion

The results of this study indicate that a pre-exercise S containing a combination of BCAAs, creatine monohydrate, taurine, glucuronolactone, and caffeine does enhance performance during a resistance exercise training session as reflected by an increase in the number of repetitions performed and training volume. Considering that acutely ingested BCAAs and creatine are not known to have an effect on acute exercise performance, the mechanism stimulating the enhanced training session is likely the result of the high energy compounds (e.g., taurine, glucuronolactone, and caffeine) found in the S. These compounds are typically marketed as sports energy drinks, and this study appears to be one of the first investigations to demonstrate the efficacy of these Ss during a heavy resistance exercise session.

The S used in this study was developed to enhance acute resistance training performance. The inclusion of creatine monohydrate and BCAAs in the S was to enhance muscle recovery (i.e., BCAAs) from acute exercise stress and the maintenance of high muscle creatine levels. The amount of creatine found in this S is the equivalent of a typical maintenance dose. To ensure that all subjects had maximized muscle creatine stores, subjects were required to creatine load for 1 week before the experimental sessions. Subjects continued to supplement with creatine (maintaining the loading phase regimen) until they completed both experimental sessions (within 5 days). As a result, any differences seen between experimental sessions are not considered to be influenced by the creatine content of the S.

The physiological importance of BCAAs in regards to an acute training session is limited. Although BCAAs are reported to reduce muscle soreness and muscle fatigue after resistance training exercise protocols (30), this appears to be only effective after several weeks of supplementation (24). There are no studies to date that have demonstrated acute performance improvements from BCAA supplementation during an acute resistance training session. Thus, the performance improvements seen in this study appear to be attributed to the other compounds associated with this S.

Previous studies have shown that taurine ingestion can improve endurance performance (27,35); however, the ability of taurine supplementation to enhance resistance training performance has not been examined. Although in laboratory studies using mammalian skeletal muscle, taurine has been shown to enhance force production in skinned fast-twitch fibers (3,15), these studies have not been conducted on humans. Furthermore, previous studies examining the efficacy of taurine ingestion in humans have used longer duration study protocols, and no studies appear to have been conducted focusing on the acute ingestion of taurine and its potential ergogenic benefits. This study appears to be the first to provide evidence suggesting that acute ingestion of taurine when combined with caffeine and glucuronolactone may improve resistance training performance.

The role that caffeine has in enhancing resistance training performance is not well understood. Some have suggested that caffeine can enhance force and power production through enhanced calcium release from the sarcoplasmic reticulum (26), while others have suggested that caffeine can increase catecholamine secretion (8). Although several investigations were unable to see any improvements in power performance after caffeine ingestion during a Wingate anaerobic power test (5,18), other studies have demonstrated improved isometric force production (21) and 6-second maximal cycle ergometer sprints (2) from caffeine supplementation. Recently, Beck et al. (4) demonstrated that acute caffeine ingestion resulted in significant improvements in maximal upper body strength, but no improvements in maximal lower body strength or training volume for both modes of exercise were found.

No studies appear to have examined the effect of glucuronolactone ingestion on exercise performance. However, when ingested with taurine and caffeine, the combined S has been shown to improve cognitive function, alertness, and physical performance (1). This study appears to be the first examination of a S marketed as an energy drink designed to enhance resistance training performance. The individual ingredients of this S have either not been examined or have shown inconsistent results as it relates to anaerobic exercise performance. However, the significantly greater number of repetitions performed and total volume of training for set 5 and the trend seen toward a greater number of repetitions (p = 0.08) performed and total volume of training (p = 0.09) during the workout indicate that the combination of these ingredients appears to provide an effective stimulus in improving acute resistance exercise performance. The improvement in training volume also appears to be reflected in the hormonal response to the exercise protocol. A greater anabolic hormone response (e.g., testosterone, growth hormone, and insulin) could have important implications in the repair and recovery of skeletal muscle after resistance exercise sessions and subsequently play a vital role in the muscle remodeling.

The significantly greater growth hormone response to the exercise protocol is similar to that of other studies that demonstrated the importance of training volume on growth hormone increases (17,22). Growth hormone secretion patterns have been shown to be responsive to changes in acid-base balance of muscle (14). Although lactate concentrations IP were not significantly different, the 8% difference seen between S and P IP (p = 0.07) and the 9% difference (p = 0.11) seen in the AUC analysis does suggest a trend toward a greater metabolic strain associated with the higher training volume seen with S.

The response of testosterone in this study is similar to that found in other investigations that showed that differences in training volume can influence the total testosterone response to exercise (22). However, the lack of any significant difference in total testosterone concentrations between S and P may be the result of the pre-exercise S as most studies examining the acute hormonal response to resistance exercise are conducted with subjects who initially fasted. Amino acid ingestion before exercise has been shown to attenuate the testosterone response to a resistance exercise session (7,13,19). Protein supplementation before a workout may result in a decrease in secretion rates or an increase in metabolic clearance rates (19). It has also been suggested that pre-exercise protein intake may lead to an increase in testosterone uptake by cells during exercise (32). These mechanisms may have contributed to a possible reduction in the magnitude of the total testosterone response seen during this study and perhaps also explain the free testosterone response observed during the immediate and post-exercise recovery periods.

Most studies show elevated concentrations of cortisol during an acute resistance exercise session (24). Increases in cortisol with S appear to reflect the higher training volumes seen during this training session. This is supported by other studies showing the sensitivity of cortisol to increases in training volume (28). Although no differences in cortisol concentrations were observed between S and P, it has been suggested that carbohydrate ingestion could reduce the cortisol response to exercise due to a lower demand for gluconeogenesis (23). The placebo was composed of 14.9 g of carbohydrate, which may have attenuated the cortisol response during that training session.

Insulin concentrations tend to parallel changes in blood glucose and amino acid concentrations (23). Insulin has generally been seen to decrease during an acute bout of resistance exercise (23); however, when a protein S is consumed before exercise, insulin concentrations have been shown to be increased during the post-exercise period (19). Increases in insulin concentrations seen IP with S appeared to reflect the amino acid content of the supplement.

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Practical Applications

In conclusion, the use of a pre-exercise energy S containing taurine, caffeine, and glucuronolactone consumed 10 minutes before resistance exercise appears to provide an enhanced training response that is reflected by increases in the number of repetitions performed and training volume. These changes appear to result in greater increases in growth hormone and insulin, indicating an augmented anabolic hormone response to such supplementation.

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Acknowledgments

This study was supported by a grant from Champion Nutrition, Concord, CA.

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References

1. Alford, C, Cox, H, and Wescott, R. The effects of red bull energy on human performance and mood. Amino Acids 21: 139-150, 2001.
2. Amselme, F, Collomp Mercier, KB, Ahmaidi, S, and Prefaut, C. Caffeine increases maximal anaerobic power and blood lactate concentration. Eur J Appl Physiol 65: 188-191, 1992.
3. Bakker, AJ and Berg, HM. Effect of taurine on sarcoplasmic reticulum function and force in skinned fast-twitch skeletal muscle fibres of the rat. J Physiol 538: 185-194, 2002.
4. Beck, TW, Housh, TJ, Schmidt, RJ, Johnson, GO, Housh, DJ, Coburn, JW, and Malek, MH. The acute effects of a caffeine-containing supplement on strength, muscular endurance, and anaerobic capabilities. J Strength Cond Res 20: 506-510, 2006.
5. Bell, DG, Jacobs, I, and Ellerington, K. Effect of caffeine and ephedrine ingestion on anaerobic exercise performance. Med Sci Sports Exerc 33: 1399-1403, 2001.
6. Borsheim, E, Tipton, KD, Wolfe, SE, and Wolfe, RR. Essential amino acids and muscle protein recovery from resistance exercise. Am J Physiol Endocrinol Metab 283: E648-E657, 2002.
7. Chandler, RM, Byrne, HK, Patterson, JG, and Ivy, JL. Dietary supplements affect the anabolic hormones after weight-training exercise. J Appl Physiol 76: 839-845, 1994.
8. Costill, DL, Dalsky, GP, and Fink, WJ. Effects of caffeine ingestion on metabolism and exercise performance. Med Sci Sports Exerc 10: 155-158, 1978.
9. Cottrell, GT, Coast, JR, and Herb, RA. Effect of recovery interval on multiple-bout sprint cycling performance after acute creatine supplementation. J Strength Cond Res 16: 109-116, 2002.
10. Dill, DB and Costill, DL. Calculation of percentage changes in volume of blood, plasma and red cells in dehydration. J Appl Physiol 37: 247-248, 1974.
11. Eckerson, JM, Stout, JR, Moore, GA, Stone, NJ, Nishimura, K, and Tamura, K. Effect of two and five days of creatine loading on anaerobic working capacity in women. J Strength Cond Res 18: 168-172, 2004.
12. Froiland, K, Koszewski, W, Hingst, J, and Kopecky, L. Nutritional supplement use among college athletes and their sources of information. Int J Sport Nutr Exerc Metab 14: 104-120, 2004.
13. Fry, AC, Kraemer, WJ, Stone, MH, Warren, BJ, Kearney, JT, Maresh, CM, Weseman, CA, and Fleck, SJ. Endocrine and performance responses to high volume training and amino acid supplementation in elite junior weightlifters. Int J Sport Nutr 3: 306-322, 1993.
14. Gordon, SE, Kraemer, WJ, Vos, NH, Lynch, JM, and Knuttgen, HG. Effect of acid-base balance on the growth hormone response to acute high-intensity cycle exercise. J Appl Physiol 76: 821-829, 1994.
15. Hamilton, EJ, Berg, HM, Easton, CJ, and Bakker, AJ. The effect of taurine depletion on the contractile properties and fatigue in fast-twitch skeletal muscle of the mouse. Amino Acids 31: 273-278, 2006.
16. Hoffman, JR. Norms for Fitness, Performance and Health. Champaign, Ill: Human Kinetics; 2006. pp. 27-40.
17. Hoffman, JR, Im, J, Rundell, KW, Kang, J, Nioka, S, Spiering, BA, Kime, R, and Chance, B. Effect of muscle oxygenation during resistance exercise on anabolic hormone response. Med Sci Sports Exerc 35: 1929-1934, 2003.
18. Hoffman, JR, Kang, J, Ratamess, NA, Jennings, PF, Mangine, G, and Faigenbaum, AD. Effect of nutritionally enriched coffee consumption on aerobic and anaerobic exercise performance. J Strength Cond Res 21: 456-459, 2007.
19. Hulmi, JJ, Volek, JS, Selanne, H, and Mero, AA. Protein ingestion prior to strength exercise affects blood hormones and metabolism. Med Sci Sports Exerc 37: 1990-1997, 2005.
20. Kalmar, JM. The influence of caffeine on voluntary muscle activation. Med Sci Sports Exerc 37: 2113-2119, 2005.
21. Kalmar, JM and Cafarrelli, E. Effects of caffeine on neuromuscular function. J Appl Physiol 87: 801-808, 1999.
22. Kraemer WJ, Marchitelli L, Gordon S, Harmon E, Dziados J, Mello R, Frykman P, McCurry, D, and Fleck, S. Hormonal and growth factor responses to heavy resistance exercise protocols. J Appl Physiol 69: 1442-1450, 1990.
23. Kraemer, WJ and Ratamess NA. Hormonal responses and adaptations to resistance exercise and training. Sports Med 35: 339-361, 2005.
24. Kraemer, WJ, Ratamess, NA, Volek, JS, Häkkinen, K, Rubin, MR, French, DN, Gómez, AL, McGuigan, MR, Scheett, TP, Newton, RU, Spiering, BA, Izquierdo, M, and Dioguardi, FS. The effects of amino acid supplementation on hormonal responses to resistance training overreaching. Metab Clin Exp 55: 282-291, 2006.
25. Kreider, RB, Ferreira, M, Wilson, M, Grindstaff, P, Plisk, S, Reinhardy, J, Cantler, E, and Alamada, AL. Effects of creatine supplementation on body composition, strength, and sprint performance. Med Sci Sports Exerc 30: 73-82, 1998.
26. Lopes, JM, Aubier, M, Jardim, J, Aranda, JV, and Macklem, PT. Effect of caffeine on neuromuscular function. J Appl Physiol 54: 1303-1305, 1983.
27. Miyazaki, T, Matsuzaki, Y, Ikegami, T, Miyakawa, S, Doy, M, Tanaka, N, and Bouscarel, B. Optimal and effective oral doses of taurine to prolong exercise performance in rat. Amino Acids 27: 291-298, 2004.
28. Ratamess, NA, Kraemer, WJ, Volek, JS, Maresh, CM, Vanheest, JL, Sharman, MJ, Rubin, MR, French, DN, Vescovi, JD, Silvestre, R, Hatfield, DL, Fleck, SJ, and Deschenes, MR. Androgen receptor content following heavy resistance exercise in men. J Steroid Biochem Mol Biol 93: 35-42, 2005.
29. Riazi, R, Wykes, LJ, Ball, RO, and Pencharz, PB. The total branched-chain amino acid requirement in young healthy adult men determined by indicator amino acid oxidation by use of L-(1-13C) phenylalanine. J Nutr 133: 1383-1389, 2003.
30. Shimomura, Y, Yamamoto, Y, Bajotto, G, Sato, J, Murakami, T, Shimomura, N, Kobayashi, H, and Mawatan, K. Nutraceutical effects of branched-chain amino acids on skeletal muscle. J Nutr 136: 529S-532S, 2006.
31. Stone, MH, Sanborn, K, Smith, LL, O'Bryant, HS, Hoke, T, Utte, r AC, Johnson, RL, Boros, R, Hruby, J, Pierce, KC, Stone, ME, and Garner, B. Effects of in-season (5-weeks) creatine and pyruvate supplementation on anaerobic performance and body composition in American football players. Int J Sport Nutr 9: 146-165, 1999.
32. Volek, S. Diet and hormonal responses: potential impact on body composition. In: The Endocrine System in Sports and Exercise. Kraemer, WJ and Rogol, AD, eds. Oxford: Blackwell Publishing; 2005. pp. 426-433.
33. Volek, JS, Duncan, ND, Mazzetti, SA, Staron, RS, Putukian, M, Gomez, AL, Pearson, Fink, DR, and Kraemer, WJ. Performance and muscle fiber adaptations to creatine supplementation and heavy resistance training. Med Sci Sports Exerc 31: 1147-1156, 1999.
34. Wolfe, RR. Skeletal muscle protein metabolism and resistance exercise. J Nutr 136: 525S-528S, 2006.
35. Zhang, M, Izuma, I, Kagamimori, S, Sokejima, S, Yamagami, T, Liu, Z, and Qi, B. Role of taurine supplementation to prevent exercise-induced oxidative stress in healthy young men. Amino Acids 26: 203-207, 2004.
Keywords:

amino acids; endocrine; ergogenic aid; strength; athletic performance

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