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The Effectiveness of a Pre-Exercise Performance Drink (PRX) on Indices of Maximal Cardiorespiratory Fitness


This study examined the effectiveness of a pre-exercise drink (PRX) called EM·PACT on indices of maximal cardiorespiratory fitness. Twenty-four males (n = 12) and females (n = 12) ages 18–24 years (20.25 + 1.42), volunteered as subjects. Each subject performed two randomized trials of a VO2max treadmill test within a week of each other. Subjects in this randomized, placebo controlled, counter balanced, crossover design, ingested either a placebo (water) or PRX 20 minutes before each exercise bout. VO2max and time to exhaustion (Time) during graded exercise testing were evaluated. Using paired samples t-tests, significantly greater mean values were found in VO2max and Time for the PRX trial compared to the placebo trial (p < .05). Results indicate that indices of cardiorespiratory fitness; specifically VO2max and Time are enhanced by ingestion of PRX prior to exercise testing. The combined results of this investigation may provide meaningful practical applications for coaches and athletes alike regarding ergogenic hydration options.


The human body's maximal ability to use or consume oxygen for aerobic metabolism during exercise, better known as VO2max, is an important predictor of athletic performance in endurance activities [1]. In addition, ventilatory threshold or the nonlinear increase in ventilation that coincides with the beginning of glycolysis (anaerobic threshold) for energy and the onset of blood lactate is considered by many exercise physiologists to be even a better indication of an endurance athlete's capacity for aerobic power [2]. This is particularly true when examining the metabolic demands of middle distance runners and other similar athletes in their respective sports. Either way, the ability of an individual to reduce or tolerate more lactate production or the metabolic end product caused by the excessive metabolism of carbohydrates (CHO) is an important factor in the performance of endurance athletes as well as other sports that rely heavily upon aerobic metabolic pathways [3].

Previously, research has demonstrated that CHO ingestion during aerobic exercise can improve performance during exercise sessions lasting longer that 90 minutes performed at intensities greater than 70% VO2max by preventing a decline in blood glucose concentration and facilitating glucose oxidation late, whereas the timing and type of CHO ingestion following exercise influences muscle glycogen restoration[4, 5]. This information is especially important for endurance athletes since CHO type and blood glucose response is important in order to optimize CHO intake either pre or post exercise.

For example, CHO ingestion immediately prior to exercise has been reported to have a negative effect on exercise performance [6]. If an athlete consumes carbohydrate-rich foods or sport drinks within 60 minutes of the beginning of an endurance exercise performance, the glucose from the ingested food or drink enters the circulation within minutes of ingestion. The subsequent rise in blood glucose concentration causes the release of the hormone insulin, which assists in clearing glucose from the circulation. A peak in insulin concentration in the blood occurs at the time exercise begins. Consequently glucose uptake by the muscles reaches an abnormally high rate during the exercise performance. Therefore, the consumption of simple CHO, which are digested and absorbed quickly, can be detrimental to exercise performance [6].

This high rate of clearance glucose from the blood can potentially cause hypoglycemia which in turn can produce symptoms of acute fatigue. In summary, consuming high-glycemic CHO immediately before exercising causes blood glucose to rise rapidly (glycemic response) which may trigger excessive insulin release (insulinemic response) [7, 8].

In contrast, consuming low glycemic carbohydrate-rich foods (starch with high amylose content or moderate glycemic CHO with high dietary fiber content) in the immediate 45–60 minute pre-exercise period allows for slower glucose absorption, reducing the potential for rebound glycemic response. Typically, the optimal forms of CHO have been combinations of glucose, fructose, sucrose, and maltodextrins with or without protein or amino acids and it has been further suggested that the glycemic index of food may be a key determining factor for when food is ingested relative to exercise participation [914].

Currently there are many sport drinks that help the body replenish CHO levels during exercise including pre-exercise formulas whose purpose is to promote the sparing of CHO by facilitating fat substrate utilization during exercise. EM·PACT (Mannatech, Inc.) is an energy and endurance pre-exercise drink (PRX) purported to increase oxygen consumption, reduce lactate production, and improve fat utilization during aerobic activity. Although anecdotal and case study evidence exists giving merit to these claims, little or no laboratory evidence has been available to support the usage of the potentially performance enhancing product for aerobic or endurance performance.

The purpose of this study was to examine the effectiveness of a pre-exercise sport drink (PRX) on indices of maximal cardiorespiratory fitness. Specifically, VO2max and Time to Exhaustion (Time) during graded exercise testing were evaluated.



In this investigation, twelve male and twelve female college students (n = 24), ages 18–24 years (20.25 ± 1.42), volunteered as subjects. Subjects signed university-approved informed consent statements in compliance with the institution's research review board on the campus in which the study was conducted.

Study Design

Subjects involved in this study were asked to submit to "two" maximal oxygen consumption tests (VO2max) within a week of each other with at least 48 hours between trials. Subjects were required to perform each maximal effort exercise test on a motor-driven treadmill. In addition, expired lung gases were examined for the purpose of determining the amount of oxygen used during exercise for VO2max. Expired lung gases were collected by sampling air exhaled from the mouth into a mouthpiece connected to sampling hoses and gas analyzers (CPX system, Medgraphics, Maple Grove, MN). The exercise intensity began at a low level and was advanced every three minutes by increasing the speed and incline of the treadmill belt using Bruce protocol [15]. During the test, heart rate and time were measured continuously while blood pressure and ratings of perceived exertion were measured toward the end of each three minute stage. VO2max was considered to have been achieved if the subject met at least two of the following criteria: 1) an RER equal to or greater than 1.15 2) plateau of VO2 during the last stage of exercise 3) maximal heart rate within ± 10 beats per minutes of predicted values.

Prior to test participation, subjects were asked to adhere to the following pre-test instructions: 1) wear comfortable, loose-fitting clothing 2) drink plenty of fluids over the 24-hour period preceding the test 3) avoid food, tobacco, alcohol, and caffeine for 3 hours prior to taking the test 4) avoid exercise or strenuous physical activity the day of the test and 5) get an adequate amount of sleep (6 to 8 hours) the night before the test [15].

Testing Procedures

Each subject arrived thirty minutes prior to each exercise trial and was given either the recommended dosage (1 Tablespoon/18 g per 8 ounces/.24 L water) of PRX or a placebo (citrus flavored water) twenty minutes prior to test participation. Administration of PRX and placebo trials were randomized with half of the subjects ingesting the placebo during the first trial and PRX during their second trial with the order reversed for the remaining subjects. Total participation time for each test was approximately 1 hour. The PRX supplement (EM·PACT) was provided from Mannatech, Inc., Coppell, TX in sealed bottles and an open-label format with an unbiased assistant (not involved in the testing or data analysis) mixing the PRX and providing it to the subjects. Both the PRX and water placebo were provided by an assistant blinding both subjects and investigators as to the order in which water placebo or PRX was ingested. At the end of the study investigators were provided information as to the order in which the subjects were provided either the PRX or water placebo. EM·PACT is a citrus flavored 7.2% carbohydrate-electrolyte concentration energy and endurance pre-exercise drink containing a proprietary blend of the following ingredients (Total 18 g/dose): Energy and Endurance Complex Fructose (14.58 g), citric acid (1.55 g), silicon dioxide (.54), medium chain triglycerides (.25 g), creatine monohydrate (.19 g), calcium citrate (.16 g), magnesium aspartate (.13 g), magnesium succinate (.13 g), potassium aspartate (.13 g), potassium succinate (.13 g), lemon oil powder (.09 g), choline bitartrate (.05 g), L-carnitine (.05 g), Ambrotose (Aloe vera extract: Manapol), Powder (.03) and lecithin (.01 g).

Statistical Analysis

Data were analyzed with paired sample t-tests. VO2max (ml·kg-1·min-1) and Time (minutes) were the specific dependent variables examined in this study. In addition measures of effect size were also calculated. An alpha level of .05 was used in determining statistical significance. Statistical analyses were performed using SPSS for Windows version 12.0 statistical package (SPSS, Inc., Chicago, IL) [16]. Data are presented as means ± SE for water and PRX trials.


Initial results indicated significantly greater mean values in VO2max (ml·kg-1·min-1) for the PRX trial compared to the placebo (water) trial t(23) = 5.82, p < .001. It was also found that the PRX trial exhibited significantly higher mean values in time to exhaustion (minutes) when compared to the placebo trial t(23) = 4.47, p < .001. Overall differences in the various parameters are depicted in Figure 1. Changes in mean values among the reported variables are displayed in Table 1.

Figure 1

Figure 1

Table 1 Mean changes in various parameters


The main findings of this study were that indices of cardiorespiratory fitness, specifically VO2max, and time to exhaustion were significantly (p < .05) enhanced by ingestion of PRX prior to graded exercise testing. In particular, overall increases were observed in VO2max (15.5%) and time to exhaustion (8.7%). The results of this study also support the use of the PRX as examined in this investigation in tests of aerobic power as well as support earlier reports of ingesting a PRX consisting of low glycemic sugars (5–8%) before exercise [914].

Improvement of time to exhaustion claims also could possibly be substantiated as the data of this investigation support a recent study in which a mixture of CHO and medium-chain triglycerides (MCTs) resulted in increased aerobic function as marked by increases in length of time trials to exhaustion [17]. Marketing for the sports enthusiast hypes MCTs as fat burners, energy sources, glycogen sparers, and muscle builders. Although MCTs do not inhibit gastric emptying as does common fat, conflicting research supports the efficacy of using MCTs solely as a means of improving fat oxidation during exercise[18, 19] and because of its minimal contribution to the formula, the individual contribution of MCTs in improving performance is highly unlikely. However, subsequent research investigating possible metabolic and ergogenic effects of combining MCTs with CHO may hold promise. It was concluded that, during bouts of exercise requiring aerobic power, the combined results of this investigation provide meaningful practical applications for coaches and athletes alike regarding possible alternative hydration options.

Future research is warranted investigating blood lactate levels, fuel substrate utilization, gender differences, fitness levels, comparisons with other products, as well as use under various environmental and competitive conditions. In addition, further research is needed to determine if other ingredients or combinations of ingredients included in the mixture of this PRX may have influenced the results. For example, creatine monohydrate is included in this mixture, however; its amount (.15 g/dose) is probably negligible in producing any ergogenic benefit due to the limited amount and timing of the ingestion period and lack of any prior loading of creatine monohydrate by the subjects. Also, L-carnitine (.04 g/dose) has been demonstrated to have an ergogenic effect (mixed reports) however; as with the creatine the amount and the timing are probably prohibitive concerning any benefit [3]. Although the results of this study favor using this particular PRX, caution should be taken regarding the findings since it is difficult to provide a feasible scientific rationale why any significant findings occurred based on the content of the product.

To the author's knowledge, no previous research has shown similar significant acute results utilizing a proprietary blend of ingredients primarily designed for use as a concentrated sports drink. In effort to substantiate or refute the efficacy of this product noted in this study, additional studies are most certainly warranted.


  1. 1.

    National Strength and Conditioning Association: Essentials of strength and conditioning. 2000, Champaign, IL: Human Kinetics, 2

    Google Scholar 

  2. 2.

    Nieman DC: Exercise testing and prescription: a health-related approach. 2002, Boston, MA: McGraw-Hill, 5

    Google Scholar 

  3. 3.

    McArdle WD, Katch FI, Katch VL: Exercise physiology: energy, nutrition, and human performance. 2001, Baltimore, MD: Lippincott, William, and Wilkins, 5

    Google Scholar 

  4. 4.

    Sherman WM, Jacobs KA, Leenders N: Carbohydrate metabolism during endurance exercise. Overtraining in Sport. Edited by: R Kreider AF, O'Toole M. 1998, Champaign: Human Kinetics, 289–293. 300–302.

    Google Scholar 

  5. 5.

    Zachwieja JJ, Costill DL, Fink WJ: Carbohydrate ingestion during exercise: effects on muscle glycogen resynthesis after exercise. Int J Sport Nutr. 1993, 3: 418–430.

    CAS  Article  Google Scholar 

  6. 6.

    Brown SP, Miller WC, Eason J: Exercise physiology: basis of human movement in health and disease. 2006, Baltimore, MD: Lippincott, William, and Wilkins

    Google Scholar 

  7. 7.

    Brand-Miller J: The G.I. factor: the glycemic index solution. 1996, Sydney, Australia: Hodder and Stoughton

    Google Scholar 

  8. 8.

    Fontvieille AM: The use of low glycemic index foods improves metabolic control of diabetes patients in a 10 week study. Diabet Med. 1992, 9: 444.

    CAS  Article  Google Scholar 

  9. 9.

    Costill DL, Sherman WM, Fink WJ, Maresh C, Witten M, Miller JM: The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am J Clin Nutr. 1981, 34: 1831–1836.

    CAS  Article  Google Scholar 

  10. 10.

    Blom P, Hostmark A, Vaage O, Kardel K, Maehlum S: Effects of different post-exercise sugar diets on the rate of muscle glycogen synthesis. Med Sci Sports Exerc. 1987, 9: 491–496.

    Google Scholar 

  11. 11.

    Burke LGC, Hargreaves M: Muscle glycogen storage after prolonged exercise: Effect of the glycemic index of carbohydrate feedings. J Appl Physiol. 1993, 75: 1019–1023.

    CAS  Article  Google Scholar 

  12. 12.

    Chandler RM, Byrne HK, Patterson JG, Ivy JL: Dietary supplements affect the anabolic hormones after weight-training exercise. J Appl Physiol. 1994, 76: 839–845.

    CAS  Article  Google Scholar 

  13. 13.

    Ivy JL: Muscle glycogen synthesis before and after exercise. Sports Med. 1991, 11: 6–19. 10.2165/00007256-199111010-00002.

    CAS  Article  Google Scholar 

  14. 14.

    Ivy JL: Glycogen resynthesis after exercise: effect of carbohydrate intake. Int J Sports Med. 1998, 19 (Suppl 2): S142–145. 10.1055/s-2007-971981.

    CAS  Article  Google Scholar 

  15. 15.

    American College of Sports Medicine: Guidelines for exercise testing and prescription. 2000, Philadelphia, PA: Lippincott, Williams, and Wilkins, 6

    Google Scholar 

  16. 16.

    SPSS: Statistical package for the social sciences. [software version 11.5]. 2003, Chicago, IL: SPSS

    Google Scholar 

  17. 17.

    Beckers EJ: Gastric emptying of carbohydrate-medium chaintriglyceride suspensions at rest. Int J Sports Med. 1992, 13: 58-10.2165/00007256-199213010-00006.

    Article  Google Scholar 

  18. 18.

    Jeukendrup AE: Fat metabolism in exercise: a review-part III: effects of nutritional interventions. Int J Sports Med. 1998, 19: 371-10.1055/s-2007-971932.

    CAS  Article  Google Scholar 

  19. 19.

    Jeukendrup AE: Effect of medium-chain triacylglycerol and carbohydrate ingestion during exercise on substrate utilization and subsequent cycling performance. Am J Clin Nutr. 1998, 67: 397.

    CAS  Article  Google Scholar 

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Gratitude is expressed to the subjects that participated in this study as well as to each of the laboratory assistants who were instrumental in the collection of the data. This study was funded by a product grant from Mannatech Inc. (Coppell, TX). The researchers in this study independently collected, analyzed, and interpreted the results from this study and have no financial interests in the results of this study. Dissemination of the results in this study does not constitute endorsement by the researchers or their institutional affiliations.

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Correspondence to Allyn Byars.

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Byars, A., Greenwood, M., Greenwood, L. et al. The Effectiveness of a Pre-Exercise Performance Drink (PRX) on Indices of Maximal Cardiorespiratory Fitness. J Int Soc Sports Nutr 3, 56 (2006).

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  • sports nutrition
  • ergogenic aids
  • VO2max
  • aerobic performance
  • sport drink