VO₂max or the ability of the human body to use or consume oxygen for aerobic metabolism during exercise is an important predictor of athletic performance in endurance activities 1 . In addition, ventilatory threshold and the onset of blood lactate are considered to be even better indicators of an endurance athlete's capacity when examining the metabolic demands of middle distance runners and other similar athletes for aerobic power 2 . As such, 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. Therefore, it is generally accepted that by using less CHO and more fat during activity with a concomitant decrease in lactate, aerobic performance of the individual should therefore be enhanced 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% VO2 max 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 6 . 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 7 . 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 7 .

This high clearance rate of 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) 8 9 10 . Endurance performance could be compromised by this rebound hypoglycemia, depressed fat catabolism, and possible earlier than expected depletion of glycogen stores.

In contrast, consuming low-glycemic CHO 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 11 12 13 14 15 16 17 18 .

Gastric emptying also affects fluid hydration and absorption of nutrients. Gastric emptying slows when ingested fluids contain a high concentration of particle in solution (osmolality) or possess high caloric content. The rate the stomach empties greatly affects intestinal absorption of fluid and nutrients. Little negative effect of exercise on gastric emptying occurs up to an intensity of about 75% of maximum, after which emptying rate slows 19 . Gastric volume, however, greatly influences gastric emptying; the emptying rate increases exponentially as fluid volume in the stomach increases. A major factor to speed gastric emptying (and compensate for any inhibitory effects of the beverage's carbohydrate content) involves keeping a relatively high fluid volume in the stomach. Consuming 150-250 ml of fluid immediately before exercise optimizes the beneficial effect of increased stomach volume on fluid and nutrient passage into the intestine. Prior research has also indicated that colder fluid emptied from the stomach at a faster rate than fluid at room temperature 3 . As a general rule, a 5 to 8% CHO-electrolyte beverage consumed during exercise in the heat contributes to temperature regulation and fluid balance as effectively as plain water by providing an intestinal energy delivery rate of approximately 5.0 kilocalories per minute in helping maintain glucose metabolism and glycogen reserves in prolonged exercise 20 21 .

Another factor influencing absorption is the consumption of triglycerides composed of predominantly long-chain fatty acids (12-18 carbons) significantly delays gastric emptying. This affects the rapidity of fat availability negatively and also slows fluid and CHO replenishment, both crucial factors in high intensity endurance exercise. Consequently, the relatively slow rate of gastric emptying and subsequent digestion, absorption, and assimilation of long-chain triglycerides makes this energy source an undesirable supplement to augment energy metabolism 22 .

Medium-chain triglycerides (MCTs) on the other hand provide a more rapid source of fatty acid fuel. MCTs are processed oils frequently produced for patients with intestinal malabsorption and tissue wasting diseases. Unlike long chain triglycerides, MCTs contain saturated fatty acids with 8-10 carbon atoms along the fatty acid chain. During digestion, lipase in the mouth, stomach, and intestinal duodenum hydrolyzes MCT to glycerol and medium chain fatty acids (MCFAs). Their water solubility allows MCFAs to move rapidly across the intestinal mucosa directly into the blood stream (portal vein) without first being transported slowly as chylomicrons by the lymphatic system as long chain triglycerides require 3 .

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. Athletes, in particular those participating in sports requiring aerobic power, commonly use pre-exercise drinks (PRX) and/or other ergogenic aids prior to training workouts and competition. Although this practice is commonplace among athletes, many of the effectiveness claims associated with these products appear to lack solid evidence substantiated by appropriately designed research trials. Additionally, there may be concerns over the purity and amounts of the listed ingredients in the drink formulations including their distribution to athletes in meeting compliance standards set forth by various athletic organizations that regulate the use of nutritional supplements.

EM·PACT™ (Mannatech, Inc., Coppell, TX) is an energy and endurance pre-exercise drink (PRX) purported to increase oxygen consumption and improve fat utilization during aerobic activity. In previous studies, ingestion of EM·PACT™ significantly enhanced indices of maximal aerobic performance when compared to a water placebo as well as fat substrate utilization when compared to another nationally marketed sports drink 23 24 .

Therefore, the purpose of this study was to examine the effects of a modified PRX formulation (modified version of EM·PACT™) from earlier investigations on factors related to maximal aerobic performance during a graded exercise test. Specifically, VO₂max, heart rate (HR), time to exhaustion (Time), and estimated non-protein fat substrate utilization (FA) during two a priori submaximal stages of a graded exercise testing were evaluated. The modification consisted of removing creatine monohydrate to meet the compliance standards set forth by various athletic organizations that regulate the use of nutritional supplements.

Initial results indicated significant mean differences in VO₂max (ml·kg-1·min-1) between PRX (50.49 ± 10.02) and PL (48.49 ± 9.91) trials for the total group (p = 0.007), which was not affected by gender (p > 0.05). Overall differences in the various parameters are depicted in Figure 1.

No significant mean differences in maximal HR (beats·min-1) were found between the PRX (188.66 ± 9.48) and PL (189.66 ± 9.49) trials for all subjects nor for either gender (p > 0.05). Significant mean differences in Time were found between the PRX (11.74 ± 1.72) and PL (11.44 ± 1.65) trials for all subjects (p = 0.034) and was not affected by gender (p > 0.05).

Significant mean differences in FA were found between PRX (60.30 ± 18) and PL (47.62 ± 17.08) in stage 1 (3rd minute, p = 0.009) and in stage 2 (6th minute, p = 0.008), PRX (25.79 ± 16.11) and PL (16.42 ± 112.37) of the graded exercise protocol for all subjects and was not affected by gender (p > 0.05). Overall differences in the two stages are depicted in FIGURE 1. Differences in mean values among all of the reported variables are displayed in Table 2.

The main findings of this study were that aerobic performance, specifically mean VO₂max, FA, and Time were significantly (p < .05) improved by ingestion of PRX prior to graded exercise testing. In particular, overall increases were observed in VO₂max (4.1%), non-protein FA (stage 1 = 26.6%, stage 2 = 57.1%), and time to exhaustion (2.6%). The findings of the present study support an earlier investigation of the PRX used in this study without the inclusion of creatine monohydrate in the drink formulation 23 . In addition, non-protein FA was also similar to an earlier study involving the PRX used in this investigation compared to another nationally marketed sports drink during the early stages of maximal exercise treadmill protocol 24 . Although the differences in the aforementioned parameters (VO₂max & Time) between PRX and PL trials were not as marked as the original investigation, the inclusion of subjects with higher levels of fitness in the later study may account for this disparity since the window of potential improvement in these individuals may not have been as great 23 . The results of this study also support the use of the PRX as examined in this investigation in tests of aerobic power. This appears to be consistent with earlier reports of ingesting a PRX consisting of low glycemic sugars before exercise including a recent study examining the effects of CHO on performance changes (i.e., time and fuel substrate utilization) and overreaching in trained cyclists 12 13 14 15 16 17 18 27 .

Improvement in time to exhaustion claims may also be substantiated as the data of this investigation support another investigation 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 6 28 . It is also fairly common for the nutritional supplement industry to market MCTs as fat burners, energy sources, glycogen sparers, and muscle builders to fitness and sports enthusiasts. Although MCTs do not inhibit gastric emptying as does common fat, conflicting research supports the efficacy of using MCTs solely or in combination with CHO as a means of improving oxidation during exercise and because of its limited amount in the formula studied in this investigation, its contribution may be minimal 29 30 . However, Subsequent research investigating possible metabolic and ergogenic effects of combining MCTs and CHO may have value. For instance, researchers in a recent study examining the effects of ingesting small additional amounts of MCTs in the diet for two weeks found that recreational athletes increased their time to exhaustion at pre-determined workloads along with increases in fat oxidation while yet another investigation reported no further improvements when combined with CHO 31 32 . As such, additional research may be needed in regards to the concentrations and timing of MCTs and CHO in the diet/supplements and their role in human performance.

As a result of these findings, it was concluded that aerobic performance, specifically VO₂max, Time, and FA may be significantly improved by ingestion of PRX 30 minutes prior to exercise testing. The modified PRX formulation utilized in this investigation supports the previous investigation and its efficacy for enhancing indices of aerobic performance (VO₂max, Time, and FA) during graded exercise testing, indicating that creatine monohydrate is not necessary for product effects. During aerobic exercise bouts, the combined results of this investigation may provide meaningful practical applications for coaches and athletes alike regarding ergogenic hydration options. Future research is warranted investigating the efficacy of PRX with further emphasis on other variables such as fuel substrate utilization, gender differences, fitness levels, comparisons with other products, as well as use under various environmental and competitive conditions including timing of ingestion (both long and short-term), and the intensity/duration of various activities. Although the results of this investigation favor using this particular PRX, caution should be taken regarding the findings as further research is needed to provide a feasible scientific rationale why any significant finding occurred based on the content of the product. To the author's knowledge, no previous investigations have shown similar significant acute findings utilizing a proprietary blend of ingredients primarily designed for use as a concentrated sports drink.

The authors declare that they have no competing interests.

AB developed the concept of the study, contributed to its design, data collection, statistical analysis, and manuscript preparation. SK & WK contributed in the design of the study, data collection, and manuscript preparation. AM & MG provided background work for the manuscript and contributed to its preparation.

All authors have read and approved the final manuscript.