Discussion
The primary objectives of this study were to determine whether CHO+P beverages produced improvements in performance time to fatigue and muscle damage compared with CHO beverages in endurance athletes. The beverages used for this comparison were matched for total carbohydrate content but not total calories. This approach is highly generalizable, because commercially available CHO+P beverages have typically added protein to beverages that already contain 6-10% carbohydrate (a typical level for CHO-only beverages). Mechanistically, this approach is advantageous because the additional protein content of the CHO+P drink is the only energy substrate difference between the CHO+P and CHO beverages. By matching the carbohydrate portion, any difference in performance or recovery can be attributed to something other than the absolute carbohydrate content of the fluids. A limitation of this approach is that the increased availability of total calories from the CHO+P beverage may have contributed to differences between trials. To minimize the effects of caloric or carbohydrate intake between trials, subjects in this study were asked to maintain a consistent diet before each performance bout. Subjects also provided dietary records for 3 d before each performance ride, which verified the consistency of dietary macronutrients and total caloric content between trials.
One of the main purposes of this study was to determine whether a CHO+P beverage could enhance athletic performance during prolonged endurance bouts to a greater extent than a CHO beverage. When utilizing the CHO+P beverage, subjects maintained an intensity of 75% of their O2peak 29% longer (P < 0.05) than when consuming a CHO beverage. In a comparable study, Ivy et al.[11] compared CHO and CHO+P beverages during an exercise bout that simulated a competitive cycling event. After 180 min of varying intensity cycling, athletes in the CHO+P trial sustained exercise at 85% O2peak for 36% longer than during a CHO trial (26.9 ± 4.5 vs 19.7 ± 4.6 min). The agreement in findings between these studies is particularly relevant because both studies utilized beverages that contained a 4:1 ratio of carbohydrate to protein, in nearly identical concentrations. In addition, both studies matched beverages for total carbohydrate content but not total caloric content.
It is possible that increased availability of total calories may have contributed to the performance improvements observed in the CHO+P trial. A hypothetical 70-kg cyclist received 37.2 kcal of CHO and 9.2 kcal of protein during the CHO+P trial every 15 min of cycling. The CHO trial was matched for CHO calories with no protein calories. Thus, cyclists consumed approximately 46 kcal more calories in the CHO+P trial (232 kcal) than the CHO trial (186 kcal) at the time of CHO fatigue (82 min). During the entire duration of the first performance bout, the cyclists consumed 139 kcal more during the CHO+P trial, due to the longer time to exhaustion. Although we cannot entirely discount the potentially beneficial effects of these additional calories in the CHO+P trial, they cannot explain the 318 kcal of additional energy expended during the prolonged performance time (calculated using mean subject values of +24 min at 2.7 L·min-1 O2, and an RER of 0.88). There is general agreement in the literature that the addition of CHO above 6-10% concentration in a sports beverage does not produce additional performance benefits.[6,7] Thus, if adding protein to a beverage of similar CHO concentration produces performance benefits, it is a practically significant finding. Future studies should examine combinations of isocaloric and isocarbohydrate beverages to elucidate specific mechanisms for these differences in performance.
Although not specifically addressed in this study, there are at least two other mechanisms by which endurance performance may be improved by the addition of protein to carbohydrate beverages. The protein in the CHO+P beverage may facilitate faster fuel-medium transport across the lining of the intestine, as implied by hydration studies from Shi and Gisolfi.[17] In addition, performance may have been aided via improved insulin stimulation during exercise. Although insulin is normally regulated during exercise to the point where even a high glycemic index sugar in a CHO beverage will only cause a small rise in insulin levels,[8] scientists have observed significantly higher postexercise insulin levels after CHO+P consumption than with CHO.[14,20,21] In addition, Ivy et al.[11] observed augmented insulin levels during variable-intensity cycling using a CHO+P beverage compared with water. However, the elevated insulin levels were no different than those observed during a CHO trial and thus could not explain the differences in performance observed between the CHO and CHO+P beverages in that study.
Time to fatigue was improved by the CHO+P beverage to a significantly greater extent during the second performance ride (40% increase) than during the first ride (29% increase). This is consistent with the proposed benefits of the CHO+P beverage. A few studies[10,21] have observed significantly faster glycogen resynthesis rates after exercise in those consuming CHO+P versus those consuming CHO. Similarly, Williams et al.[20] showed significantly greater muscle glycogen storage (128%) after CHO+P consumption, compared with a CHO beverage. These increases in muscle glycogen may have been mediated by elevated post-exercise insulin levels observed in two of these studies,[20,21] which may facilitate greater glucose uptake via translocation of GLUT4 transporters or activation of glycogen synthase.[4,21] Increased muscle glycogen storage could explain why CHO+P beverages have been associated with 21% [14] to 55%[20] increases in postrecovery endurance performance, when compared with CHO beverages.
Although the proposed mechanism for improved postrecovery performance is plausible, it should be noted that some investigators have observed no differences in glycogen storage between CHO+P and CHO treatments.[5,18,19] Ivy et al.[10] discussed numerous methodological variances that could explain these inconsistencies, including differences in carbohydrate and protein concentrations in the beverages, beverage administration protocols, and the time period of recovery measurements. It is possible that the improved postrecovery performance in this study was related to the higher total calories consumed in the CHO+P trial, due to the added protein and longer time to fatigue in the initial ride to exhaustion. During and after the first performance ride, the cyclists consumed 190 more calories during the CHO+P trial (581 kcal) than the CHO trial (391 kcal). However, as discussed previously, the added calories consumed do not match the additional calories expended during the longer CHO+P trial. Thus, it is unlikely that the additional calories consumed played a major role in the differences in performance between trials, as the net energy difference between trials (i.e., calories expended vs calories consumed) is in favor of the CHO trial. This hypothesis is supported by data from Ivy et al.,[10] who showed that glycogen resynthesis after cycling was improved after consumption of a CHO+P beverage versus an isocarbohydrate CHO beverage or an isocaloric CHO beverage. This suggests that CHO+P supplements may augment muscle recovery through mechanisms that are independent of total calories consumed.
The second purpose of the present study was to determine whether postexercise creatine phosphokinase (CPK) levels, a common indicator of muscular damage, differed between the two beverage trials. CPK levels measured 12-15 h after the CHO+P trial were reduced 83% (P < 0.05) compared with the CHO trial. We are aware of no peer-reviewed studies specifically comparing CPK levels between similar beverage treatments. However, the trends in the present study agree with data presented by Ready et al.[16] These investigators observed that CPK levels 24 h after a run/cycle duathlon were 36% lower when consuming a CHO+P beverage than when consuming a CHO beverage. Although the measured reduction was significantly greater in this study, the exercise methods and CPK measurement times were different between the studies. In a pilot for the present study, postexercise plasma CPK concentration peaked between 12 and 15 h after exercise; thus, this time period was utilized for postexercise CPK measurements. The measurements by Ready et al.[16] were obtained 24 h postexercise, which may explain some of the variance in magnitude of CPK changes between studies.
The postexercise CPK levels observed in the CHO trial were comparable to those reported by Prou et al.[15] 6 and 24 h after an endurance triathlon that included swimming, cycling, and running. Due to the primarily concentric muscular actions of cycling, more moderate CPK responses are expected after endurance cycling than endurance running.[12] However, the seemingly high postcycling CPK levels observed in this study may have been related to the type of endurance protocol utilized. Fatigue was defined as the point at which the cyclist could no longer maintain a cadence of 50 rpm for 30 s. As the subjects began to fatigue over the course of the trial, their cadence dropped from approximately 90-105 rpm to 50 rpm. As cadence decreased, resistance to pedaling increased to maintain a constant workload. This created a somewhat novel cycling environment where muscular force production was considerably higher than cyclists would typically experience for a given workload. In addition, the clipless pedals utilized by the cyclists allowed them to pull the pedals through the lower portion of the pedal stroke as cadence decreased, potentially increasing the extent of eccentric muscular contractions. These factors likely explain why the postexercise CPK values were somewhat higher than expected during typical cycling exercise.
The decreased postexercise CPK levels observed in our CHO+P trial indicated reduced muscle damage 12-15 h after exhaustive endurance exercise. The current study did not address specific mechanisms regarding how muscle damage was reduced in the CHO+P trial. However, Bloomer and Goldfarb[3] have suggested that nutritional supplements theoretically function to minimize secondary or delayed onset damage, as opposed to reducing the initial mechanical damage from exercise. It seems plausible that the CHO+P beverage in our study increased protein concentrations outside the cell, potentially driving increased protein synthesis and repair. This and other specific mechanisms should be examined by future investigators.
In summary, administration of a carbohydrate beverage with additional protein calories resulted in significant improvements in cycling time to fatigue and reductions in postexercise muscle damage (measured indirectly using plasma CPK levels) in comparison with a carbohydrate-only beverage. Further research is necessary to determine whether these effects were the result of higher total caloric content in the CHO+P beverage or due to specific protein-mediated mechanisms. Regardless of the specific mechanisms, these findings may have important implications for endurance athletes because it suggests that the effective caloric concentration of sports beverages can be elevated in CHO+P beverages to a greater degree than the 6-8% typically observed in CHO beverages.
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