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Original Research

Effects of Isocaloric Carbohydrate vs. Carbohydrate-Protein Supplements on Cycling Time to Exhaustion

Richardson, Kristy L.; Coburn, Jared W.; Beam, William C.; Brown, Lee E.

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Journal of Strength and Conditioning Research: May 2012 - Volume 26 - Issue 5 - p 1361-1365
doi: 10.1519/JSC.0b013e318248d774
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It is important to have adequate amounts of carbohydrates available for physical activity. Carbohydrates supply energy for the conversion of adenosine diphosphate to adenosine triphosphate, which is a critical step for supplying energy to working skeletal muscles. Glycogen depletion leads to fatigue during prolonged exercise; hence, to recover from prolonged exercise, glycogen stores need to be replenished (11). This is especially important for athletes looking for ways to recover more quickly between double-day practice sessions or back-to-back competitive events. It has long been known that time to exhaustion is directly related to initial muscle glycogen stores (1) and that the quantity of carbohydrate in the diet directly alters the concentration of glycogen in muscle (2).

The timing of nutrient intake after exercise has an effect on the rate of glycogen replenishment. Insulin sensitivity is increased after exercise, making the rate of glycogen replenishment higher during this time (10). For example, there is a significantly higher rate of glycogen replenishment when nutrient supplementation is given immediately after exercise when compared with 2–3 hours after exercise (7,12). If only carbohydrates are consumed, supplementation every 15–30 minutes has been shown to increase the rate of glycogen replenishment compared with less frequent administrations (17,21,22).

The effect of adding protein to a carbohydrate (CHO) supplement has also been investigated for its effect on glycogen replenishment (6,9,21–23). For example, it was found that there was an enhanced ability to restore muscle glycogen when protein was added to a CHO supplement after exercise (6,23). The potential for enhanced glycogen storage with the addition of protein is usually attributed to an enhanced insulin secretion (8,13). However, comparisons of CHO versus carbohydrate-protein (CHO-Pro) recovery drinks are often done without using isocaloric (similar number of calories) CHO and CHO-Pro supplements (23). The CHO-Pro supplements typically contain more calories, which may have been the reason for the enhanced glycogen replenishment. One study did use isocaloric supplements, but the supplements were only administered at 10 minutes and 2 hours after exercise (6). Others have concluded that when carbohydrates (1.2 g of carbohydrate per kg body weight per hour) were consumed on a frequent basis after exercise (every 15–30 minutes, thereby maintaining blood glucose and insulin), the addition of protein did not affect muscle glycogen replenishment (4,9,13,21,22).

In addition to glycogen replenishment, consumption of different nutrients after exercise may also affect subsequent exercise performance after recovery. Studies have observed increased time to exhaustion when protein was added to a CHO supplement (16,18,23). However, the supplements in these studies were not isocaloric (18,23). The longer time to exhaustion observed in these studies may have been because of the higher amount of calories provided by the CHO-Pro supplements. When the supplements were isocaloric, the supplements were only administered 0 and 60 minutes after exercise (16). As mentioned previously, a more frequent consumption of carbohydrates has been shown to enhance glycogen replenishment (17,21,22). The less frequent administration of CHO supplement may not have optimized the rate of glycogen replenishment and thus hindered performance on the time-to-exhaustion test.

Several studies (6,16,18,23) have shown enhanced glycogen replenishment or exercise performance when protein was added to a CHO supplement, whereas others (9,21,22) showed no difference in glycogen replenishment if a sufficient amount of carbohydrate (1.2 g of carbohydrate per kg body weight per hour) was consumed on a frequent basis (every 15-30 minutes). No studies, to our knowledge, have been conducted to look at whether there is a difference in time to exhaustion when the supplements are isocaloric and when they are consumed on a frequent basis (i.e., every 30 minutes postexercise). Therefore, the present study was conducted to examine the effect of frequent postexercise consumption of commercially available isocaloric CHO and CHO-Pro supplements on time to exhaustion during a subsequent bout of exercise.


Experimental Approach to the Problem

Previous studies that have compared carbohydrate with combined carbohydrate-protein recovery drinks have either used drinks that differed in caloric content or used infrequent administration of the drinks. Therefore, we administered isocaloric recovery drinks on a more frequent basis (every 30 minutes) to determine the effects on recovery as measured by repeated time-to-exhaustion cycle ergometry tests.


Seven men (mean ± SD: age = 27.9 ± 7.2 years; height = 175.9 ± 9.0 cm; body mass = 78.8 ± 15.3 kg; V[Combining Dot Above]O2max = 50.0 ± 7.3) and 4 women (age = 27.0 ± 1.6 years; height = 164.3 ± 8.3 cm; body mass = 60.9 ± 4.7 kg; V[Combining Dot Above]O2max = 45.2 ± 3.9) volunteered to participate in this double-blind study. Participants were physically active and participated in at least 1 hour of continuous aerobic activity, including cycling, at least 2 times per week. The procedures and the potential benefits and risks were fully explained to the participants before they signed the informed consent to participate in this study. The study was approved by the university institutional review board.


During the first visit to the laboratory, participants performed a maximal cycle ergometer (Monark Ergomedic 839E, Monark Exercise AB, Varburg, Sweden) test for the determination of V[Combining Dot Above]O2max and maximal power output. After a 5-minute warm-up at 50 W, the power level on the cycle ergometer was increased 30 W every 2 minutes until voluntary exhaustion or V[Combining Dot Above]O2max was reached. Expired air was collected throughout the test using an open-circuit spirometry Parvo Medics metabolic cart (Model 2400; Parvo Medics, Inc., Sandy, UT, USA). Heart rate was monitored every minute using a Polar heart rate monitor. Subjects were asked to rate their level of exertion every minute using Borg's rating of perceived exertion scale (6–20). The V[Combining Dot Above]O2max test was considered a maximal effort when at least one of the following criteria was met: (a) a plateau in oxygen uptake with increased rate of work, (b) heart rate within 5–10 beats of the age-predicted maximum heart rate (maximum heart rate = 220 – age), or (c) a respiratory exchange ratio (RER) > 1.10 (20). Assessment of V[Combining Dot Above]O2max has been shown to be highly reliable, with an intraclass correlation coefficient of 0.80 (14).

To maintain a consistent amount of muscle glycogen, participants were asked to refrain from exercise and follow a prescribed diet during the 48 hours before the experimental protocol. The diets were planned using the dietary exchange system (19). Subjects were instructed on how to use the exchange system and how to record a diet log. Participants were given exchange lists and diet log recording sheets. All recorded diet logs were analyzed for carbohydrate, protein, and fat content using Diet Analysis Plus (version 8.0; ESHA Research, Salem, OR, USA). The aim was to have participants consume a diet within the macronutrient distribution ranges of 45–65% carbohydrates, 10–35% protein, and 20–35% fat during the 48 hours before each of the experimental days (Table 1).

Table 1
Table 1:
Percent of daily caloric intake for energy-yielding nutrients during 24 and 48 hours preceding the time-to-exhaustion tests (mean ±SD).*

Experimental Protocol

At least 72 hours after the first visit, the participants came to the laboratory after a 12-hour fast to perform 2 cycle ergometer time-to-exhaustion tests. The fast served to lower liver glycogen levels, effectively isolating muscle glycogen content as the variable of interest. The purpose of the first time-to-exhaustion test was muscle glycogen depletion. After a 5-minute warm-up at 50 W, the power level was increased to the power output that corresponded to the power output when participants were at 75% of their V[Combining Dot Above]O2max. The participants exercised at this power level until they voluntarily stopped or were no longer able to pedal at the prescribed cadence (80 revolutions per minute). The participants were protected from seeing the time during this test and equal levels of verbal encouragement were given to all participants. Heart rate and perceived exertion were determined every 2 minutes using the same procedure used during the V[Combining Dot Above]O2max test. After a 5-minute cooldown at 50 W, participants rested in the laboratory for 3 hours. During this time, they were given a CHO or CHO-Pro supplement at 0, 30, 60, 90, and 120 minutes after the first time-to-exhaustion test. The supplement administered (CHO vs. CHO-Pro) was randomly determined. The CHO supplement provided 1.5 g of carbohydrate per kg body weight per hour and the CHO-Pro supplement provided 1.2 g of carbohydrate per kg body weight per hour and 0.3 g of protein per kg body weight per hour (Table 2). To avoid investigator bias, the supplements (CHO and CHO-Pro) were premixed each day by trained laboratory personnel who did not speak with the subjects or administer the testing or training protocols. Both the CHO and the CHO-Pro supplements were mixed with a ratio of 4.7 g of CHO or CHO-Pro for each 1 fluid oz of cold water. The bottle containing the supplement was marked to indicate how much of the supplement should be ingested at 0, 30, 60, 90, and 120 minutes. The total amount of fluid ingested, therefore, was identical relative to the body mass of the participant. Three hours after the first time-to-exhaustion test was completed, a second time-to-exhaustion test was administered to assess the effects of the supplement on recovery. The same protocol used for the first time-to-exhaustion test was used for the second time-to-exhaustion test. At least 1 week after the second visit, each participant visited the laboratory for a third visit. The testing protocol used for the second visit was repeated for the third visit, but the alternative supplement (CHO or CHO-Pro) was given. Repeated time-to-exhaustion tests have been shown to be highly correlated (r = 0.864) in subelite aerobic athletes (3).

Table 2
Table 2:
Nutrient composition in 100 kilocalories of carbohydrate (CHO) and carbohydrate-protein (CHO-Pro) supplements.

Statistical Analyses

A 2 × 2, time (first time-to-exhaustion test, second time-to-exhaustion test) × supplement (CHO, CHO-Pro), repeated measures analysis of variance (ANOVA) was used to compare differences in time to exhaustion. Differences were considered significant at p ≤ 0.05. All data were analyzed using SPSS (version 19.0; SPSS, Inc., Chicago, IL, USA).


Figure 1 shows the mean (±SEM) pretest and posttest time-to-exhaustion times for the CHO and CHO-Pro groups. The results of the 2 × 2 repeated measures ANOVA indicated that there was no significant interaction between supplements and time to exhaustion (p > 0.05). Furthermore, there was no significant main effect for supplements or time to exhaustion (p > 0.05).

Figure 1
Figure 1:
Mean (±SEM) pretest and posttest time-to-exhaustion scores for the carbohydrate (CHO) and carbohydrate-protein (CHO-Pro) supplements. There was no significant interaction or main effect for type of supplement or pretest to posttest time to exhaustion (p > 0.05).


The results of this study indicated that isocaloric CHO and CHO-Pro supplements consumed at 0, 30, 60, 90, and 120 minutes after exercise have the same effect on recovery and subsequent exercise endurance during a second bout of exercise. These results are consistent with the findings of others (9,21,22) who concluded that when 1.2 g of carbohydrate per kg body weight per hour is consumed on a frequent basis (every 15–30 minutes) after exercise, the addition of protein does not affect muscle glycogen restoration. Based on the results of these studies, the present study provided an adequate amount of carbohydrates for muscle glycogen restoration with both the CHO (1.5 g of carbohydrate per kg body weight per hour) and the CHO-Pro (1.2 g of carbohydrate per kg body weight per hour) supplements and the supplements were consumed on a frequent basis (every 30 minutes) after the first time-to-exhaustion test.

Several previous studies have compared differences in time to exhaustion after CHO and CHO-Pro supplementation (16,18,23). The results of these studies indicated a longer time to exhaustion when a CHO-Pro supplement was consumed after exercise compared with a CHO supplement. However, the supplements used by these investigators were not isocaloric because the CHO-Pro supplements supplied more calories than the CHO supplements. Others found a greater insulin response when the CHO-Pro supplement was consumed compared with the CHO supplement (16,23). They concluded that the greater insulin response would likely result in enhanced glycogen replenishment, which would explain the increased time to exhaustion observed when the CHO-Pro supplement was consumed. Another study (18) did not test the insulin response but suggested that an increase in insulin enhanced glycogen replenishment, which may have ultimately enhanced time to exhaustion.

One study did provide recovery beverages that were isocaloric, providing 1.5 g of carbohydrate per kg body weight and 0.6 g of protein per kg body weight for the CHO-Pro supplement and 2.1 g of carbohydrate per kg body weight for the CHO supplement (16). However, they administered the supplements less frequently than the present study (0 and 60 minutes after exercise vs. 0, 30, 60, 90, and 120 minutes). This may not have been an optimal method of administering the CHO supplement to achieve glycogen restoration. Others have concluded that when adequate amounts of carbohydrates (1.2 g of carbohydrate per kg body weight per hour) are consumed on a frequent basis (every 15–30 minutes) after exercise, the addition of protein does not affect muscle glycogen restoration (9,21). Therefore, the longer time to exhaustion found by some (16) may have been because of the supplements being provided on a frequent basis. If the supplements had been consumed more frequently, every 15–30 minutes instead of only 0 and 60 minutes after exercise, the results may have been consistent with our results.

It should be noted that the time-to-exhaustion values in this study were significantly shorter than the previous studies that used similar exercise protocols (5). There are several plausible explanations for these differences. In the present study, subjects were instructed to fast overnight and the testing took place the next morning. Although this may have lowered liver glycogen levels substantially, it may not have caused significant muscle glycogen depletion. Thus, the relatively short time to exhaustion likely reflects fatigue mechanisms other than muscle glycogen depletion. For example, the overnight fast and subsequent exercise may have increased reliance on amino acids and fats for energy (15). Decreased plasma branched-chain amino acids levels (as a result of increased uptake into exercising muscle cells) and increased fatty acid release into the blood can increase free tryptophan levels and result in increased movement of tryptophan across the blood-brain barrier. It has been suggested that acute increases in tryptophan in the brain can lead to perceptions of fatigue and increase the mental effort needed for exercise. This is an example of a central fatigue mechanism (15). Ingestion of both carbohydrates and amino acids may be effective in combating these mechanisms of central fatigue. For example, carbohydrate ingestion might reduce the need for free fatty acid release, and amino acid ingestion might offset the loss of plasma branched-chain amino acids during and after exercise.

Practical Applications

This investigation provided isocaloric CHO and CHO-Pro supplements on a frequent basis (every 30 minutes) after exhaustive exercise and found no significant difference in time to exhaustion on a subsequent exercise bout between the 2 supplements. We conclude that CHO and CHO-Pro ingestion after exercise provides the same amount of restoration provided that isocaloric supplements are consumed and the supplements are consumed at least every 30 minutes, and that athletes such as wrestlers and some track athletes who perform repeated bouts of strenuous exercise with relatively short recovery periods can benefit from either type of recovery beverage.


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recovery drinks; endurance; fatigue; supplements

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