Carbohydrates are the primary macronutrient ingested in most athletes’ diets. When carbohydrates are ingested, they are broken down into glucose and enter the bloodstream. The body uses blood glucose and stored muscle glycogen for energy during exercise.
As the duration of exercise increases, more blood glucose and less muscle glycogen is used for energy. This is why it may be important to ingest carbohydrates before and/or during exercise.
As the carbohydrates are broken down into glucose, the body can use them for energy. The body’s use of blood glucose depends on the type, intensity, and duration of exercise.
Numerous studies have been conducted on the use of carbohydrate supplementation with different types of carbohydrates, different timing of the ingestion of carbohydrates, and different duration of the exercise performed.
Human Studies
Langenfeld et al. had trained cyclists ride at their own pace for 80 miles on two separate occasions during a simulated time trial. The trials were preceded by a prescribed diet with a final feeding 3 to 4 hours before exercise. After every 10 miles, subjects ingested either a noncaloric placebo or a carbohydrate maltodextrin supplement (5% maltodextrin and 2% fructose). Glucose levels were significantly higher after 40 miles and free fatty acid levels were significantly lower after the completion of the race with the carbohydrate supplementation. Carbohydrate oxidation was significantly higher with carbohydrate supplementation and these riders could sustain a higher average intensity with significantly faster times (241.0 versus 253.2 min for placebo and carbohydrate groups, respectively).
Next, seven well-trained male cyclists exercised at either 45% or 75% while receiving a placebo, 10% liquid carbohydrate supplement or a solid carbohydrate supplement . The subjects performed this exercise for 124 minutes and then a second set for 190 minutes followed by a ride to exhaustion at 80% Plasma glucose and insulin responses were significantly greater in the liquid carbohydrate trial when compared with the placebo. The time to exhaustion for the liquid (233.4 min) and solid carbohydrate (223.9 min) trials did not differ, but both were significantly greater than the placebo (202.4 min).
A study of carbohydrate use in US National Field Hockey Team members also showed positive results Seven members of the team and seven team counterparts were given a carbohydrate drink containing 1 g/kg of carbohydrate four times per day while the other group ingested a placebo for 7 days of intense training. The results showed a significant pre- to post-change in maximal time to exhaustion and less psychological fatigue in the carbohydrate-supplemented group.
Tsintzas et al. had 11 male subjects run on a treadmill at 70% until exhaustion three times separated by a week. During the first two occasions, a carbohydrate-electrolyte solution (5.5% or 6.9%) was ingested for the first hour of exercise while water was ingested for the remaining time. On the third occasion, water was ingested throughout the entire run. There were no differences between the two carbohydrate trials, but the time to exhaustion was greater in the 5.5% carbohydrate trial when compared to the water trial . Overall, the average performance times for the two carbohydrate trials were significantly longer than the water trial.
Another study showing positive results of carbohydrate supplementation was done by Tarnopolsky et al Supplementation trials were
a) 177 kcal of 81 % carbohydrate and 19% protein consumed during the 3-day pretest and less than 10 minutes after exercise plus 600 mL 8% glucose polymers/fructose 1 hour before and the same supplementation during testing
b) Placebo during 3-day pretest plus the remainder the same as trial A
c) Placebo at all points,
d) Same as trial B with 8% glucose 1 hour before the test and the same supplementation during the test.
With this complex supplementation protocol, the results showed a greater time to fatigue at 85% (9.6 versus 7.8 min for group A and placebo, respectively) and total carbohydrate oxidation for trial A versus the placebo trial. Plasma glucose concentrations were higher for trials A and B versus the placebo. Overall, these results show that 3 days of carbohydrate and protein supplementation followed by 1-hour pre- and during-exercise supplementation with mixed carbohydrates increases time to fatigue and carbohydrate oxidation.
A study on carbohydrates with or without BCAAs has shown positive results. Eight subjects drank carbohydrate drinks 1 hour before (5 mL/kg, 18% carbohydrate) and during exercise (2 mL/kg, 6% carbohydrate), the same carbohydrate solutions with BCAAs, or a placebo. The subjects ran longer when fed either carbohydrate solution when compared to the placebo (9.66, 9.00, and 6.36 min for the carb, carb plus BCAAs, and placebo, respectively), but there were no differences between the carbohydrate groups.
Next, McConell et al. conducted a study on eight endurance-trained men cycling to exhaustion at 69% The subjects ingested an 8% carbohydrate solution or a placebo. No differences were noted in oxygen uptake, heart rate, or respiratory exchange ratio during exercise, however, time to exhaustion was 30% greater when the carbohydrates were ingested.
Moreover, the effects of carbohydrate supplementation before and during exercise were examined using five moderately trained subjects A high-intensity exercise test (90% VO2max ) was performed for 60 minutes under the following conditions-
a) Pre-exercise glucose polymer and placebo during exercise
b) Glucose polymer before and during exercise or
c) Placebo before and during exercise
The subjects ingested 300 mL of a placebo or 10% glucose solution immediately before and every 15 minutes during exercise. There were no differences in power output in the groups for the first 40 minutes, but power output was greater with glucose during the final 20 minutes (total power output = 619 kg for the glucose/placebo, 599 kg for the glucose/glucose, and 560 kg for the placebo). followed a similar pattern, and the authors concluded that the ingestion of glucose before exercise results in less of a decline in power output, but no further benefit is seen with the ingestion of glucose during exercise.
Other studies have shown that carbohydrate ingestion increases performance to a greater degree than a combination of carbohydrate, fat, and protein. Six highly trained endurance cyclists rode for 330 minutes at 50% peak power output while ingesting a sports bar containing 7 g of fat, 14 g of protein, and 19 g of carbohydrate or an equicaloric amount of carbohydrate before performing a time trial. Rates of fat oxidation were greater after exercise during the combination trial compared to the carbohydrate trial. However, two subjects could not complete the time trial after they ingested the combination of fat, protein, and carbohydrates, while all of the subjects completed the time trial after the ingestion of just carbohydrate. The authors concluded that the combination of fat, protein, and carbohydrate can increase fat metabolism during prolonged exercise, but may impair subsequent high-intensity performance.
On the other hand, a study by Madsen et al. showed no improvement in performance with carbohydrate ingestion. Nine well-trained cyclists cycled 100 km as fast as possible after ingesting glucose, glucose plus BCAAs, or a placebo. The results showed no significant differences between groups for performance time.
Another recent study by Burke et al. also showed no difference in endurance performance. Six well-trained cyclists consumed 2 g/kg of carbohydrate of either high glycemic index (GI) (potato), low GI (pasta), or a lowenergy jelly (control) 2 hours before exercise. The exercise consisted of 2 hours of cycling at 70% , followed by a performance ride at 300 kg. Also, immediately before and during exercise, the subjects consumed 10 g/100 mL of a glucose solution. The results showed no significant differences between any of the trials for carbohydrate oxidation or time to complete the performance ride.
Carbohydrates are the most frequently used endurance enhancer. With the ingestion of carbohydrates before and/ or during exercise, there will be more glucose available for energy. This should result in an increase in endurance performance.
Safety and Toxicity
One might suffer abdominal cramping or gastrointestinal problems after ingesting too great of an amount of carbohydrate before and/or during exercise. The use of a liquid carbohydrate solution may prevent this.
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