A high-carbohydrate training diet is a must for optimum sports performance because it produces the biggest stores of muscle glycogen. Unlike the fat stores in the body, which can release almost unlimited amounts of fatty acids, the carbohydrate stores are small. They are fully depleted after two or three hours of strenuous exercise. This depletion of carbohydrate stores is called “hitting the wall.” The blood glucose concentration begins to decline at this point. If exercise continues as the same rate, blood glucose may drop to levels that interfere with brain function and cause disorientation and unconsciousness.
All else being equal, the eventual winner is the person with the largest stores of muscle glycogen. It is important to maximize your muscle glycogen stores by ingesting a high-carbohydrate training diet and by carb loading in the days prior to the competition.
There are times when low G.I. foods provide an advantage and times when high G.I. are better. For best performance a serious athlete needs to learn which foods have high and low G.I. factors and when to eat them. Understanding the glycemic index and making the best food choices can give you an advantage.
Low-GI Foods: Before the Event
Low-GI foods have been proven to extend endurance when eaten alone one or two hours before prolonged strenuous exercise. Low-GI foods are best eaten about two hours before the big event –so that the meal will have left the stomach but will remain in the small intestine, slowly releasing glucose energy, for hours afterwards. The slow rate and steady stream of glucose trickles into the bloodstream during the event. Most importantly, the extra glucose will still be available toward the end of the exercise, when muscle stores are running close to empty. In this way, low-GI foods increase endurance and prolong the time before exhaustion hits.
When a pre-event meal of lentils (low GI value) was compared with one of potatoes (high GI value), cyclists were able to continue cycling at high intensity (65 percent of their maximum capacity) for twenty minutes longer when the meal had a low G value. Their blood-glucose and insulin levels were still above fasting levels at the end of exercise, indicating that carbohydrates were continuing to be absorbed from the small intestine even after ninety minutes of strenuous exercise.
In any sport context, it’s critical to select low-GI foods that do not cause gastrointestinal discomfort (stomach cramps, etc.). Some low-GI foods, such as legumes that are high in fiber or ingestible sugars, may produce symptoms in people not use to eating large amounts of them. There are plenty of low-fiber, low-GI choices, including pasta, noodles, and Basmati rice.
High- GI Foods: During and After the Event
While the pre-event meal should have a low GI value, scientific evidence indicates that there are times when high-GI foods are preferable. This includes during the event, after the event, and after normal training sessions. This is because high-GI foods are absorbed faster and stimulate more insulin, the hormone responsible for getting glucose back into the muscles for either immediate or future use.
During the event
High-GI foods should be used during events lasting longer than ninety minutes. This form of carbohydrate is rapidly released into the bloodstream and ensures that glucose is available for oxidation in the muscle cells. Liquid foods are usually tolerated better than solid foods, for endurance racing for example, because they are emptied more quickly from the stomach. Sports drinks are ideal during the race because they replace water and electrolytes as well. If you feel hungry for something solid during a race, try jelly beans (GI value of 80) or another form of high-glucose candy. Consume 30 to 60 grams of carbohydrate per hour during the event.
After the event (recovery)
In some competitive sports, athletes compete on consecutive days, and glycogen stores need to be at their maximum each time. Here it is important to restock the glycogen store in the muscles as quickly as possible after each day’s events. High-GI foods are best in this situation. Muscles are more sensitive to glucose in the bloodstream in the first hour after exercise, so a concerted effort should be made to get as many high-GI foods in as soon as possible.
Suggested foods include most of the sports drinks which replace water and electrolyte losses, or high-GI rice (e.g., jasmine), breads, and breakfast cereals such as cornflakes or rice krispies. Potatoes cooked without fat are good choice too but their high satiety means it is hard to eat lots of them.
Carbohydrate Loading For Training & Understanding
Why This Is Important…
It’s not just your pre- and post-event meals that influence your performance. Very active people need to eat much larger amounts of carbohydrates than inactive people. Consuming a high-carbohydrate diet every day will help you reach peak performance. When athletes fail to consume adequate carbohydrates each day, muscle and liver glycogen stores eventually become depleted. Dr. Ted Costill at the University of Texas showed that the gradual and chronic depletion of stored glycogen may decrease endurance and exercise performance. Intense workouts two to three times a day draw heavily on the athlete’s muscle glycogen stores. Athletes on low-carbohydrate diet will not perform their best because muscle stores of fuel are low.
If the diet provides inadequate amounts of carbohydrate, the reduction in muscle glycogen will be critical. An athlete training heavily should consume about 500 to 800 grams of carbohydrate a day (about two to three times normal) to help prevent carbohydrate depletion. Typically, American adults consume between 200 to 250 grams of carbohydrates each day.
Could a High-GI Diet Be Harmful to Athletes?
By virtue of their high activity levels, athletes have optimal insulin sensitivity. When they eat high-carbohydrate, high-GI foods, blood glucose and insulin levels rise far less in them than in the average person. This also provides the athlete with a bonus by not exposing their bodies to dangerous levels of blood glucose which produce disease in sedentary, insulin resistant individuals.
Adapted from the Book: The New Glucose Revolution
Written by: Jennie Brand-Miller, PhD
Thomas M.S. Wolever, MD PhD
Stephen Colagiuri, MD
Kaye Foster-Powell, M Nutr & Diet