The Glycemic Index in Sport
By Ellen Coleman, RD, MA, MPH
The glycemic index (GI) provides a way to rank carbohydrate-rich foods according
to the blood glucose response following their intake. The GI is calculated by
measuring the incremental area under the blood glucose curve following ingestion
of a test food (glucose or white bread) providing 50 g of carbohydrate, compared
with the area under the blood glucose curve following an equal carbohydrate
intake from the reference food. All tests are conducted after an overnight fast
(1).
The GI reflects the rate of digestion and absorption of a carbohydrate-rich
food. Thus, the GI is influenced by: the food form (including particle size,
presence of intact grains, texture, and viscosity); the degree of food processing
and cooking; the presence of fructose or lactose (both have a low GI); the ratio
of amylopectin and amylose in starch (amylose has a slower rate of digestion);
starch-protein or starch-fat interactions; and the presence of antinutrients
such as phytates and lectins (1).
Generally, foods are divided into those that have a high GI (glucose, bread,
potatoes, breakfast cereal, sports drinks), a moderate GI (sucrose, soft drinks,
oats, tropical fruits such as bananas and mangos), or a low GI (fructose, milk,
yogurt, lentils, pasta, cold climate fruits such as apples and oranges). Tables
of the GI of a large number of foods have been published internationally (2).
Some practitioners have recommended manipulating the GI of foods and meals to
enhance carbohydrate availability and improve athletic performance. For example,
low GI foods are often recommended before exercise to promote sustained carbohydrate
availability. Moderate to high GI foods are recommended during exercise to promote
carbohydrate oxidation and following exercise to promote glycogen repletion.
Thomas and colleagues initially raised interest in the use of GI in sport by
manipulating the glycemic response to pre-exercise meals (3). They reported
that the consumption of 1 g of carbohydrate/kg from a low GI food (lentils)
one hour prior to cycling at 67% of VO2max increased endurance compared to an
equal amount of carbohydrate from a high GI food (potatoes). The low GI lentils
promoted lower postprandial blood glucose and insulin responses and more stable
blood glucose levels during exercise compared with the high GI potatoes (3).
In a second study, Thomas and associates provided 1 g of carbohydrate/kg from
two low GI meals and two high GI meals (powdered foods and breakfast cereals)
one hour prior to cycling to exhaustion at 70% of VO2max (4). The low GI meals
were associated with higher blood glucose levels after 90 minutes of exercise
compared to the high GI meals and appeared to provide a sustained source of
carbohydrate throughout exercise. However, there were no differences in time
to exhaustion between the low GI meals and high GI meals and no correlation
between exercise time and meal GI.
Sparks and colleagues provided 1 g of carbohydrate/kg from a low GI food (lentils)
and a high GI food (potatoes) 45 minutes prior to exercise (5). The subjects
cycled for 50 minutes at 67% of VO2max and then underwent a 15 minute performance
trial. The high GI meal caused an increase in blood glucose before exercise
and a decline in blood glucose at the onset of exercise compared to the low
GI meal. Despite these alterations in exercise metabolism, there were no differences
in work output during the performance trial.
DeMarco and associates provided 1.5 g of carbohydrate/kg from a low GI food
and a high GI food 30 minutes prior to exercise (6). The subjects cycled for
two hours at 70% of VO2max and then rode to exhaustion at 100% of VO2max. Plasma
insulin levels were lower for the low GI meal through 20 minutes of cycling
and the exercise time to exhaustion was longer for the low GI meal compared
to the high GI meal. The low GI meal also maintained higher blood glucose levels
at the end of two hours of exercise, which may have improved subsequent maximal
effort.
Sherman et al (7) compared the ingestion of 1.1 g/kg and 2.2 g/kg of a high
glycemic carbohydrate beverage 1 hour prior to exercise. The subjects cycled
at 70% of VO2max for 90 minutes and then underwent a performance trial. Serum
insulin was initially elevated at the start of and during exercise and blood
glucose initially decreased. However, time-trial performance was significantly
increased 12.5% by the carbohydrate feedings, presumably via increased carbohydrate
oxidation.
There is insufficient evidence to support the recommendation that all athletes
consume low GI index foods before exercise. The hypoglycemia and hyperinsulinemia
following pre-exercise carbohydrate feedings are transient and will not harm
performance unless the athlete reacts negatively to high glycemic foods. A low
glycemic index pre-exercise meal may be beneficial for athletes when consuming
carbohydrate during exercise is not practical or possible. Athletes should evaluate
their responses to high-carbohydrate foods with both low and high glycemic indexes
in training to find what works the best (1).
Athletes who react negatively to high glycemic foods can choose from several
strategies: consume a low glycemic index carbohydrate before exercise; take
in carbohydrate a few minutes before exercise; or wait until exercising to consume
carbohydrate. The exercise-induced rise in the hormones epinephrine, norepinephrine,
and growth hormone inhibit the release of insulin and thus counter insulin’s
effect in lowering blood glucose.
Consuming carbohydrate during prolonged exercise enhances carbohydrate availability
and improves performance. Although it makes sense that athletes should consume
carbohydrate sources that are rapidly digested and absorbed to promote carbohydrate
oxidation, the glycemic response to carbohydrate feedings during exercise has
not been systematically studied. However, most athletes choose carbohydrate-rich
foods (sports bars and gels) and fluids (sports drinks) that would be classified
as having a moderate to high GI (1).
Consuming adequate carbohydrate following exercise helps to replenish glycogen
stores between daily exercise sessions. Burke and associates investigated the
effect of GI on muscle glycogen repletion following two hours of cycling at
75% of V02max (7). The subjects consumed 10 g of carbohydrate/kg/day from high
GI or a low GI meals. The high GI diet promoted greater muscle glycogen storage
(106 mmol/kg) than the low GI diet (71.5 mmol/kg) after 24 hours.
The most rapid increase in muscle glycogen during the first 24 hours of recovery
may be achieved by consuming foods with a high GI. However, Burke and colleagues
note that the total amount of carbohydrate consumed is the most important consideration
for glycogen repletion. They recommend an intake of 7 to 10 g of carbohydrate
per kg for maximum daily glycogen restoration (1).
The GI concept has limitations. The GI is based on equal grams of carbohydrate
(50 g), not average serving sizes. The numbers that are available are also largely
based on tests using single foods. The blood glucose response to high GI foods
may be blunted when combined with low GI foods in the meal. However, the GI
can be applied to mixed meals by taking a weighted mean of the GI of the carbohydrate-rich
foods that make up the meal (1).
The GI may be useful in sport by helping to fine tune food choices. However,
the GI should not be used exclusively to provide guidelines for carbohydrate
and food intake before, during, and after exercise. There are other features
of foods that are important to the athlete, such as the food’s nutritional content
and the practical issues of palatability, portability, cost, gastric comfort,
and ease of preparation. Since food choices are specific to the individual athlete
and exercise situation, athletes should choose foods according to their nutritional
goals (1).
References
1. Burke LM, Collier GR, Hargreaves M. The glycemic index – a new tool in sport
nutrition? Int J Sport Nutr. 1998; 8:401-415.
2. Foster-Powell K, Brand Miller J. International tables of glycemic index.
Am J Clin Nutr. 1995; 62(Suppl): S871-S893.
3. Thomas, DE, Brotherhood JR, Brand JC. Carbohydrate feeding before exercise:
effect of glycemic index. Int J Sport Med. 1991; 12:180-186.
4. Thomas DE, Brotherhood JR, Brand Miller, J. Plasma glucose levels after prolonged
strenuous exercise correlate inversely with glycemic response to food consumed
before exercise. Int J Sport Nutr. 1994; 4:361-373.
5. Sparks MJ, Selig SS, Febbraio MA. Pre-exercise carbohydrate ingestion: effect
of the glycemic index on endurance exercise performance. Med Sci Sport Exerc.
1998; 30:844-849.
6. DeMarco HM, Sucher KP, Cisar CJ, GE Butterfield. Pre-exercise carbohydrate
meals: application of glycemic index. Med Sci Sport Exerc. 1999; 31:164-170.
7. Sherman WM, Peden MC, Wright DA. Carbohydrate feedings 1 hour before exercise
improves cycling performance. Am J Clin Nutr. 1991;54:866-870.
8. Burke LM, Collier GR, Hargreaves M. Muscle glycogen storage after prolonged
exercsie: effect of glycemic index. J. Appl. Physiol. 1993; 75:1019-1023.