by Charlene Boudreau, USA Swimming

While there are plenty of studies addressing the use of carbohydrate ingestion by trained athletes, most of these studies have involved cycling or running, rather than swimming.  In addition, many studies have focused on the ingestion of carbohydrates during exercise, as opposed to before it.  Research in this area is also difficult to interpret because there are several factors that can influence a carbohydrate’s ability to achieve the performance-enhancing effect.  In other words, the extent to which carbohydrate ingestion “works” depends on various combinations of the following:

    Type of exercise

• Intensity of the exercise

• Amount of carbohydrate ingested

• Type of carbohydrate ingested

• Timing of the carbohydrate ingestion

• Pre-exercise nutritional status

• Pre-exercise training status

Timing is particularly important because of the effect most carbohydrates have on blood sugar.  Following their ingestion, carbohydrates tend to raise blood sugar levels significantly.  This reaction is immediately followed by an equally dramatic rise in insulin, which causes (1) the uptake of blood glucose by the muscles and (2) a reduction in glucose production by the liver.  Once glucose is moved from blood to muscle, it can then be converted to glycogen for storage.  Although the initial net result of carbohydrate ingestion is a significant drop in blood glucose, levels eventually rise to surpass baseline values.  The total time for this chain of events to occur is about one hour.  Once at their peak, glucose levels decline gradually at a rate that depends on the activity at the time.

The extent to which a carbohydrate increases blood glucose, and therefore insulin, in the bloodstream can be quantified by a number called the Glycemic Index.  A higher glycemic index translates to a faster and more dramatic response.  A lower glycemic index indicates a more gradual response.  For example, the ingestion of glucose (glycemic index=100) quickly invokes the dramatic chain of reactions described above.  On the other hand, the ingestion of fructose (glycemic index=23) results in a more prolonged response because its glycemic index is much lower.  During exercise, high glycemic index carbohydrates help maintain blood sugar levels, while low glycemic index carbohydrates do not because the response is not dramatic enough.  Prior to exercise, high glycemic index carbohydrates have the potential to induce a poorly timed state of hypoglycemia, while low glycemic index carbohydrates do not.  Hence we see the importance of timing.

Fueling for a workout is different than fueling for a meet.  Fueling for a meet is different from fueling for a race. Therefore, fueling for performance must be looked at from different perspectives.

Workouts and Training - The typical pool workout for competitive swimmers lasts 90 minutes to 2 hours.  There are two mechanisms responsible for fatigue during this type of prolonged exercise, (1) hypoglycemia (low blood sugar) and (2) glycogen depletion (glycogen is the storage form of carbohydrate).  Since both can be alleviated by eating carbohydrate-rich foods like bagels, fruit and sports drinks, many research papers on preventing fatigue during exercise have focused on the effects of carbohydrate ingestion on maintaining blood sugar levels.  The availability of this “fuel” during exercise allows the body to spare glycogen and prolongs the time an athlete can exercise before tiring.  It has been demonstrated that athletes who eat a high-carbohydrate meal an hour or two before exercise experience an increase in glycogen storage and benefit less from ingesting carbohydrate during exercise than athletes who fasted.  In other words, for those who do not fuel up ahead of time, the intake of a carbohydrate-containing product during a workout is much more critical in preventing fatigue.  The more glycogen the body has stored, the more accessible the fuel is in a time of need, such as during high-intensity exercise.  The less glycogen the body has stored, the more important it is to provide exogenous (outside) sources of carbohydrate during a tough workout.  Therefore, it makes sense to fuel the body beforehand.

Meet Day and Pre-Race - As mentioned previously, high glycemic index carbohydrates help maintain blood sugar levels when eaten during exercise, but when eaten at rest, they result in an insulin “spike” within 15 minutes that can lead to a dramatic drop in blood sugar within 45-60 minutes, and a temporary period of hypoglycemia.  At rest, this response may facilitate the storage of glycogen (a desirable effect), but it would not be desirable to experience low blood sugar levels during the hour immediately prior to a race.  Before a race, low glycemic index foods prevent the sharp drop in blood sugar.  Therefore, carbohydrates with a low to moderate glycemic index have been recommended as part of the race day menu, particularly when there is less than an hour to go before the start.

There has been some controversy regarding the use of glycemic index in designing meals because of factors that can influence it (amount eaten, amount of added fiber, amount of added fat, food preparation).  Most of these arguments stem from the healthcare perspective because the glycemic index was originally introduced to help diabetics control their blood sugar levels.  In swimming, the issue is not long-term disease prevention, but rather utilizing the blood sugar response to (1) maximize carbohydrate storage during training and (2) to weaken blood sugar “spikes” and drops during competition.  Therefore, the application of glycemic index to the pre-race meal and training performance is very worthy of consideration.

Applications of recent research (Kirwam, O’Gorman & Evans, 1998;Stannard, Thompson & Brand Miller, 2000; Sparks, Selig & Febraio, 1998; Febbraio, Keenan, Angus, Campbell & Garnham, 2000; DeMarco, Sucher, Cisar & Butterfield, 1999; Mitchell, Braun, Pizza & Forrest, 2000.)

• Eating a breakfast cereal 45 minutes prior to a moderate intensity pool practice can be beneficial in comparison to eating no carbohydrate at all.

• Eating carbohydrates with a glycemic index in the 60-70 range can limit the use of fat as a fuel during the first hour of activity.  To limit this effect and maximize the carbohydrate-sparing effect, whole-grain oats cereal may be a better choice then whole-oat flour cereal for this pre-workout meal.

• Adding dietary fiber may be another strategy to enhance the use of fat as fuel in the early stages of a workout.

• The glycemic index of the pre-exercise meal has little impact on maximal performance lasting 21 minutes or less.

• Eaten at least an hour in advance, carbohydrates are not likely to impact most non-open-water races.

• The type of carbohydrate ingested 45 minutes prior to exercise may not impact performance in sub-maximal activities lasting less than 65 minutes (a short practice).

• Eating low glycemic index foods before workouts limits carbohydrate preferencing that comes with eating high glycemic index foods.  Using carbohydrate as fuel in the early stages of sub-maximal exercise may limit the amount of carbohydrate available during the latter stages of long workouts.

• Eat moderate-low glycemic index foods before workouts.  Ingesting high glycemic index foods 30 minutes prior to sub-maximal exercise stimulates the body to burn carbohydrate instead of fat.

• Increasing the use of carbohydrate as fuel in the early stages of sub-maximal exercise may limit the amount of carbohydrate available during the latter stages of long workouts.

• To stimulate the body to burn fat instead of carbohydrate, eat low glycemic index foods 45 minutes prior to long workouts.

• Sparing carbohydrate for later stages of exercise may improve the quality and feel of performance as the workout approaches the two-hour mark.

• Adding dietary fiber to the pre-workout meal may be another strategy to enhance the use of fat as fuel in the early stages of a workout, thus allowing for better performance later on.

• The type of carbohydrate ingested 60 minutes prior to exercise may not impact performance in activities lasting less than 45 minutes.

• The inclusion of carbohydrate in the pre-exercise drink can help maintain fluid balance. (The small percent of carbohydrate in these drinks enhances fluid absorption.)

The Final Word

Studies that show direct performance benefits from low glycemic index carbohydrate ingestion are not directly applicable to swim meets because competitive swimming involves completing a pre-determined amount of work in the shortest time possible.  It’s not about which swimmer goes the longest distance before fatiguing.  The glycemic index has practical applications in the sense that proper pre-exercise food selection can (1) improve performance during workouts that are 2 hours or more in duration and (2) prevent undesired effects close to race time.

    In terms of fuel, the pre-race meal contributes little to performance, but the pre-   workout meal can affect the quality of training day-to-day.

    During meet days, athletes are encouraged to eat high glycemic index (>60) foods if they have 2 or more hours before their next race.  When the time remaining until the next race is less than 2 hours, swimmers should try to find low glycemic index (<40) food choices.

Talking Point:  There may be an opportunity for swimmers to maintain leanness by optimizing the use of fat as a fuel source in the early stages of practice, while sparing glycogen for the latter sets.  This strategy will not solve the problem of excess body fat, but it might help.  It may also help the caloric deficit and spare protein.

The following table provides the glycemic index ratings of some common foods:

*Glycemic Index ratings complied from Bernadot, D.  Nutrition for Serious Athletes.  Human Kinetics:Champaign, 2000; Clark, N.  Nancy Clark’s Sports Nutrition Guidebook.  Human Kinetics:Champaign, 1997; Foster-Powell, K. and J. Brand-Miller.  International tables of glycemic index.  International Journal of Clinical Nutrition 62:871S-893S, 1995; Walberg Rankin, J.  Gatorade Sports Science Exchange 26(1), 1997.

For information on what to eat AFTER workouts, see our review on Carbohydrate, Protein and Recovery (CARBOHYDRATE, PROTEIN AND RECOVERY LINK), and for maintaining hydration and energy DURING workouts, see our review on Hydration and Fueling During Exercise (LINK TO HYDRATION AND FUELING DURING EXERCISE).

References

Kirwan, J.P., D. O’Gorman and W.J. Evans.  (1998).  A moderate glycemic meal before endurance exercise can enhance performance.  Journal of Applied Physiology 84(1):53-59.

Stannard, S.R., M.W. Thompson and J.C. Brand Miller.  (2000).  The effect of glycemic index on plasma glucose and lactate levels during incremental exercise.  International Journal of Sport Nutrition and Exercise Metabolism 10:51-61.

Sparks, M.J., S.S. Selig and M.A. Febraio.  (1998).  Pre-exercise carbohydrate ingestion: effect of the glycemic index on endurance exercise performance.  Medicine and Science in Sports and Exercise 30(6):844-849.

Febbraio, M.A., J. Keenan, D.J. Angus, S.E. Campbell and A.P. Garnham.  (2000).  Pre-exercise carbohydrate ingestion, glucose kinetics, and muscle glycogen use: effect of the glycemic index.  Journal of Applied Physiology 89:1845-1851.

DeMarco, H.M., K.P. Sucher, C.J. Cisar and G.E. Butterfield.  (1999).  Pre-exercise carbohydrate meals: application of glycemic index.  Medicine and Science in Sports and Exercise 31(1):164-170.

Mitchell, J.B., W.A. Braun, F.X. Pizza and M. Forrest.  (2000).  Pre-exercise carbohydrate and fluid ingestion: influence of glycemic index response on 10-km treadmill running performance in the heat.  Journal of Sports Medicine and Physical Fitness 40(1):41-50.

Recommended Reading

Bjorck, I., Y. Granfeldt, H. Liljeberg, J. Tovar and N-G. Asp.  (1994).  Food properties affecting the digestion and absorption of carbohydrates.  American Journal of Clinical Nutrition 59(suppl):699S-705S.

Foster-Powell, K. and J.B. Miller.  (1995).  International tables of glycemic index.  American Journal of Clinical Nutrition 62(suppl):871S-893S.

Hawley, J.A. and L.M. Burke.  (1997).  Effect of meal frequency and timing on physical performance.  British Journal of Nutrition 77(suppl 1):S91-S103.

Tsintzas, K. and C. Williams.  (1998).  Human muscle glycogen metabolism during exercise – Effect of carbohydrate supplementation.  Sports Medicine 25(1):7-23.

Tufts University.  (2000).  Should you be choosing your foods by their glycemic index?  Tufts University Health and Nutrition Letter November:4-5. 

Walberg Rankin, J.  (1997).  Glycemic index and exercise metabolism.  Sports Science Exchange 10(1):. 

Williams, C. and C. Chryssanthopoulos.  (1997).  Pre-exercise food intake and performance.  World Reviews of Nutrition and Dietetics 82:33-45.

Wolever, T.M.S., D.J.A. Jenkins, A.L. Jenkins and R.G. Josse.  (1991).  The glycemic index: methodology and clinical implications.  American Journal of Clinical Nutrition 54:846-854.