High intensity exercise of short du-ration, such as weight lifting, relies onthe anaerobic pathway for adenosinetriphosphate (ATP is the fuel sourceused directly by the muscles to powermuscular contractions.) Under theseconditions, only glucose can be used asfuel (1). Exercise of low to moderateintensity (up to 60 percent of aerobiccapacity) can be fueled almost entirelyaerobically (1). This means that carbo-hydrate, fat, and amino acids can all beused as fuel for low intensity exercise.Hormones are released into thebloodstream during exercise, and thesepromote fat mobilization and the use offat for energy. At low to moderate in-tensity exercise, fat and carbohydrateeach supply about half of the energy (1).Fat cannot be metabolized rapidlyenough to meet the energy requirementsof intense exercise (above 70 percentaerobic capacity). Furthermore, it takesmore oxygen to burn fat than to burncarbohydrate (1,2). This makes carbo-hydrate a better fuel choice for intenseeffort, when oxygen supply is limited.For a given amount of oxygen, moreenergy can be obtained from carbo-hydrate than from fat. Muscleglycogen is the fuel source usedfor most forms of exercise, es-pecially weight training.
It takes30 to 60 minutes of exercise forfatty acids to be available to themuscles to use as fuel (1). Upuntil this time, glucose derivedfrom muscle glycogen is the pri-mary fuel.This is why it’s best to doyour aerobics when you’re gly-cogen depleted, and the hor-mones released during exercisehave had time to mobilize fatty ac-ids. An excellent time for aero-bics is in the morning before break-fast, because you’re glycogen de-pleted then. And, the longer you spendexercising, the more fat will be used asthe fuel source.Exercise training, especially endur-ance exercise, increases aerobic capac-ity. As the heart, lungs, and blood sys-tem get bigger and stronger, they candeliver more oxygen to the tissues. Thisallows relatively more fat to be used ata given level of exercise intensity. Theanaerobic threshold is the intensity ofeffort at which lactic acid begins to ac-cumulate and is usually expressed as apercentage of aerobic capacity (1).Lactic acid is produced from anaerobicmetabolism of glucose, so the anaero-bic threshold is a measure of how wellyour body is trained for aerobic energyproduction.
Trained athletes start toaccumulate lactic acid at about 70 per-cent of aerobic capacity, while untrainedindividuals begin to accumulate lacticacid in their blood at about 50 percentaerobic capacity.Furthermore, if aerobically trainedand untrained individuals exercise at therelative intensity (the same % VO2max),the untrained individual will accumulatemore lactic acid in the muscles and bloodthan the trained individual (3). The dif-ference would be even greater if exer-cise was performed at the same abso-lute oxygen requirement. This is ex-plained in part by decreased clearanceof lactic acid through the liver by un-trained individuals (3) and also by moreefficient aerobic metabolism of carbo-hydrates and fat in the trained athlete(4).In addition to increasing muscleglycogen storage, endurance trainingalso increases the muscles’ ability to usefat as an energy source. This increasedability to burn fat spares glycogenstores, thus further increasing endur-ance (4). Endurance training increasesthe size and number of mitochondria inthe muscles and activates enzymes in-volved in the Krebs cycle and oxidativephosphorylation – the central energyproducing pathways of the body (1,3).
This is one reason why bodybuildersshould engage in aerobic exercise: Itincreases the ability to burn fat for en-ergy. This not only helps you stay leanbut also spares glycogen so you can trainharder and longer.Muscle glycogen reserves becomeprogressively lower during exercise.During long bouts of exercise, glyco-gen reserves may drop to critically lowlevels – to the point of glycogen deple-tion (1). The athlete then feels ex-hausted and must stop exercising ordramatically reduce the intensity. Thepoint of muscular fatigue coincides withglycogen depletion (5). This is sepa-rate from momentary muscular failureat the end of a set which is due to lacticacid accumulation. Glycogen reservescan also be depleted gradually over a period of days if carbohydrate intakedoes not match that utilized during ex-ercise. This feeling of fatigue from fail-ure of adequately replenish glycogen re-serves is often interpreted as overtrain-ing. In some cases, overtraining maybe alleviated by increased carbohydrateconsumption. Not getting a good pumpin the gym is a clue that you’re prob-ably glycogen deficient.The amount of carbohydrates youtake in affects your training intensity.A group of athletes consuming 300 to350 grams of carbohydrate per day wasseen to become progressively more gly-cogen depleted during successive daysof training (6). After several days, theseathletes were unable to continue withheavy training. In contrast, a diet pro-viding 500 to 600 grams carbohydrateper day was seen to result in completerepletion of glycogen reserves, and ath-letes on this diet were able to maintain aheavy training schedule.Of course, these numbers are notprescriptive.
An individual athlete’s car-bohydrate requirement depends on hisenergy needs, which in turn depend onthe type, intensity, duration, and fre-quency of exercise. Endurance athletesrequire the most energy and the mostcarbohydrates. The longer and harderyou train, the more carbohydrate calo-ries you need.Some athletes train so heavily thatthey have trouble consuming enoughhigh carbohydrate foods to fuel their ac-tivities and replenish glycogen stores(1). Also, consuming a huge volume offood can cause gastrointestinal distress,bloating, or discomfort, and is not con-ducive to optimal exercise performance. Carbohydrate drinks are veryuseful in this situation, as well as forathletes trying to further increase ca-loric intake. Carbohydrate beveragesare also useful during training and ath-letic competitions to help maintain en-ergy. The best carb drinks containslow-release glucose polymers (dex-trins) rather than simple sugars such asglucose, sucrose, or fructose. This isthe formula contained in our Pro-Carb™powder drink mix made frommaltodextrin, a slow-releasing carbohy-drate. To order Pro-Carb™, simply callour toll-free orderline at 1-800-344-3404.
1. Coleman,E. Carbohydrates: themaster fuel. In: Sports Nutrition for the90’s, eds. Berning, JR and Stenn, SN.Aspen Publishers, 1991.
2. Guyton, Textbook of MedicalPhysiology. Published by W.B.Saunders, chapter 71, 1976.
3. Nagle FJ, Bassett DR Jr. En-ergy Metabolism in Exercise. In; Nu-trition in Exercise and Sport, eds.Hickson, JF and Wolinsky, I. CRCPress, 1989.
4. Sherman, W. Carbohydrates,muscle glycogen, and muscle glycogensupercompensation. In: Ergogenic Aidsin Sport, ed. Williams, M. Human Ki-netics Publishers, 1983.
5. Vander, Sherman, and Luciano,Human Physiology – The Mechanismsof Body Function, p. 236. Publishedby McGraw-Hill Book Company, 1980.
6. Costill DL, Bowers R, Branam G,Sparks K. Muscle glycogen utilizationduring prolonged exercise on succes-sive days. J. Appl. Physiol. 31: 834-838, 1977.