In carbohydrate metabolism, therate of glycogen utilization is directly pro-portional to the intensity of the exercise(1). In other words, the faster the bodyneeds to produce energy, the more it re-lies on carbohydrates. This is becausefat cannot be metabolized fast enough tomeet the energy demands of high exer-cise. Furthermore, carbohydrates can bemetabolized to produce energy anaerobi-cally (in the absence of oxygen) while fatmetabolism requires oxygen. Also, dur-ing aerobic metabolism, it takes less oxy-gen to burn carbs than to burn fat. For agive amount of oxygen, the body can pro-duce more ATP from carbs than from fat.This makes carbohydrate a better fuelsource during intense exercise when oxy-gen is a limiting factor.The respiratory quotient (RQ) isthe ratio of carbon dioxide produced tooxygen consumed. One liter of oxygen isable to produce 5 calories from carbohy-drates but only 4.7 calories from fat (2,3).Since carbohydrates and fat require dif-ferent amounts of oxygen to burn, youcan tell from the respiratory quotientwhat kind of fuel is being used by thebody. An RQ of 1.0 indicates carbsare being used exclusively for en-ergy (4).Intermediate values ofRQ indicates that a mixtureboth fuels are being used. Us-ing this sort of measurementduring cycling exercise, re-search found that carbohydrate,when available, is the body’spreferred fuel. Radioisotopestudies and muscle biopsy as-sessments of energy stores be-fore and after exercise have con-firmed this finding (4).In general, as intensity of exer-cise increases, the relative contri-bution of carbohydrate as the fuelsource increases (1,2).
Furthermore,a low-carbohydrate diet reduces exercisetime to exhaustion (1). At rest, musclesrely mostly on fat as their energy source(5).During low intensity exercise (40-50%VO2max) such as walking, the primarymetabolic fuel is fat, while muscle glyco-gen degradation is minimal (1). As theexercise intensity increases, more muscleglycogen is used as fuel. Moderate inten-sity exercise (50-60% VO2max) is fueledby roughly equal amounts of fats and car-bohydrates (2, 4). At 90-95% VO2max,carbohydrate provides as much as 95%of the energy and the RQ approaches 1.0(1, 2, 4). Many athletes train at intensi-ties above 70% VO2max which precludesthe use of fat as fuel.Although high-intensity exerciseburns more calories per hour, more ofthose calories come from carbs. Low-intensity exercise is fueled mostly by fat,but doesn’t burn very many calories perhour. One of the best exercises for losingfat is running. Although running is highintensity and is fueled partially by carbs,it has the additional benefit of loweringyour set point — the amount of fat yourbody is programmed to store.When glycogen stores are limit-ing, the body also draws on amino acidsas fuel (4, 5). The liver can convert mostamino acids into glucose in a processcalled “gluconeogenesis.” The branchedchain amino acids (BCAAs) leucine, iso-leucine and valine can be oxidized as fueldirectly in the muscles (5). One studyshowed that as much as much as 57grams of protein — equivalent to the rec-ommended daily allowance — could beburned as fuel during a 10-12 mile run(6). Lemon and Mullen showed that dur-ing a cycling effort lasting over an hourat 61% VO2max, 10.4% of the energywas derived from protein if the subjectswere in a glycogen-depleted state (7). Ina glycogen-loaded state, only 4.4% ofenergy derived was from protein.
Thisis another important reason why body-builders need to ensure adequate carbo-hydrate intake: Carbs have a protein-spar-ing effect, meaning that if adequate carbsare available, they will be used instead ofprotein. Obviously, bodybuilders wanttheir amino acids to be used for buildingprotein, not as fuel. Pro-Carb is the ultimate carbo-hydrate supplement. Carbohydrates area great energy source for fueling athleticactivities and supporting weight gain.Pro-Carb is ideal for carb loading, for useduring training and athletic competitionand as a source of extra calories. Notonly is it high in carbohydrates, but a Pro-Carb drink also supplies water which isneeded for glycogen storage. Liquid car-bohydrate meals can be consumed closerto competition than solid foods becauseof their shorter gastric emptying time (2).Liquid meals also produce a low stoolresidue and thus minimize intestinal bulk.Liquid meals are a good way to get calo-ries in without filling up your intestines.Pro-Carb is also an excellent supplementto add extra calories during heavy train-ing. Pro-Carb utilizes maltodextrin as itscarbohydrate source. Maltodextrin is acomplex carbohydrate derived fromstarch with a low glycemic index (rang-ing from 22 to 29), meaning that it is re-leased into the bloodstream more slowlythan simple sugars (glucose has a glyce-mic index of 100). That way, you don’tget a large insulin surge with subsequenthypoglycemia. Using Pro-Carb, you geta more even energy level because thecarbs are released slowly, minimizing thechance that any will “spill over” into fatstores. Maltodextrin is a glucose poly-mer, making it ideal for replenishing gly-cogen stores. Pro-Carb mixes instantlyand tastes great, and contains no simplesugars or artificial sweeteners. Onescoop provides 22 grams of complexcarbs, 4 grams of high quality protein,with less than a gram of fat. Mixed without Hi-Protein powder, it makes a balancedliquid meal.
1. Sherman, W. Carbohydrates, muscleglycogen, and muscle glycogensupercompensation. In: Ergogenic aidsin sport, ed. Williams, M. Human Kinet-ics Publishers, 1983
2. Coleman, E. Carbohydrates: the mas-ter fuel. In: Sports Nutrition for the 90s,eds. Berning, JR and Stenn, SN. AspenPublishers, 1991.
3. Guyton, Textbook of Medical Physi-ology. Published by W.B. Saunders, chap-ter 71, 1976.
4. Nagle FJ, Basset, DR Jr. Energy Me-tabolism in Exercise. In: Nutrition in Ex-ercise and Sports, eds. Hickson, JF andWolinsky, I. CRC Press, 1989.
5. Miller, GD and Massaro, EJ. Carbo-hydrate in ultra-endurance performance.In: Nutrition in Exercise and Sports, eds.Hickson, JF and Wolinsky, I. CRC Press,1989
6. Dohm, GL, Williams, RT, Kasparek,GJ and Van Rij, AM. Increased excre-tion of urea and N-methylhistidine by ratsand humans after a bout of exercise. JAppl Physiol: Respirat Environ ExcerisePhysiol 52: 27, 1982.
7. Lemon, PWR and Mullen, JP. Effectof initial muscle glycogen levels on pro-tein catabolism during exercise. J ApplPhysiol 48: 624-629, 1980.