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Bulletin #57 – Carbohydrates — The Optimal Fuel For Success

Recently we talked about the ben-efits of a high carbohydrate diet as com-pared to the high fat diet in terms of get-ting lean. Not all calories are cre-ated equal. Dietary fat is preferen-tially stored as body fat, whereascarbohydrates do not significantlycontribute to fat stores (1,2,3). Mostof your body fat is derived from thefat you eat and very little comesfrom conversion of protein or car-bohydrate into fat (1,2,3). So itmakes sense that if you want to re-duce body fat the first place to startis to eat less fat. Proponents of thehigh fat diet say that carbohydratescause insulin release which in turnstimulates fat storage. On the Parrillodiet we teach you how to select car-bohydrate sources and structureyour meals so that carbohydrates arereleased into the bloodstream veryslowly, so this isn’t a problem. Tolook at the big picture, if you con-sume too many calories from anysource for a prolonged period oftime some of these calories will end up asbody fat. Conversely, if you operate in acalorie deficit for a prolonged period oftime you will lose some fat. However, theresults you will get in either of these situ-ations are different depending on if yousupply the bulk of those calories in theform of fat or carbohydrate. We havefound that if you eat a diet higher in carbsand low in fat you will end up being leanerand more muscular whether you are us-ing the diet to gain weight or lose weight.

Parrillo Performance

This month I want to talk aboutthe other half of the story, that carbohy-drates are a superior fuel for exercise per-formance compared to fat. Not only willcarbs make you leaner, they’ll also giveyou more energy, strength, and endurance.We’ll start with a review of energy sub-strate utilization during exercise, and thentalk about dietary strategies to maximizeyour exercise performance. This discus-sion will apply equally well to bodybuild-ers and endurance athletes.Adenosine triphosphate (ATP) is the mol-ecule that directly provides energy for yourmuscles. Before any foods can be usedto fuel exercise, the energy they containmust be converted into ATP. When ATPis broken down it forms adenosine diphos-phate (ADP) and free phosphate. To dothis a phosphate bond is broken. Whenthis happens energy is released and it isthis energy which powers muscular con-tractions.

You know that when you burnwood in your fireplace heat is released.Heat is a form of energy. When food isburned inside your body the energy whichis released is used to manufacture ATP.The ATP is then broken down releasingthat energy which it captured from thefood, and this energy is the fuel your bodyuses at the cellular level to do its work,including muscular contractions.The amount of ATP stored insideyour cells is very small. Muscle cells con-tain enough ATP to power a maximal con-traction for about two to four seconds(4). So the ATP has to be continu-ally replenished as it is used. Thereare three energy systems in placefor making new ATP so you cancontinue to exercise beyond fourseconds. These are the phosphagensystem, the lactic acid system, andthe aerobic system. The phosph-agen system is comprised of the“high energy phosphate com-pounds,” which include ATP andcreatine phosphate (CP). This isby far the fastest energy systemand can provide for rapid burstsof high intensity exercise such asweight lifting, sprinting, football,and some track and field events.Muscle cells contain from three tofive times more CP than ATP, andthe phosphagen system can sus-tain maximal exercise for about sixto eight seconds. After that, the CPis also used up and we have to ac-tivate another energy system. You can seewhy creatine is such an effective andpopular sports supplement. It helps to pro-vide more immediate energy allowing forhigher intensity exercise.

This is why cre-atine increases strength in weight lifters.As a nice “side effect” it also attracts waterinto the cell, making the muscles fullerand harder.The next energy system is the lactic acidpathway. This can provide maximal en-ergy for periods lasting up to one-and-a-half or two minutes. This pathway con-sists of the anaerobic conversion of glu-cose into lactic acid (anaerobic means“without oxygen”). This is actually a rela-tively long and complicated process in-volving several steps, and the overall path-way is called “glycolysis.” The end resultis that one glucose molecule is brokendown to form two lactic acid molecules, releasing energy in the process. This en-ergy is used to form ATP. Where doesthis glucose come from? By far the mostimportant source of glucose for this pro-cess is stored muscle glycogen (4,5).Let’s think about that for a minute. Howlong do most of your sets last in the gym?I’m not talking about the rest period, butthe amount of time you’re actually liftingweight with the muscle under tension.Probably anywhere from 30-60 seconds,generally.

This is definitely longer than thephosphagen system can hold out withoutbeing replenished from some other energysource. Without calling in reinforcements,the phosphagen system by itself couldonly get you through the first rep or two.So what this means is that for very in-tense (maximal or near maximal) exerciselasting about a minute or so, such asweight lifting, the lactic acid system is theprimary energy-producing pathway atwork (4,5,6). And the primary fuel sub-strate for the lactic acid system is storedmuscle glycogen (4,5,6). When you do aset in the gym most of the energy comesfrom stored muscle glycogen. So muscleglycogen is the most important fuel sourcefor weight lifters, sprinters, football play-ers, and other athletes performing shortbursts of maximal exercise. A high car-bohydrate diet is best for athletes becauseit helps maintain a high level of stored gly-cogen in muscle.The problem with fat as an en-ergy source for resistance exercise is thatfat cannot be converted into glucose orglycogen, at least in humans. So a highfat diet cannot maintain muscle glycogenand therefore cannot support as high alevel of exercise performance. You mightwonder how people on extremely lowcarb diets can manage to lift weights atall.

While fat cannot be converted into glu-cose, amino acids from protein break-down can. So even if you don’t eat anycarbs at all you can still get a little muscleglycogen from breakdown of protein andconversion of amino acids into glucose.Also, the high fat diet results in the pro-duction of ketones which can be used bymuscles as fuel for weight lifting. I sus-pect that most people who follow the highfat diets have extremely low muscle gly-cogen levels and in fact are able to per-form some degree of resistance trainingusing ketones as fuel. We have seen thatwhen CapTri is used as an energy supple-ment during low carb “cutting” diets thisgreatly improves the ability to continue lift-ing. And we know that CapTri works bybeing converted into ketones by the liver.So I suspect the same process is goingon with the high fat diets, except that itdoesn’t work nearly as well with conven-tional fats as it does with CapTri becauseCapTri is converted into ketones muchmore rapidly and completely than are con-ventional fats. On the Parrillo Programeven when we use CapTri in this way,the bodybuilder is still eating some carbs,just not as much as usual. So he still has asignificant store of muscle glycogen.

Theproblem with the high fat diet is not onlythat you have virtually no muscle glyco-gen, but also that the ketones which areavailable are not stored to any appreciableextent in muscle cells. So on low carbdiets you have no muscle glycogen, mean-ing that you’ve knocked out the primaryenergy pathway used in weight lifting-typeexercise. You do have ketones, but thesecan’t be stored inside the muscle. You haveless strength, less endurance, and sinceglycogen stores are depleted your musclesare flat and you have no pump. Not a verygood way to go. There can be no doubtthat carbohydrates are a much better fuelsource for bodybuilders and enduranceathletes than fat, and this is backed up bybiochemistry, by the scientific literature,and by our own testing here at ParrilloPerformance.The third energy pathway is theaerobic system. This system can provideenergy continuously for hours, but at alower level of intensity compared to theother pathways. This pathway can useboth carbohydrate and fat, and in fact isthe only energy system that can use fatas a fuel substrate. As explained above,very brief and intense exercise such asweight lifting and sprinting is fueled bythe lactic acid system (an anaerobic en-ergy-producing pathway) and very long,low intensity exercise such as walking isfueled by the aerobic system. Moderateintensity exercise such as jogging or bik-ing is fueled by a combination of both path-ways.

When the energy requirement de-manded by the exercise does not exceedthe ability of the aerobic system to supplyATP, then this is the primary energy sys-tem. A leisure walk is powered by theaerobic pathway, and you can sustain thislevel of activity for hours. If you pick upthe pace and start jogging at a comfort-able pace you can maintain for some time,this is still fueled by the aerobic pathway.If now you begin running fast, the aero-bic energy system can no longer supplyenough ATP to meet the energy require-ment of your muscles and the lactic acidsystem kicks in. You can keep up thispace for several minutes – longer than the2 minutes the lactic acid system could lastby itself because it is being supplementedby the aerobic system. Now if we moveup one more notch of intensity, such asweight lifting or an all-out sprint, the lac-tic acid system is operating at full boreand when muscle glycogen becomes de-pleted (among other factors) the exercisewill stop. The aerobic system simply can-not supply energy fast enough to keep upwith the demands of the muscle.Now let’s discuss how your bodyselects energy substrates during exercise.After we understand the patterns of en-ergy substrate utilization we can design effective dietary strategies to maximize ourresults. Glucose stored as glycogen inmuscle and liver, and fatty acids storedas triacylglycerols (fat molecules) in adi-pose depots, are both important fuelsources for exercise.

Factors which de-termine the balance between carbohydrateversus fat which is used as fuel includethe intensity of exercise, the duration ofexercise, the athlete’s level of condition-ing, and initial glycogen levels (4). Aminoacids can also be oxidized (burned) to pro-vide fuel, but normally contribute less than10% of the energy cost of exercise. Onthe Parrillo Program we do everything pos-sible to prevent the use of amino acids asfuel, because we want to maximize musclemass. So for people on our program thecontribution of amino acids to the fuel mixis even less than 10%.By far the most important thingyou can do to prevent use of protein asfuel is to make sure your muscle glyco-gen stores are always “topped off.” That’sanother serious draw-back of the verylow carb diets. Glycogen levels are so lowthe muscle protein is very vulnerable tobe broken down so that the aminoacids can be converted to glucoseand used as fuel. Adequate glyco-gen availability “spares” aminoacids, meaning that if your bodyhas carbs available to use as fuelit won’t need to use any protein.The next thing you can do is touse a scoop of Hi-Protein Powderright before your aerobics so thatif any amino acids are going to beused as fuel they will be derivedfrom the protein powder insteadof your muscles.

The Hi-Proteinis probably superior to regularfood for this purpose since it isdigested rapidly and the aminos arereleased into the bloodstreamfaster than from whole, solid pro-tein foods. I recommend doingyour aerobics first thing in themorning before breakfast, be-cause it’s at that time that bloodsugar levels are low and muscleglycogen is at the lowest level itwill be throughout the day. Thisresults in greater use of stored body fatas a fuel source during your aerobics sinceless carbohydrate energy is available. Thisdoes however put you at some risk forbreaking down muscle, so that’s why it’sa good idea to have a serving of Hi-Pro-tein Powder before your morning aero-bics. Keep in mind however that this con-dition of relative glycogen depletion I’mtalking about is a far cry from the near-zero levels resulting from low carb diets.Even first thing out of bed before break-fast, Parrillo bodybuilders have a lot ofglycogen on board. This just turns out tobe the time which it will be lowest duringthe day, so it’s the best time to burn fat.So, if we control things such that we usevery little or no protein as fuel, the energycost of our exercise is supplied by a mix-ture of carbohydrate and fat. The relativecontribution of carbohydrate and fat tothe substrate mix being oxidized can bedetermined by measuring the respiratoryquotient (RQ). This gets a little technicalhere, so I’ll be brief.

Carbohydrate mol-ecules such as glucose contain oxygen,so the carbon atoms in the carbohydratemolecule are already partially oxidized.However, the carbon atoms in a fatty acidmolecule are not partially oxidized. There-fore, when fat and carbohydrate areburned (oxidized) separately differentamounts of oxygen are consumed peramount of carbon dioxide produced. Soif we measure the amount of oxygen con-sumed and carbon dioxide produced byan athlete while he’s exercising, we can“back calculate” if he’s burning fat orcarbs as fuel. (I hope that wasn’t too pain-ful.) An RQ of 1.0 indicates that essen-tially pure carbs are being used as fuel,and an RQ of 0.7 indicates that fat is thefuel source. Intermediate values of RQdemonstrate that a mixture of carbohy-drate and fat is serving as the fuel supply.What experiments like this haveproven is that during very low intensityexercise (like walking) fat is the predomi-nant fuel source (4,5,6). During sleep orrest almost exclusively fat is used. So ifyou believe these guys who advocate lowintensity aerobics because it uses a higherpercentage of calories from fat, then justsitting around and watching TV shouldget you ripped. As the intensity of exer-cise increases, we see that more carbo-hydrate is used as fuel (4,5,6).

This pro-gression continues until exercise intensityreaches V02max, at which time carbohy-drate becomes the sole energy substrate(4). You will recall that V02max (vee-oh-two-max) represents the maximal rate ofoxygen consumption by an athlete. Thismeans that the aerobic energy system iscompletely maximized, and any furtherincrease in energy needs must be met bythe anaerobic (lactic acid) system. Sincefat cannot be used by the lactic acid sys-tem, the energy at this point can only besupplied by carbs. This all makes goodsense. We know that we can maintain lowintensity exercise for prolonged periodsof time (you could walk all day if youwanted to). That’s because this activityis powered by the aerobic energy path-way, which can use fat as a fuel source.Your body has many more calories storedas fat than it can store as glycogen, be-cause fat is a much more compact wayto store energy and because you have limited space to store glycogen. You can carryon this low level of activity indefinitelybecause you won’t run out of fat. But wecan only sustain maximal exercise for arelatively short time, because this relieson the lactic acid system which can onlyuse glucose as fuel (other factors con-tributing to muscle fatigue are also at playhere).

After the glucose (stored as glyco-gen) is used up the fat can’t burn fastenough to meet the demands of the exer-cise. At very high exercise intensity oxy-gen cannot be delivered to the muscle fastenough to allow the aerobic energy sys-tem to operate, so the muscle must relyon the anaerobic (lactic acid) system,which can only use carbohydrate. Thebottom line: fat cannot serve as the fuelsource for very high exercise intensity,because it cannot undergo anaerobic me-tabolism. Carbs are the only fuel that cansupport maximal exercise intensity.As you would expect, moderateintensity exercise uses a mixture of carbsand fat. As the duration of exercise pro-ceeds, muscle glycogen gradually be-comes depleted so oxidation of fat beginsto make a greater contribution. We alsosee greater uptake of glucose from thebloodstream. This is attributable to greatermuscle blood flow during exercise as wellas more efficient extraction of glucosefrom the blood by the muscle. After 20minutes of exercise muscle glycogenstores become partially depleted and theuse of muscle glycogen slows (4). Thisis accompanied by increased use of bloodglucose. The liver acts to help maintainblood glucose levels by breaking down itsglycogen stores and releasing glucoseunits into the bloodstream. After bothmuscle and liver glycogen stores are de-pleted, which takes about two or threehours of moderate intensity exercise(marathon running or long distance cy-cling for example), is when we really getin trouble.

At this point one of three thingsmust happen: the exercise must be stoppedor significantly reduced in intensity, bloodglucose must be maintained by carbohy-drate ingestion during exercise, or muscletissue will be destroyed to supply aminoacids as fuel.Carbohydrates—The Optimal Fuel For Success, Part IIt is well known that aerobictraining allows one to perform “more aero-bically” at the same absolute level of ex-ercise intensity (4,7). This means that asyour level of cardiovascular conditioningimproves you can derive more and moreof the required energy from fat and relyless and less on carbohydrates. This hap-pens because cardiovascular training in-creases the number of mitochondria inmuscle cells and the level of fat-metabo-lizing enzymes (4,7). In other words, thecellular fat-burning machinery is built upand your muscle learns to use less carbsand more fat. This is a great benefit ofregular aerobic exercise. However, itwon’t happen if all you do is walk. In ourexperience here at Parrillo we have seenthat there is an intensity threshold requiredto elicit this metabolic adaptation. Youcan’t get your muscles to grow unlessyou lift intensely, and you can’t train yourmuscles to rely more heavily on fat un-less you do your aerobics intensely. In-tense aerobics will have a much moremarked effect in helping you get lean thanmall walking.

Trust me on this one.An added benefit here is that asyou train your muscles to use a higherproportion of fat in the substrate mix, thisspares muscle glycogen. If you can burnmore fat you don’t need to burn as muchglycogen. So you’ll have more enduranceplus greater strength as your workoutproceeds. Depletion of muscle glycogenis associated with exercise fatigue (4). Theglycogen-sparing effect resulting fromincreased lipid oxidation appears to be animportant mechanism explaining whyaerobic exercise causes an increase inendurance capacity (4). Furthermore,aerobically trained individuals seem tostore more fat inside their muscle cells,as well as increasing their ability for in-tramuscular glycogen accumulation (4).You want to get lean? You want to getpumped? You want to maximize yourstrength and endurance? Do your aero-bics and eat a high carb – low fat diet.You’ll be amazed at the results. Nextmonth we’ll continue our discussion ofcarbohydrates and exercise and talk aboutdietary manipulation of fuel stores and en-ergy substrates. What kind of carbs arebest? When? How much? Stay tuned.


1. Flatt JP. Dietary fat, carbohydrate bal-ance, and weight maintenance: effects ofexercise. Am. J. Clin. Nutr. 45: 296-306,1987.

2. Flatt JP. Use and storage of carbohy-drate and fat. Am. J. Clin. Nutr. 61: 952s-959s, 1995.

3. Acheson KJ, Flatt JP, and Jequier E.Glycogen synthesis versus lipogenesisafter a 500 gram carbohydrate meal inman. Metabolism 31: 1234-1240, 1982.

4. Liebman M and Wilkinson JG. Carbo-hydrate metabolism and exercise. Chap-ter 2 from Nutrition in Exercise and Sport,edited by Wolinsky I and Hickson JF, CRCPress, Boca Raton, 1994.

5. Miller GD. Carbohydrates in ultra-en-durance exercise and athletic perfor-mance. Chapter 3 from Nutrition in Exer-cise and Sport, edited by Wolinsky I andHickson JF, CRC Press, Boca Raton,1994.

6. Hargreaves M. Skeletal muscle carbo-hydrate metabolism during exercise.Chapter 2 fromExercise Metabolism, ed-ited by Hargreaves M, Human KineticsPublishers, Champaign, IL, 1995.

7. Coggan AR and Williams BD. Meta-bolic adaptations to endurance training:substrate metabolism during exercise.Chapter 6 fromExercise Metabolism, ed-ited by Hargreaves M, Human KineticsPublishers, Champaign, IL, 1995.

2018-03-13T11:10:33-04:00 June 3rd, 2009|Technical Supplement Bulletins|

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