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Bulletin #45 – Fructose: The Ideal Carbohydrate Source For Gaining Fat

I still get a lot of questions aboutfructose, the sugar that occurs naturallyin fruit. Recently I saw an article adver-tising a supplement bar based on fructose,explaining why this low glycemic indexcarb is great for bodybuilders. I decidedit was time to revisit this is-sue and try to set therecord straight. I hateto see people bemisled and workhard in the gymjust to have theirresults ruined byeating the wrongthing.The prob-lem is simple: fruc-tose is converted tofat in the liver. That’sreally all there is to it.Some people point tothe fact the fructose has a lowglycemic index (which it does) and that itgenerates a small insulin response, sug-gesting that this makes it a good carbo-hydrate source for athletes. The reasonfructose has a low glycemic index is be-cause a large proportion of it is releasedfrom the liver as fat instead of carbohy-drate.Can you eat some fruit now and then andstill have a good physique? Sure you can.But the athetes I work with want THEBEST RESULTS POSSIBLE. Profes-sional bodybuilders don’t want good phy-siques – they want perfect physiques. Ofcourse, fruit is generally a healthy food -high in fiber, vitamins, and minerals, andlow in fat. But try to think of fruit asnature’s candy, because that’s exactly whatit is. If your goal is to build a lean andmuscular physique, then you don’t wantto eat candy. Sugar and fat are natural,but that doesn’t mean they’ll make youlean and muscular.I originally learned that fruitmakes you fat not by reviewing the bio-chemical pathways of metabolism, but byactually doing nutritional experiments withreal bodybuilders. Rather than being sometheory out of a book, this is an experi-mental fact. For a long time I didn’t un-derstand it – I just knew fromour work in the gym thatcertain foods madebodybuilders get inbetter shape whileother foods madethem get fat.

Parrillo Performance

Theexperiment goeslike this: As abodybuilder getsclose to a contest,his body fat levelgets very low -maybe 3-5% for amale and 8-9% for afemale. At this point hisskin is paper thin (in the hu-man most fat is stored just under the skin).You can see the striations of his musclesclearly through his skin. As you can imag-ine, any little change at this point reallyshows up. This is why I like to use com-petitive bodybuilders for the most demand-ing nutritional experiments – they are a verysensitive indicator of what works andwhat doesn’t. With the athlete in contestshape, we measure his body weight andpercent body fat every day. We weigh hisfood and calculate how many calories heis consuming, and break it down into calo-ries from protein, carbohydrate, and fat.If his weight doesn’t change, this meanscaloric intake exactly balances caloric ex-penditure, so we have a direct measure-ment of his total daily energy expenditure.Everything is measured and controlled,and nothing is left to chance.Okay, here’s the deal: Let’s saywe remove 300 calories worth of com-plex carbohydrates from his diet in formof rice, and replace it with 300 caloriesworth of fruit. His total caloric intake re-mains the same, as does his percent ofcalories from protein, carbohydrate, andfat.

His training program remains exactlythe same. The only change is the type ofcarbohydrate supplying 300 of the calo-ries: rice has been replaced with bananas.You expect his body weight and percentbody fat to remain the same, right? Toevery one’s surprise, the bodybuilderstarts to gain fat. We let this go on for acouple of weeks and the athlete contin-ues to gain fat. Now we pull the bananasout of the diet and put the rice back in -i.e., go back to the original diet. Guesswhat? He loses the fat. Amazing, but true.We’ve done countless experiments likethis with just about every food imagin-able. That’s how we came up with ourdiet – by finding what really works. TheParrillo Performance Nutrition Manual tellsyou which foods work to build a lean,muscular physique, and which foodsdon’t. The competitive bodybuilder is ourlaboratory. The same diet developed tohone champion bodybuilders worksequally well for anyone seeking to losefat and gain muscle. So far, no one whohas given our program a sincere efforthas said that it did not work for them.Granted, some specific parameters haveto be adjusted to optimize the program foryour individual metabolism. We tell youhow to do that too.

The optimum numberof calories and the optimum ratio of pro-tein to carbohydrate varies among indi-viduals. For example, some people, espe-cially ones who have trouble losing fat,do better with more protein and less car-bohydrate. People who have suffered re-peated bouts of yo-yo dieting have lost alot of muscle mass and consequently havea slow metabolic rate. They may actuallyneed to increase calories and put on somemuscle before they have any machineryto burn fat.Well, back to the fruit story. Whydoes it make any difference what kind of food you eat? For a given number of calo-ries it seems like it shouldn’t matter whatfoods they come from. This is one of themost common mistakes people makewhen trying to lose fat. They think that ifthey reduce calories they will automati-cally lose weight. This is true, but onlyfor a little while. And if you lose weightby drastically reducing calories, about 50%of the weight lost will be muscle. Whatpeople fail to realize is that the types offoods you eat is just as important as howmany calories you consume. If cuttingcalories was the answer, then those lowcalorie weight loss drinks would work,but they don’t.The key point is that differentfoods have different chemical composi-tions and therefore have different effectsinside your body. Of course, all food isfuel, but what type of fuel it is matters alot.

Try putting kerosene in your car some-time and see how it runs. For any ma-chine to run optimally, including the hu-man body, it requires the right kind of fuel.The problem with fruit is that virtually allof the calories it supplies come in the formof simple sugars. The most abundantsugar in fruit is fructose (commonlyknown as fruit sugar), although somefruits (oranges and grapes for example)also contain a lot of glucose. The prob-lem with fructose is that it bypasses theenzyme phosphofructokinase-I (PFK-I),the rate limiting step of glycolysis, thepathway responsible for the conversionof carbohydrate into energy (1). In otherwords fructose bypasses the control pointthat decides if a dietary sugar is going tobe stored as glycogen or fat. Complexcarbohydrates such as rice, oatmeal, andpotatoes, are preferentially stored as gly-cogen until glycogen stores are full. Fruc-tose, on the other hand, gets directly con-verted to fat in the liver, then gets whiskedoff in the bloodstream to be stored in fatcells (1).As you know from our previousarticles about carbohydrate metabolismand thermogenesis, the dietary energy(calories) supplied by carbs is used forseveral purposes. Some of it is simply lostas heat during its digestion and metabo-lism in a process we know as diet-inducedthermogenesis. You can loosely think ofthis as “friction” in the metabolic path-way, and this energy loss contributes tothe generation of body heat.

Most of thedietary energy is used to maintain the basalmetabolic rate (BMR) – the energy costof keeping your body alive. Some of theenergy is used to perform work, like ex-ercise and just tending to the activities ofdaily life. After that, any energy left isstored as glycogen in muscles and in theliver. If you consume too many caloriesfrom carbohydrate, after glycogen storesare full the rest will be con-verted to fat (triglycerides) inthe liver, and transported bythe blood to fat cells (adiposetissue) for storage.So after glycogen stores arefull, excess calories from anytype of carbohydrate can beconverted to fat. The enzymethat regulates whether dietaryenergy supplied by carbohy-drate is stored as glycogen orfat is PFK-I. It shuttles carbsinto glycogen stores untilthey’re full, then it switchesthe flow of carbohydratesfrom glycogen synthesis tofat synthesis. Glycogen is thestorage form of carbohy-drate in animals, and theamount of glycogen you canstore is quite limited. The up-per limit is generally believedto be 250-400 grams, de-pending on the amount ofskeletal muscle mass youhave. (Very massive bodybuilders may beable to store as much as 600 grams.)

Thisamounts to only 1000-1600 calories – noteven enough energy to fuel your body forone day. The deal with fructose is that ittotally skips the enzyme PFK-I, which isthe regulatory step responsible for mak-ing sure glycogen stores are full beforefat synthesis is switched on. Instead ofbeing stored as glycogen, fructose getsdirectly converted to fat by the liver. NowI think you can see why I have a problemwith recommending fruit for bodybuild-ers.To get a detailed understandingof fructose metabolism we should startat the beginning. Fructose is absorbedfrom the small intestine and transportedto the liver by the portal vein. You have torealize that fructose itself is not releasedfrom the liver into the bloodstream to reachthe rest of the body. Any carbohydratesource you ingest is first converted to glu-cose by the liver, and glucose (“bloodsugar”) is the carbohydrate source usedby muscles and the form of carbohydratewhich is converted into glycogen. The firstenzyme to act on fructose is fructokinase,which adds a phosphate group to thesugar to form fructose-1-phosphate(F1P). Glucose is similarly phosphory-lated at the 6 position by the enzyme hex-okinase, forming G6P. All cells have hex-okinase, and thus have the ability to phos-phorylate glucose. This means that all cellscan metabolize glucose for energy. On theother hand, fructokinase is virtually con-fined to the liver (1). So while glucose isa general substrate for all body tissues,fructose represents a carbohydrate load targeted for the liver (1).

The next thingthat happens is F1P is split by the enzymealdolase to form glyceraldehyde (GA) anddihydroxyacetone phosphate (DHAP).This means that the products of fructosemetabolism enter the glycolytic pathwayat the triose phosphate level (i.e., as threecarbon sugars). Glucose, on the otherhand, is phosphorylated to yield G6P,which may proceed directly to glycogensynthesis (1). To be broken down forenergy glucose must first pass throughthe rate-limiting PFK-I step. Fructose me-tabolites enter below this step, and thusbypass an important point of regulation.Fructose therefore is more prone to beconverted to fat, while glucose is moreprone to be converted to glycogen.The biochemistry is much morecomplex than is appropriate for this ar-ticle, but I have pointed out the salient fea-tures of the pathway to explain why glu-cose-based carbohydrate sources are bet-ter than fructose, especially for peopletrying to minimize body fat stores. Scien-tific studies have proven that starch (glu-cose polymer) is much more efficient atreplenishing skeletal muscle glycogenstores than fructose (2).

Now you un-derstand why — muscle cells don’t havethe enzyme needed to phosphorylate fruc-tose, so its metabolism is essentially lim-ited to the liver.When we were designing theParrillo Performance Bar, we surveyedevery available sports supplement bar wecould find. We found that 25 out of the26 bars had fructose in either the first orsecond ingredient. (If you use somebodyelse’s bar, go read the label.) Why? Be-cause corn syrup and fruit juice (goodsources of fructose) are real cheap, andthey’re also very sweet. We pioneered theuse of a new carbohydrate source in theParrillo Bar called rice dextrin. It’s a short-chain glucose polymer made from rice.This gives you the quick energy you wantfrom a sports bar, but without the fruc-tose. Each Bar also contains CapTri(which is legendary by now) and an ul-tra-high efficiency protein source.As we discussed in an earlier article aboutcarbohydrate metabolism, complex car-bohydrates (such as starch andmaltodextrin) are more effective in replen-ishing glycogen stores than simple sug-ars (3). This makes sense because com-plex carbs are released into the blood-stream slowly whereas simple sugars arereleased very rapidly, potentially over-whelming the glycogen synthesis path-ways and “spilling over” into fat stores.

Furthermore, the increased insulin releaseresulting from simple sugars causes moreof the sugar to be converted to fat.This is why Parrillo Performance ProCarbis based on maltodextrin instead of sugar,like most other carbohydrate supplements.Maltodextrin is a medium-chain glucosepolymer. It has been found thatmaltodextrin is 15% more efficient at re-storing muscle glycogen levels than con-ventional carbohydrate foods like rice andpasta (4). This makes ProCarb ideal forglycogen supercompensation (carb load-ing). Maltodextrin beverages like Pro-Carbhave also been demonstrated to increaseblood glucose levels during exercise andto increase exercise time to exhaustion(4,5).At this point, I think I can antici-pate a question from the biochemists inthe crowd. You’ve probably heard thatfructose is low on the glycemic index,which means it raises blood sugar veryslowly and elicits only a small insulin re-sponse. From your reading of our serieson endocrinology, you know that a slow,steady insulin response is good. Since in-sulin is a potent stimulus for fat storage,we want to keep insulin levels fairly low,so by this reasoning it seems like fruc-tose would be good.

The problem is thatthe REASON fructose has a low glyce-mic index and results in a small insulinrelease is that it is converted to fat in theliver. It doesn’t raise blood sugar verymuch because it is released from the liveras fat instead of sugar. Fructose has aMUCH greater tendency to be convertedto fat than other carbohydrate sources,so why use it? Now you understand thebiochemistry behind my controversialstance on fruit.Now I’d like to go into a littlemore detail about carbohydrate metabo-lism and glycogen storage. After exerciseis completed dietary carbohydrate is di-rected toward restoring muscle and liverglycogen and returning blood glucose tonormal levels. Dietary starches and sug-ars are digested to simple sugars (glucoseand fructose) which are then available forglycogen formation (2). Until recently itwas believed that glycogen was made justfrom glucose extracted from the bloodby the liver and muscle, but the actualdynamics of glycogen restitution turn outto be much more complicated. In fact,glucose serves primarily to replenishmuscle glycogen stores, while the liver ismore versatile in its choice of substrates(building blocks) for glycogen synthesis.The liver is able to make glycogen fromfructose, lactate, glycerol, alanine, andother three-carbon metabolites (2). Amaz-ingly, most glucose absorbed from the gutactually travels through the liver withoutbeing absorbed and preferentially is usedto replenish muscle glycogen stores (2).

This is cutting edge stuff, folks, that youprobably won’t hear anywhere else. Idon’t know of anyone besides ParrilloPerformance who researches nutritionalbiochemistry and metabolism at this levelof molecular detail to instruct bodybuild-ers on exact techniques to control the flowof energy and nutrients through the meta-bolic pathways. Who else teaches you how to control the metabolic pathwaysto optimize bodybuilding results?Anyway, what happens is this: After ex-ercise liver and muscle glycogen (carbo-hydrate) stores are depleted. The liver isa very versatile metabolic engine and isable to recover its glycogen stores frommany different sources, including lactate,fructose, glucose, and amino acid metabo-lites. Muscle tissue, on the other hand,has a specialized function (contraction)and doesn’t have all of the various en-zymes the liver does which allow the liverto inter-convert so many different meta-bolic intermediates. Muscle tissue relieson glucose to recover its glycogen store,so the liver is kind enough to let the glu-cose pass on through so it can be deliv-ered to muscle.

The liver makes due withother carbon compounds which muscleis unable to use. What does this mean forus bodybuilders? Think about it – the an-swer is right in front of you.What this means is if you feedyour body glucose – the form of carbo-hydrate in starch, Pro-Carb, and theParrillo Bar – the carbohydrate will be di-rected to your muscles and stored thereas glycogen. Dietary carbohydrate in theform of glucose will be directed to yourmuscles until muscle glycogen stores arefull. This will make your muscle full, hard,and pumped. Also, this will give you moreenergy and strength during your work-outs, since muscles rely on their internalglycogen stores as fuel for anaerobic ex-ercise. After muscle glycogen stores arefull additional glucose will be used to re-store liver glycogen. Only after bothmuscle and liver glycogen stores are fullyrepleted will further excess glucose be con-verted to fat. (Studies have shown thatoverfeeding as much as a 500 gram car-bohydrate load leads to practically no fatsynthesis, because the carbs are storedas glycogen.) The story with fructose isvery different, indeed. Muscle does nothave the enzymatic machinery needed toconvert fructose into glycogen, so fruc-tose represents a dietary carbohydrate loadtargeted for the liver (1,2). In the liver,two things can happen with fructose.First, fructose can be absorbed by the livercells, converted to glucose, and thenstored as glycogen. Second, the fructosecan be converted to fat.

You rememberwhat I said earlier about the enzyme PFK-I. This is the “rate limiting” enzyme thatoperates as a switch to decide if a sugargets stored as glycogen or converted tofat. Fructose completely bypasses thisenzyme and is readily converted to fat bythe liver. This is why ordinary table sugarincreases blood triglyceride levels andmakes you fat. Table sugar is a disaccha-ride known as sucrose, and is made fromone molecule of glucose connected to onemolecule of fructose. Recent thought oncarbohydrate metabolism suggests that itis the fructose portion of sucrose that isresponsible for making sweets so fatten-ing.So a large portion of the fruc-tose simply gets converted directly to fatand released into the bloodstream. Bam.You get a dose of fat. But the damagedoesn’t stop there. The rest of the fruc-tose gets converted into liver glycogen.That sounds okay, until you stop to thinkabout it. You see, once liver glycogenstores are full the liver says, “We’ve gotall the glycogen we can hold, so any morecarbs coming in here we’ll just convertto fat.” Fructose preferentially repletesliver glycogen instead of muscle glyco-gen (2) and shifts the liver into fat-stor-ing mode.

This is exactly what we don’twant. We need some liver glycogen, tobe sure, because this is what keeps bloodsugar levels steady. But when liver glyco-gen stores are full, this is when dietarycarbs start to “spill over” into fat stores.The third problem is that fructose cannotbe used to replenish muscle glycogen, soon a high fructose diet liver glycogenstores are filled and we start convertingcarbs into fat without ever filling muscleglycogen stores. This scenario is a car-bohydrate nightmare. Fructose is theworst carb source for bodybuilders youcan imagine. If you wanted to design asupplement to ruin a bodybuilder’s phy-sique, it would be a fructose-based en-ergy bar. Unfortunately, the vast majorityof the bars out there rely on fructose astheir major carb source, because it’scheap.In summary, fructose does threethings: a large portion of it is converteddirectly to fat by the liver, it preferentiallyfills liver glycogen stores so that evengood carbs are more prone to spill overinto fat, and it cannot be used by muscleto recover glycogen. Calorie for calorie,the only nutrient that will make you fatterthan fructose is fat itself. Besides that, Idon’t have a problem with it.Glucose then has some specialmetabolic properties that you can use toyour advantage.

Exercise induces muscleto be more sensitive to the effects of in-sulin, so blood glucose is shuttled prefer-entially to glycogen-depleted muscle (2).Interestingly, and fortunately for the body-builder, high blood glucose and insulin lev-els do relatively little to stimulate hepatic(liver) glycogenesis (glycogen produc-tion). Instead, most glucose passes onthrough the liver and is extracted by skel-etal muscle. This means that if you sup-ply your body with carbohydrate in the form of glucose post-exercise that thecalories will be preferentially stored asmuscle glycogen. An excellent recoverytrick is to eat a Parrillo Bar or a scoop ofPro-Carb right after your workout. Theseare essentially “free” calories which youknow will end up in muscle and not beconverted to fat. If you eat a fructose-based supplement bar, however, the calo-ries just stay in your liver and get con-verted either to liver glycogen or fat. Itwon’t help recover muscle glycogen.In closing I’d like to talk aboutthe kinetics (time course) of glycogen stor-age. Glycogen recovery following exer-cise is highly dependent on the carbohy-drate content of the diet, up until 500-600grams of carbohydrate are provided.Above this intake, glycogen synthesizingpathways appear to be saturated (2). In-terestingly, not only is the absolute car-bohydrate content of the diet important,but the type of dietary carbohydrate con-sumed is key also.

A study by Costill (re-viewed in reference 2) compared the rela-tive efficacies of simple sugars versuscomplex carbohydrates in restoringmuscle glycogen following exercise. Bothwere equally effective during the first 24hours of glycogen synthesis, but by 48hours post-exercise the complex carbsresulted in significantly better recover ofmuscle glycogen stores. Another studylooked at the effects of different carbo-hydrate types on liver glycogen, and dem-onstrated that fructose is more efficientthan glucose at replenishing liver glyco-gen. The question is, do you want yourcarbs to be stored in muscle or in yourliver?Basically, this means if you relyon simple sugars or fructose, within 24hours your muscles will have stored asmuch glycogen as they can, and any fur-ther carbohydrate you consume will beconverted to fat. On the other hand if youuse starches (complex carbs made fromglucose polymers) not only can youachieve higher levels of muscle glycogenstorage, but also less of the carbs will beconverted to fat. In other words, if youstore more of your dietary carbs as muscleglycogen, less will be available for con-version to fat. Pretty neat, huh?You know that weight lifting is an anaerobicexercise that relies on muscle glycogenas the primary fuel source. You also knowthat muscles fully loaded with glycogenare bigger, harder, and stronger.

What canyou do to target dietary carbohydrate tomuscle? Use complex carbohydrates madefrom glucose polymers as your carbsource. This is the type of carbohydratefound in starchy foods (potatoes, rice,beans, oatmeal, etc.) and in the ParrilloBar and in Pro-Carb. Eat a high carbohy-drate diet; usually 60-70% of your calo-ries should come from complex carbs. Eata high carbohydrate meal immediately af-ter training, when muscles are glycogendepleted and are primed to store carbo-hydrate. A convenient way to do this is toput a Parrillo Bar in your gym bag or elsea shaker bottle with a scoop of Pro-Carbin it. This is also an ideal time to consumesome protein to provide amino acids torebuild your muscles. If you’re trying topack on a few more pounds of muscle itmay be as simple as eating a scoop ofPro-Carb plus a scoop of Hi-Protein Pow-der right after your workout. Do this fora month and I can virtually guaranteeyou’ll see a difference. After a workout,these calories will go straight to musclewith virtually no risk of being convertedto fat. Finally, avoid fructose. Most of thesupplement bars out there (probably 90%of them at least) use fructose, high fruc-tose corn syrup, or fruit juice as one oftheir main ingredients. Beware. Theseproducts are not effective in recovery ofmuscle glycogen and instead are targetedto the liver. If you’re looking for a goodway to replenish your fat stores after ex-ercise, fructose would be an excellentchoice.Parrillo supplements are made theway they are for a reason.

We use ricedextrin in the Parrillo Bar and maltodextrinin Pro-Carb. Both are glucose polymers.Sure, high fructose corn syrup or fruitjuice concentrate would be cheaper, butwe’ve designed our supplements to be thebest, not the cheapest. Our supplementsare designed for the professional body-builder whose career depends on his (orher) physique. You might be surprised thata seemingly small difference like using glu-cose instead of fructose would be impor-tant, but it can make the difference be-tween winning and losing. Now you knowhow to control the traffic of carbohydratesthrough the metabolic pathways of yourbody and direct carbs to muscle whileminimizing their conversion to fat. Youalso know when somebody starts tellingyou how wonderful their fructose bar is,you’d better put on your hip waders.


1. Shafrir E. Fructose/sucrose metabo-lism, its physiological and pathologicalimplications. Sugars and Sweeteners,Kretchmer N and Hollenbeck CB, Eds.CRC Press, 1991, pp. 63-98.

2. Paige DM. Clinical Nutrition. C.V.Mosby Company, St. Louis, 1988, pp.703-704.

3. Costill DL, Sherman WM, Fink WJ,Witten MW, and Miller JM. The role ofdietary carbohydrates in muscle glycogenresynthesis after strenuous running. Am.J. Clin. Nutr. 34: 1831-1836, 1981.

4. Lamb DR, Snyder AC, and Baur TS.Muscle glycogen loading with a liquidcarbohydrate supplement. Int. J. SportNutr. 1: 52-60, 1991.

5. Snyder AC, Lamb DR, Baur T, ConnorsD, and Brodowicz G. Maltodextrin feed-ing immediately before prolonged cyclingat 62% VO2max increases time to exhaus-tion. Med. Sci. Sports Exerc. 15: 126,1983

6. Buskirk, ER and Puhl, S. Nutritionalbeverages: exercise and sport. Nutritionin Exercise and Sport, Hickson JF andWolinsky I, Eds. CRC Press, 1989.

2018-03-13T11:10:36-04:00 May 28th, 2009|Technical Supplement Bulletins|

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