Fructose: The Ideal Carbohydrate Source For Gaining Fat

July 29, 2010 by  

by John Parrillo

I still get a lot of questions aboutfructose, the sugar that occurs naturallyin fruit. Recently I saw an article advertising a supplement bar based on fructose,explaining why this low glycemic indexcarb is great for bodybuilders. I decidedit was time to revisit this issueand try to set therecord straight. I hateto see people bemisled and workhard in the gymjust to have their results ruined byeating the wrong thing.

The problem is simple: fructoseis 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, suggestingthat this makes it a good carbohydratesource for athletes. The reasonfructose has a low glycemic index is becausea large proportion of it is releasedfrom the liver as fat instead of carbohydrate.
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. Professionalbodybuilders don’t want good physiques- 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 biochemicalpathways of metabolism, but byactually doing nutritional experiments withreal bodybuilders. Rather than being sometheory out of a book, this is an experimentalfact. For a long time I didn’t understandit – I just knew fromour work in the gym thatcertain foods madebodybuilders get inbetter shape whileother foods madethem get fat. 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 humanmost fat is stored just under the skin).You can see the striations of his musclesclearly through his skin. As you can imagine,any little change at this point reallyshows up. This is why I like to use competitivebodybuilders for the most demandingnutritional 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 caloriesfrom protein, carbohydrate, and fat.If his weight doesn’t change, this meanscaloric intake exactly balances caloric expenditure,so we have a direct measurementof 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 complexcarbohydrates from his diet in formof rice, and replace it with 300 caloriesworth of fruit. His total caloric intake remainsthe 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 calories: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 continuesto 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 like this with just about every food imaginable.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 proteinto carbohydrate varies among individuals.For example, some people, especiallyones who have trouble losing fat,do better with more protein and less carbohydrate.People who have suffered repeatedbouts 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 offood you eat? For a given number of caloriesit 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 automaticallylose 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 compositionsand 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 sometimeand see how it runs. For any machineto run optimally, including the humanbody, 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 problemwith 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 glycogenuntil glycogen stores are full. Fructose,on the other hand, gets directly convertedto 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 metabolismin a process we know as diet-inducedthermogenesis. You can loosely think ofthis as “friction” in the metabolic pathway,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 exerciseand 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 convertedto 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 carbohydrateis 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 carbohydratein animals, and theamount of glycogen you canstore is quite limited. The upperlimit is generally believedto be 250-400 grams, dependingon 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 makingsure 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 bodybuilders.

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 glucoseby 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 phosphorylatedat the 6 position by the enzyme hexokinase,forming G6P. All cells have hexokinase,and thus have the ability to phosphorylateglucose. This means that all cellscan metabolize glucose for energy. On theother hand, fructokinase is virtually confinedto the liver (1). So while glucose isa general substrate for all body tissues,fructose represents a carbohydrate loadtargeted 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 metabolitesenter 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 article,but I have pointed out the salient featuresof the pathway to explain why glucose-based carbohydrate sources are betterthan fructose, especially for peopletrying to minimize body fat stores. Scientificstudies have proven that starch (glucosepolymer) is much more efficient atreplenishing skeletal muscle glycogenstores than fructose (2). Now you understandwhy — muscle cells don’t havethe enzyme needed to phosphorylate fructose,so its metabolism is essentially limitedto 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? Becausecorn 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 shortchainglucose polymer made from rice.This gives you the quick energy you wantfrom a sports bar, but without the fructose.Each Bar also contains CapTri(which is legendary by now) and an ultra-high efficiency protein source.
As we discussed in an earlier article aboutcarbohydrate metabolism, complex carbohydrates(such as starch andmaltodextrin) are more effective in replenishingglycogen stores than simple sugars(3). This makes sense because complexcarbs are released into the bloodstreamslowly whereas simple sugars arereleased very rapidly, potentially overwhelmingthe glycogen synthesis pathwaysand “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 restoringmuscle glycogen levels than conventionalcarbohydrate foods like rice andpasta (4). This makes ProCarb ideal forglycogen supercompensation (carb loading).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 anticipatea 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 response.From your reading of our serieson endocrinology, you know that a slow,steady insulin response is good. Since insulinis a potent stimulus for fat storage,we want to keep insulin levels fairly low,so by this reasoning it seems like fructosewould be good. The problem is thatthe REASON fructose has a low glycemicindex 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 metabolismand glycogen storage. After exerciseis completed dietary carbohydrate is directedtoward restoring muscle and liverglycogen and returning blood glucose tonormal levels. Dietary starches and sugarsare 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). Amazingly,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 bodybuilderson exact techniques to control the flowof energy and nutrients through the metabolicpathways. Who else teaches youhow to control the metabolic pathwaysto optimize bodybuilding results?
Anyway, what happens is this: After exerciseliver and muscle glycogen (carbohydrate)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 metabolites.Muscle tissue, on the other hand,has a specialized function (contraction)and doesn’t have all of the various enzymesthe liver does which allow the liverto inter-convert so many different metabolicintermediates. Muscle tissue relieson glucose to recover its glycogen store,so the liver is kind enough to let the glucosepass on through so it can be deliveredto muscle. The liver makes due withother carbon compounds which muscleis unable to use. What does this mean forus bodybuilders? Think about it – the answeris right in front of you.

What this means is if you feedyour body glucose – the form of carbohydratein starch, Pro-Carb, and theParrillo Bar – the carbohydrate will be directedto 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 workouts,since muscles rely on their internalglycogen stores as fuel for anaerobic exercise.After muscle glycogen stores arefull additional glucose will be used to restoreliver glycogen. Only after bothmuscle and liver glycogen stores are fullyrepleted will further excess glucose be convertedto fat. (Studies have shown thatoverfeeding as much as a 500 gram carbohydrateload 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 fructoserepresents 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 PFKI.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 disaccharideknown 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 fattening.So a large portion of the fructosesimply 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 fructosegets 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 glycogen(2) and shifts the liver into fat-storingmode. 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 glycogenstores 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 carbohydratenightmare. Fructose is theworst carb source for bodybuilders youcan imagine. If you wanted to design asupplement to ruin a bodybuilder’s physique,it would be a fructose-based energybar. 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 insulin,so blood glucose is shuttled preferentiallyto glycogen-depleted muscle (2).Interestingly, and fortunately for the bodybuilder,high blood glucose and insulin levelsdo relatively little to stimulate hepatic(liver) glycogenesis (glycogen production).Instead, most glucose passes onthrough the liver and is extracted by skeletalmuscle. This means that if you supplyyour body with carbohydrate in theFrutose: The Ideal Carbohydrate Source For Gaining FatIn the liver, two things can happen with fructose. First,fructose can be absorbed by the liver cells, convertedto glucose, and then stored as glycogen. Second,the fructose can be converted to fat. You rememberwhat I said earlier about the enzyme PFK-I. This isthe “rate limiting” enzyme that operates as a switch todecide if a sugar gets stored as glycogen or convertedto fat. Fructose completely bypasses this enzymeand is readily converted to fat by the liver.© 1999, 1992 Parrillo Performance • Cincinnati OH 45246 • (513) 874-3305 • ORDERLINE 1-800-344-3404 185form 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 fructosebasedsupplement bar, however, the caloriesjust stay in your liver and get convertedeither to liver glycogen or fat. Itwon’t help recover muscle glycogen.In closing I’d like to talk aboutthe kinetics (time course) of glycogen storage.Glycogen recovery following exerciseis highly dependent on the carbohydratecontent of the diet, up until 500-600grams of carbohydrate are provided.Above this intake, glycogen synthesizingpathways appear to be saturated (2). Interestingly,not only is the absolute carbohydratecontent of the diet important,but the type of dietary carbohydrate consumedis key also. A study by Costill (reviewedin reference 2) compared the relativeefficacies 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 carbohydratetypes on liver glycogen, and demonstratedthat fructose is more efficientthan glucose at replenishing liver glycogen.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 furthercarbohydrate 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 conversionto 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 carbohydratediet; usually 60-70% of your caloriesshould come from complex carbs. Eata high carbohydrate meal immediately aftertraining, when muscles are glycogendepleted and are primed to store carbohydrate.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 Powderright 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 fructosecorn 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 exercise,fructose would be an excellentchoice.
Parrillo supplements are made theway they are for a reason. We use rice dextrin 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 bodybuilderwhose career depends on his (orher) physique. You might be surprised thata seemingly small difference like using glucoseinstead of fructose would be important,but it can make the difference betweenwinning 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 metabolism,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 feedingimmediately before prolonged cyclingat 62% VO2max increases time to exhaustion.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.


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