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Bulletin #23 – Fruitless Dieting: More On Why Fruit Turns To Body Fat

In the last bulletin, we began ourdiscussion of fruit and fructose metabo-lism. As you know, I do not recommendfruit or fruit juice in any of my diets. Thisis a controversial issue that a lot of peopledon’t understand. Don’t get me wrong:Fruit is generally a healthy, nutritious food.Fruit is high in fiber, low in fat and calo-ries and is a good source of vitamins andminerals. If you want something sweetfor dessert, fruit is a good choice. How-ever, there are two groups of people forwhom fruit is not the best food choice:bodybuilders and  and anyone trying tomaximize fat loss.I originally discovered this byconducting carefully controlled nutritionexperiments with bodybuilders. Simplyput, I found that when fruit was addedto their diets, they got fat. This wasnot due to an increase in calories, be-cause in these experiments fruit wassubstituted for another carbohydratesource so that total calories were keptconstant. So I knew it had to be some-thing special about fruit itself that causedfat accumulation. In this bulletin I will ex-plain what it is.The problem is that 80-90 per-cent of the calories in fruit are suppliedby simple sugars, fructose and glucose.Some fruits, such as grapes and oranges,contain a lot of glucose, but most fruitssupply the bulk of their calories as fruc-tose, which is also known as fruit sugar.The bottom line is that fructose is rapidlyconverted to fat by the liver.

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Whereas mostother carbohydrate sources are preferen-tially stored as glycogen, fructose is pref-erentially converted to fat and stored inadipose tissue. This is a consequence ofthe molecular structure of fructose, whichallows it to skip a key regulatory point incarbohydrate metabolism. This regulatorypoint is a step in the glucolytic pathwaycatalyzed by the enzyme phosphofructoki-nase-I (PFK-I).As you know, from previous ar-ticles about carbohydrate metabolism andthermogenesis, the dietary energy (calo-ries) supplied by carbs is used for severalpurposes. Some of it is simply lost as heatduring its digestion and metabolism in aprocess we know as diet-induce thermo-genesis. You can loosely think of this as“friction” in the metabolic pathway, andthis energy loss contributes to the gen-eration of body heat. Most of the dietaryenergy is used to maintain the basal meta-bolic rat (BMR) — the energy cost ofkeeping your body alive.

Some of the en-ergy is used to perform work, like exer-cise and activities of daily life. After that,any energy left is stored as glycogen inmuscles and in the liver. If you consumetoo many calories from carbohydrate af-ter glycogen stores are full, the rest willbe converted to fat (triglycerides) in theliver, and transported by the blood to fatcells (adipose tissue) for storage.So excess calories from any car-bohydrate source can be converted to fat.The enzyme that regulates whether dietaryenergy supplied by carbohydrate is storedas glycogen or fat is PFK-I. It shuttlescarbs into glycogen stores until full , thenit switches the flow of carbohydratesfrom glycogen synthesis to fat synthesis.Glycogen is the storage form of carbo-hydrate in animals, and the amount of gly-cogen you can store is quite limited. theupper limit is generally believed to be be-tween 250-400 grams, depending on theamount of skeletal muscle mass you have.This amounts to only 1000-1600 calories— not even enough energy to fuel yourbody for one day.The deal with fructose is that ittotally skips the enzyme PFK-I, whichis the regulatory step responsible formaking sure glycogen stores are fullbefore fat synthesis is switched on.Instead of being stored as glycogen,fructose gets directly converted to fatby the liver.

Now I think you can seewhy I have a problem with recommend-ing fruit for bodybuilders. to get a de-tailed understanding of fructose metabo-lism, we should start at the beginning.Fructose is absorbed from the small in-testine and directly transported to the liverby the portal vein. The first enzyme toact on fructose is fructokinase, which addsa phosphate group to the sugar to formfructose-1-phosphate (F1P). Glucose issimilarly phosphorylated at the six posi-tion of the hexokinase, forming G6P. Allcells have hexokinase and thus have theability to phosphorylate glucose. Thismeans that all cells can metabolize glu-cose for energy.On the other hand, fructokinaseis virtually confined to the liver (1). Sowhile glucose is a general substrate for allbody tissues, fructose represents a car-bohydrate load targeted for the liver (1).The next thing that happens is F1P is splitby the enzyme aldolase to form glyceral-dehyde (GA) and dihydroxyacetone phos-phate (DHAP). Refer to the figure, whichis adapted from Shafrir (1).

This meansthat products of fructose metabolism en-ter the glycolytic pathway at the triosephosphate level (i.e., as three carbon sug-ars). Glucose, on the other hand, is phos-phorylated to yield G6P, which may pro-ceed directly to glycogen synthesis (1).To be broken down for energy, glucosemust first pass through the rate-limitingPFK-I step. Fructose metabolites enterbelow this step, and thus bypass an im-portant point of regulation. Fructose there-fore is more prone to be converted to fat,while glucose is more prone to be con-verted to glycogen.The biochemistry is much morecomplex than is appropriate for this ar-ticle, but I have pointed out the salientfeatures of the pathway to explain whyglucose-based carbohydrate  sources arebetter than fructose, especially for peopletrying to minimize body fat stores. Scien-tific studies have proven that while fruc-tose is effective at replenishing liver gly-cogen stores, starch (glucose polymers)is much more efficient at replenishingskeletal muscle glycogen stores (2).When we were designing theParrillo Supplement Bar, we surveyed ev-ery available sports supplement bar wecould find. We found that 25 out of the26 bars had fructose as either the first orsecond ingredient. (If you use somebodyelse’s bar, go read the label.)Why? Because corn syrup andfruit juice (good sources of fructose) arereal cheap and they’re also very sweet.We pioneered the use of a new carbohy-drate source in our bar called rice dex-trin.

It’s a short-chain glucose polymermade from rice. This gives you the quickenergy you want from a sports bar, butwithout the fructose. Each ParrilloSupplement Bar also contains CapTri®(which is legendary by now) and an ul-tra-high efficiency protein source.As we discussed in an earlierbulletin about carbohydrate metabolism,complex carbohydrates (such as starchand maltodextrin) are more effecitve atreplenishing glycogen stores than simplesugars (3). This makes sense becausecomplex carbs are released into the blood-stream slowly whereas simple sugars arereleased very rapidly, potentially over-whelming the glycogen synthesis pathwayand “spilling over” into fat stores. Fur-thermore, the increased insulin releaseresulting from simple sugars causes moreof the sugar to be converted to fat.This is why Parrillo PerformancePro-Carb™ Formula is based onmaltodextrin instead of sugar, like mostother carbohydrate supplements.Maltodextrin is a medium-chain glucosepolymer made from corn. It has beenfound that maltodextrin is 15 percent moreefficient at restoring muscle glycogen lev-els than conventional carbohydrate foodslike rice and pasta (4).This makes Pro-Carb™ ideal forglycogen supercompensation (carb load-ing).

Maltodextrin beverages like Pro-Carb™ have also been demonstrated toincrease blood glucose levels during ex-ercise and to increase exercise time toexhaustion (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 glycemic index, whichmeans it raises blood sugar very slowlyand elicits only a small insulin release.From your reading of our series on endo-crinology, you know that a slow, steadyinsulin response is good. Since insulin isa potent stimulus for fat storage, we wantto keep insulin levels fairly low, so be thisreasoning it seems like fructose would begood. The problem is that the REASONfructose has a low glycemic index andresults in a small insulin release is that it isconverted to fat in the liver. It doesn’t raiseblood sugar very much because it is re-leased from the liver as fat instead of sugar.Fructose has a MUCH greatertendency to be converted to fat than othercarbohydrate sources, so why use it?Now you understand the biochemistrybehind my controversial stance on fruit.I’m not just making this stuff up, folks.There’s a reason behind every part of myprogram.


1. Shafrir E. Fructose/sucrose metabo-lism, its physiological and patholigicalimplications. 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, Miller JM. The role of di-etary 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% VO2 Max increases time to ex-haustion. Med. Sci. Sports Exerc. 15: 126,1983.

2018-03-13T11:10:38-04:00 May 15th, 2009|Technical Supplement Bulletins|

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