The latest word on fructose and why it makes you fat
More than twenty years ago, I came out against fructose in the diet, and people thought I had gone nuts. After all, fructose is a naturally occurring sugar in fruit, which is generally a healthy food - high in fiber, vitamins and minerals and low in fat.
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Only recently, however, have nutritional scientists begun to recognize fructose for what it is: a sweetening 
agent that makes you fat. The reason for their nutritional “eureka” has to with the fact that our national consumption of fructose (as well as other simple sugars) over the decades parallels America’s rise in obesity. (1)
The form of fructose that is most prevalent in our diets now is a particularly troublesome sweetener known as high-fructose corn syrup. This sweetener is used mostly in soft drinks and refined foods, but you will also find it in the most unlikely of places: health foods such as the energy bars, sports drinks, and carbohydrate supplements you probably take every day. Further, many of the foods in your pantry are sweetened with this additive, and it might just be the baddest of the bad when it comes to simple sugars.
Made from cornstarch, high-fructose corn syrup is a liquid that is predominantly fructose but has some glucose in it. (Even the powdered fructose sold in stores is made from cornstarch and not from refining the sugar in fruit, as you might assume.) Food manufacturers love using high-fructose corn syrup because it tastes much sweeter than sugar; this means they can use less of it and save on their manufacturing costs. Consumption of high-fructose corn syrup has risen more than 21 percent since 1970, when it was introduced into the food supply. (2)
According to a U.S. Department of Agriculture analysis, from 1985 to 2000, Americans added roughly 330 calories to their daily intake, and twenty-five percent (about 83 calories) came from sweeteners, including high-fructose corn syrup. That amount of added sugar in your diet will produce a weight gain of nearly nine pounds a year. (3)
I will talk more about fructose and weight gain in a moment, but I want to alert you to something about fructose that is just now gaining attention. There is a medically recognized condition called “fructose intolerance” - a sensitivity to the fructose in fruit juices, sports drinks, or products containing high-fructose corn syrup, and sometimes to the natural fructose in fruit. Symptoms include stomach upset, diarrhea, and bloating. In rare cases in which someone lacks the enzyme needed to digest fructose, the symptoms will be more severe: vomiting, hypoglycemia (low blood sugar), jaundice, or enlarged liver. You can be tested for fructose intolerance with a breath test or a blood test. If diagnosed with this condition, you’ll have to avoid foods containing fructose, especially high-fructose corn syrup.
First reported in 1956, fructose intolerance is on the rise due to interactions between genes and environment. Globally, people are consuming so much high-fructose corn syrup that this overconsumption has caused the gene to increasingly express itself in people who have an inherited disposition to fructose intolerance. (4) Thus, the prevalence of this sweetener in the food supply is quite problematic, triggering this genetic susceptibility at increasingly alarming rates.
Now back to fructose and weight gain. I originally learned that fruit makes you fat not by reviewing the biochemical pathways of metabolism, but by actually doing nutritional experiments with real bodybuilders. 
Rather than being some theory out of a book, this is an experimental fact. For a long time I didn’t understand it - I just knew from our work in the gym that certain foods made bodybuilders get in better shape and other foods made them get fat.
In my experiments, I would remove 300 calories worth of complex carbohydrates from the subject’s diet in the form of rice, and replace it with 300 calories worth from fruit. The subject’s total caloric intake remained the same, as did the percent of calories from protein, carbohydrate and fat. The training program remained exactly the same. The only change was in the form of carbohydrate supplying 300 of the calories: Rice was replaced by bananas.
You would expect the subject’s body weight and percent body fat to remain the same, right? But that is not what happened. The subject would begin to gain fat. I would let this go on for a couple of weeks and the subject continues to gain fat. Then I’d pull the bananas out of the diet and put the rice back in - i.e., go back to the original diet. Guess what? The subject would begin losing body fat.
From a metabolic standpoint, the problem with the fructose in fruit is that it bypasses the control point that decides if a dietary sugar is going to be stored as glycogen or fat. Complex carbohydrates, such as rice, oatmeal or potatoes, are preferentially stored as glycogen until glycogen stores are full. Fructose, on the other hand, gets directly converted to fat in the liver, then gets whisked off in the bloodstream to be stored in fat cells (5).
So a large portion of the fructose simply gets turned directly into fat and released into the bloodstream. Bam. You get a dose of fat. But the damage doesn’t stop there. The rest of the fructose gets converted into liver glycogen. That sounds okay, until you stop to think about it. You see, once liver glycogen stores are full, the liver says, “I’ve got all the glycogen we can hold, so any more carbs coming in here I’ll just convert to fat.”
ructose thus preferentially repletes liver glycogen instead of muscle glycogen and shifts the liver into fat-storing mode. This is exactly what you don’t want. You need some liver glycogen, to be sure, because this is what keeps blood sugar levels steady. But when liver glycogen stores are full, this is when dietary carbs start to “spill over” into fat stores - and not to muscle cells as muscle glycogen.
Clearly, one of the keys to effectively restoring glycogen is the type of carbohydrate you eat. Natural, starchy carbohydrates such as potatoes, yams, whole grains, corn, and legumes do a better job at this than simple sugars like fructose do. Research has shown that a diet high in starchy carbohydrates can restock more glycogen in the muscles 48 hours after exercise than simple sugars can.(6)
So if you eat simple sugars like fructose, you’re not going to be able to store as much glycogen had you consumed natural, starchy carbohydrates. What implications does this have for you as an exerciser and bodybuilder?
For one thing, you won’t be able to train as hard or as long during your next workout, because you haven’t stored as much glycogen. Nor will you be able to recover from your workouts as efficiently. By contrast, eating ample amounts of starchy carbohydrates will extend your endurance and effectively re-supply your muscles with glycogen for better recovery. You’ll stay leaner too, since starchy carbs are fully utilized for energy production and glycogen synthesis.
Clearly, fructose is the worst carb source for exercisers and bodybuilders you can imagine. If you wanted to design a supplement to ruin your physique, it would be a fructose-based energy bar. Unfortunately, the vast majority of the bars out there rely on fructose as their major carb source, because it’s cheap.
When we were designing the Parrillo supplement bars, we surveyed every available sports supplement bar we could find. We found that 25 out of the 26 bars had fructose in either the first or second ingredient. (If you use somebody else’s bar, go read the label.)
Why? Because high-fructose corn syrup and fruit juice (good sources of fructose) are real cheap, and they’re also very sweet, as I noted above. We pioneered the use of a new carbohydrate source in the Parrillo Bar called rice dextrin. It’s a shortchain glucose polymer made from rice. This gives you the quick energy you want from a sports bar, but without the fructose.
In summary, a large portion of fructose is converted directly to fat by the liver, fructose preferentially fills liver glycogen stores so that even good carbs are more prone to spill over into fat, and it cannot be used by muscle to recover glycogen. Calorie for calorie, the only nutrient that will make you fatter than fructose is fat itself.
Try to think of fruit as nature’s candy, because that’s exactly what it is. If your goal is to build a lean and muscular physique, then you don’t want to eat candy. Sugar and fat are natural, but that doesn’t mean they’ll make you lean and muscular.
Parrillo Performance Weight loss and Muscle Gain Products
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REFERENCES
Elliott, S.S., et al. 2002. Fructose, weight gain, and insulin resistance syndrome. American Journal of Clinical Nutrition 76: 911-922.
Jequier, E. 1994. Carbohydrates as a source of energy. American Journal of Clinical Nutrition 59: 682S-685S.
Morris, K.L., et al. 1999. Glycemic index, cardiovascular disease, and obesity. Nutrition Review 57: 273-276.
Cox., T.M. 2002. The genetic consequences of our sweet tooth. Nature Reviews/Genetics 3: 7-13.
Shafrir E. 1991. Fructose/sucrose metabolism, its physiological and pathological implications. In Sugars and Sweeteners, CRC Press, pp. 63-98.
Costill DL, et. al. 1981. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. American Journal of Clinical Nutrition, 34: 1831-1836.
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