I have been hearing some talk lately about whether certain carbs good or bad for you and in particular fructose. Fructose is the main sugar (carb) in fruits and it is commonly labeled as high fructose corn syrup which you see in all of that processed food. Most bodybuilders will tell you to stay away from fructose like it is the plague. So once again we come to another million dollar question in the world of bodybuilding….should you stay away from all fructose, including fruit, if you are on a quest to transform your body from fat and sloppy to lean and mean? I guess you are going to have to keep reading but hopefully by the end of this discussion you will be able to make your own informed decision.

Just as an aside here, let me say that the sugar you have in the bowl on your table (sucrose) is made up of 1 part glucose and 1 part fructose. That is all I am going to say about sucrose so keep that in the back of your mind throughout this whole discussion of fructose.

Blinded by Science

Bear with me here a little bit as I try to summarize the science about carbohydrate metabolism instead of you just taking my word for it.

So why exactly does fructose have such negative reputation in the world of bodybuilding. Well, the belief is that fructose always turns into fat when ingested. This is both true and false. Whenever you consume a carb, whether it is glucose, fructose, dextrin, malto-dextrin or whatever, that carb is going to go through a bunch of processes in the body and can be converted into any number of end products, one of those being fat. So any carb can end up as fat but the kicker here is how much easier this happens with fructose.

So, on to the science. Let’s look at what happens to the most fundamental form of sugar, a glucose molecule, once we consume it. That molecule of glucose goes through a long and complicated highway of metabolic processes to wind up as a molecule of fat which is stored for energy later. One of the most important steps along this highway is the conversion of fructose-6-phosphate to fructose 1,6-bisphosphate. Now that your eyes have glazed over let me tell you the important implications this has to this whole entire discussion about fructose. That important step is what is termed the rate limiting step in fatty acid synthesis from glucose. So in layman’s terms, it limits the rate and relative ease by which that glucose molecule can be turned into fat. Fructose enters that long and complicated highway of reactions that convert carbs to fats AFTER that critical rate limiting step. Let me repeat that again. Fructose enters that long and complicated highway of reactions that convert carbs to fats AFTER that critical rate limiting step. This means that fructose can be converted to fat much more easily than glucose (and other carbs) because there is no rate limiting step to get in its way. I’m going to repeat that again. Fructose can be converted to fat much more easily than glucose (and other carbs) because there is no rate limiting step to get in its way. Fructose skips that rate limiting step!! Now this doesn’t mean that fructose will always turn into fat. What is does mean is that under the right conditions, fructose will prefer to turn into fat more so than other carb sources. What are those right conditions? Keep reading and I’ll tell you.

Fructose and Body Composition

Time to answer another $1,000,000 question. If you have stuck with me this far congratulations. Your efforts will be rewarded.

The deal with fructose and how it will ultimately affect your body composition hinges around two key factors, your overall calorie consumption (are you in a deficit or surplus) and your total carb consumption. If your carbs are low then the fructose will be filling up your glycogen reserves first and there is much less of a chance of it turning into fat. If your carbs are high, then that fructose you are eating will likely be stored as fat. Now if your carbs and cals are high it is pretty much guaranteed that the fructose you are consuming will be turned into fat. So what this boils down to is that if you are dieting, especially if you are on a low carb diet, fructose isn’t going to have a negative impact on your body composition (it’s not going to turn to fat). BUT, if you are bulking, be prepared to see the love handles growing a lot faster if you are consuming fructose. BOOM. I’ll take those million bucks now in installments if you can’t afford it all at once.

Additional consequences of Fructose

So how else does fructose uniquely affect the body? There are many ways that fructose impacts the body in comparison to other carb sources. The key ones I have been able to dig up are:

1. Thermogenesis – more fat loss stuff
2. Leptin
3. Insulin and insulin resistance
4. Athletic performance

I’ll try to touch on each of these

What the heck is Leptin

Leptin is the hormone that basically tells your body “I’m full; you don’t need to eat anymore”. A couple of the biggest setbacks to successful dieting are always feeling like you’re dying of hunger and slowing down of your metabolism. Decreases in Leptin levels are tied into those setbacks. Fructose does not induce insulin secretion (which I will talk about in bit) which causes a drop in leptin which in turn can sabotage the best of intentions when dieting. Sounds like a good thing with fructose, it doesn't spike insulin which would in turn blunt leptin. Just humor me for a bit and keep reading on.

Let’s look at the situation while bulking now. If you are eating a ton of fructose during your bulk, your body will not produce as much leptin. This will not only encourage overeating, but it will also result in a slower metabolic rate — which means the love handles are going to form faster while on your bulk.

Insulin and Insulin resistance effects

I am sure you all know that insulin is a hormone needed to absorb glucose into your muscle and fat cells. Without it, your blood sugar would be all over the place and you would probably end up dead. Another aspect of insulin is that it also blocks fat burning. Fructose does not trigger the release of insulin, whereas most other carbs do. So you have seen how fructose may lead to increased fat gain, but it may also lead to greater fat loss by keeping insulin down while still supplying the body with carbohydrates (i.e. other carbs spike insulin which can block fat burning but fructose doesn’t spike it.)

Ok then (this is why I told you to keep reading on), why don’t we just cut carbs to nothing but fructose during a diet? Seems logical in that respect doesn’t it? Not really. While low insulin levels may promote greater fat loss, low insulin also leads to decreased secretion of the hormone leptin, which I touched on above. That Leptin keeps you feeling full and keeps your metabolism revved up, especially while dieting. Plus in addition to that, fructose exerts other effects on insulin resistance and thermogenesis which make it unsuitable for fat loss.

So, speaking of insulin resistance, Fructose has an uncanny ability to rapidly induce insulin resistance, which is a condition where your body does not respond well to insulin. This results in increased fat gain and a plethora of other problems that will put a halt to all your progress in the gym.

This ability of fructose to induce insulin resistance stems from its conversion to fat, which is a lipid. Some of the most potent inducers of insulin resistance are high levels of lipids in the blood. It has been amply shown that fructose increases blood lipids in the form of triglycerides, which hang out in the bloodstream and thus cause insulin resistance to develop. Bottom line here, insulin resistance means a greater chance for fat gain so keep fructose to a minimum when calories, and in turn, the risk of fat gain, is high. You are not doing yourself any favors by loading up on fructose.

Thermogenesis-burning the fat off

Thermogenesis, yet another reason fructose is a poor choice for fat loss. Thermogenesis is the production of heat by the body. As a general rule, increased thermogenesis leads to increased fat burning – this is the principal behind taking clen, albuterol, etc. Many carbs are potent stimulators of thermogenesis, but fructose is not one of them. Thermogenesis caused by food consumption is usually pretty negligible, but while on a diet and needing every fat burning advantage we can get, it becomes important. When I consume carbs on a diet, and even some times while bulking, I actually start sweating like a pig sometimes because of that thermogenic effect of the carbs.

Is it going to help me in the gym

There are some differences between fructose and other carbs that can affect your performance in the gym. The most important difference comes by way of glycogen, which is the body’s stored form of carbohydrates (energy). This glycogen is stored in your liver and muscles. Liver glycogen is what is used as our body’s main supply of glucose for everyday activities such as cognition and baseline metabolism. Muscle glycogen is what we use when we are pounding weights in the gym.

Fructose compared to other sugars has differing abilities to replenish these muscle and liver glycogen reserves, and these differences can have profound effects on your performance. Fructose alone is ineffective in replenishing muscle glycogen which we use to push the weight, but it does rapidly replenish liver glycogen. I think you get where I am going here. Fructose is not the most efficient fuel to fuel our workouts. Here is what is happening when we get a slug of fructose in us. First, we are going to use it to fill up the glycogen reserves in our liver. Once that is full, there are two other options for the excess fructose. Put it into the muscle glycogen reserves or store it as fat. Now from all that you have read so far, you should be able to ascertain that the more likely of those two scenarios is to store it as fat. Yes, it could go into muscle glycogen under the right circumstances, but if your cals and carbs are high you muscle glycogen is likely full and it is going into fat reserves. Bottom line again, skip the preworkout touting fructose as an ingredient, it isn’t doing anything to improve your workouts and is likely turning into love handles.

The elephant in the room – FRUIT

Alright, hopefully after reading all of this, you are smart enough now to avoid anything with high fructose corn syrup on the label. But what about fruit? Unfortunately I see to many diehards in this sport avoiding even fruit like it is the plaque. I am not one of them. There are just too many beneficial aspects to them to completely exclude them from the diet. Fruit juices, yes I avoid, but not whole fruits. They are just too beneficial to avoid in the grand scheme of things. They are packed with vitamins, minerals, phytonutrients, fiber, etc., etc., which all improve your performance in the gym, health, and body composition. So unless you plan on eating a dozen bananas a day, fruits can provide vital nutrients that your body needs, without the harmful side effects of high fructose consumption. Now don’t go eating a dozen bananas a day but enjoy some blackberries or raspberries in your oatmeal in the morning.

The Main Points

These are the main points I was trying to get across about fructose. This turned out way longer than I wanted it to so I’ll sum up everything here.

1. Fructose will not always be turned into fat, but it is much more likely to be turned into fat than it is to be burned as energy, or stored as muscle glycogen.
2. Because of 1. Above, fructose is also a poor choice when bulking.
3. The fact that fructose does not increase insulin, and in turn does not increase leptin, once again makes it a poor choice while dieting because it can lead to increased hunger and a slower metabolism which is the death toll to successfully dieting.
4. The potential of fructose to induce insulin resistance makes it a poor choice when bulking. Insulin resistance means more potential for love handles when carbs and calories are high.
5. The reduced ability of fructose to replenish muscle glycogen makes it a poor choice for both pre and post-workout.
6. Fruits are not your enemy, they are your friend. They have vitamins, minerals, phytonutrients, and fiber that your body needs to stay healthy and assist your efforts in the gym. So enjoy in moderation.

References:

White, J. (2013). Challenging the Fructose Hypothesis: New Perspectives on Fructose Consumption and Metabolism. Advances In Nutrition: An International Review Journal, 4, 246-256.

Conlee, R., Lawler, R., & Ross, P. (1987). Effects of Glucose or Fructose Feeding on Glycogen Repletion in Muscle and Liver after Exercise or Fasting. Annals of Nutrition and Metabolism,31(2), 126-132.
Federico, L. (2005). Fructose, insulin resistance, and metabolic dyslipidemia. Nutrition and Metabolism, 2(5), 1316-1326.

Sievenpiper, J., De Souza, R., Cosma, A., Ha, V., & Mirrahimi, A. (2014). Fructose vs. glucose and metabolism: Do the metabolic differences matter? Current Opinion in Lipidology, 25(1), 8-19.

Ventura, J., Estruch, A., Rodas, G., & Segura, R. (1993). Effect of prior ingestion of glucose or fructose on the performance of exercise of intermediate duration. European Journal of Applied Physiology and Occupational Physiology, 68(4), 345-349.

Johnson, R., & Murray, R. (2010). Fructose, Exercise, and Health. Current Sports Medicine Reports, 9(4), 253-258.

Frank, K., & Orwell, S. (n.d.). Is high fructose corn syrup (HFCS) worse than sugar? http://examine.com/faq/is-hfcs-high-fructose-corn-syrup-worse-than-sugar...