Should You Be Afraid of Fructose?

This is originally an article I wrote for Wannabebig.  However, they decided it didn't quite fit their editorial content.  While the article was mainly aimed at a bodybuilding audience, it has information that is relevant to anyone looking to lose weight.  Thus, I am reposting the article here.


It seems that no matter where you turn, you will come across information telling you that fructose will make you fat and impair your fat loss efforts.  Certainly, there is evidence that diets high in fructose can disrupt your body’s ability to regulate appetite (3), which, in turn, would make it difficult for you to get more lean.  There is also evidence that diets high in fructose can increase fat deposition (7).  However, people have taken this information to the extreme and have concluded that, since high amounts of fructose can be a problem, then any fructose must be a problem.  It has gone as far as some bodybuilding coaches, such as John Parillo, recommending that bodybuilders avoid fruit during contest preparation because of the belief that fructose will go straight towards fat formation.  However, these thoughts are based on misunderstandings of fructose metabolism.

Fructose:  To Fat, Or Not To Fat?

To understand why people are concerned about fructose going straight towards fat formation, we need to delve into how fructose is metabolized in your body.  When you eat a meal, the carbohydrate in that meal is broken down into glucose and fructose.  Glucose goes straight into your bloodstream, where a number of things can happen to it.  It can be stored in muscle or liver as a substance called glycogen, which would then be used later to fuel your high-intensity training sessions.  The glucose can also be used immediately for energy.  Finally, it can go towards the formation of fat.  Where it goes depends upon a number of factors, such as how hard and how much you train, and how many calories you are eating overall.

Fructose cannot go straight into the bloodstream like glucose.  It goes to the liver first.  The liver can take the fructose, convert it to glucose, and then release that glucose into the blood.  It can also take that fructose and store it as glycogen.  Finally, it can convert the fructose to fat.  It is this conversion to fat that causes a lot of confusion.

Your liver can convert both glucose and fructose to fat.  There is an enzyme in your liver called phosphofructokinase (PFK), which can help put the brakes on how much glucose is converted to fat.  However, fructose bypasses this step and can move more efficiently into the pathway to be converted to fat.  This is where the arguments of people like John Parillo come in.  The claim is, since fructose bypasses PFK, it will be easily converted to fat, and thus you should minimize your intake as much as possible.

However, just because fructose bypasses PFK, doesn’t mean it will be converted to fat.  Remember I said earlier that your liver can take fructose and convert it to glucose, or that it can store it as glycogen.  Where it goes is dependent upon a number of factors.  For example, I assume that if you are reading this article, you are a hard-training bodybuilder, or at least someone who exercises frequently.  You may also be dieting for a contest.  Well, when you create a calorie deficit (either through training, dieting, or both), fructose is converted to glucose or glycogen rather than being converted to fat.  For example, one study showed that a 50% fructose diet did not impair body fat loss in exercising rats (2).  Another study showed that when humans are in a calorie deficit, fructose has no effect on fat burning (8).

Liver glycogen is a key factor here, because it reflects the overall energy status of your body.  In fact, there is a positive relationship between liver glycogen and the rate at which your liver forms fat (4).  Similarly, there is a negative relationship between how much liver glycogen you are creating, and how much fat your liver is creating (i.e., the more that fructose is directed towards the formation of liver glycogen, the less it is directed towards the creation of fat) (4).  Basically, this means that if you are in an energy surplus (eating more calories than you’re expending, so that liver glycogen is constantly full), the fructose you eat is efficiently directed towards fat formation.  However, if you are in an energy deficit (eating less calories than you’re expending, so that liver glycogen is not constantly full), then the fructose you eat will be directed more towards glycogen or being converted to glucose.

Also, it should be remembered that, even if fructose does go towards the formation of fat, it doesn’t mean there will be an increase in fat overall.  For every fat molecule that is formed, another one may be burned somewhere else.  Fat accumulation only happens when the rate of fat formation exceeds the rate of fat burning over a 24-hour period.  Remember, your body is synthesizing new fat all of the time, even when you are dieting.  However, you lose fat when dieting because the rate of fat burning exceeds the rate of fat formation.  Thus, you always need to keep the big picture in mind when talking about fructose and fat metabolism.  Don’t get hung up on the small details.

To further illustrate how it’s really the big picture that matters, let’s take an obesity clinic for which I used to work.  During the initial phase of the program, the clients were put on five high protein shakes per day.  These shakes were sweetened with fructose.  On top of that, the clients were mixing berries with the shakes.  It was estimated the men were getting up to 80-97 grams of fructose per day, and the women were getting up to 64-78 grams.  Yet, the clients were losing significant amounts of fat and weight (an average of 40 pounds in 3 months).  Their blood lipids showed dramatic improvements.  They also weren’t hungry.  This happened because the clients were in an energy deficit, exercising regularly, and eating sufficient protein to help control their appetite; thus, the amount of fructose did not impair their fat loss.

Fructose and Mass Building

Fructose really isn’t much of an issue when trying to get lean.  However, where it can become a problem is if you are trying to gain size and you are in a mass-building phase of your training.  Most bodybuilders eat a lot of extra calories when in a mass-building phase, and they don’t mind some fat gain along with it.  However, as I mentioned earlier, whether fructose gets directed towards fat formation or not depends upon the energy status of your body.  If you are building mass and consuming extra calories, the fructose you eat is much more likely to be converted to fat, and it may happen more efficiently than other types of carbohydrate.  This is because the glycogen in your liver will be full, and the fructose has nowhere to go other than fat formation.  This can lead to fat accumulation around your liver and other organs, which is not the type of tissue accumulation you are looking for when trying to add size.  While hard training may prevent some of this (1), it won’t eliminate it.

Does this mean you should avoid whole food sources of fructose like fruit when mass building?  No, because it’s difficult to consume large amounts of fructose when consuming whole fruit.  For example, while a raw apple has a lot of fructose per calorie, it is difficult to consume large amounts of calories from an apple.  A single apple has around 80 calories; you would have to consume 3 apples just to get 23 grams of fructose.  However, if you are a person who likes to make fruit smoothies as mass-building shakes, you will want to be aware of how much overall fructose you are consuming.

The biggest things to avoid are highly processed foods and sugary drinks, which are often high in fructose.  For example, one can of cola has almost 30 grams of fructose.  Also, if you are using a high calorie, mass-building shake as part of your nutritional regimen, you will want to check the ingredients.  Many mass-building shake formulations are sweetened primarily with fructose.  In fact, I’ve seen some formulations to contain up to 100 grams of fructose per serving.  You will also want to be wary of products with the so-called “natural” sweetener Agave syrup.  Agave is almost pure fructose.

To get an idea of how much fructose is contained in many foods, you can check out which contains a list of foods highest in fructose.   This list is based on a 200-calorie serving, so be aware that some of the foods, while high in fructose, would be difficult to consume in large amounts (such as an apple as I mentioned earlier).  Some of the foods listed, like ketchup, are typically consumed in very small quantities.  The amounts of fructose are listed in milligrams; divide each amount by 1000 to convert to grams.

Everything in Moderation

So what should your upper limit of fructose intake be?  This is difficult to answer in regards to bodybuilding, because most of the existing research has been on obese individuals and type 2 diabetics.  However, that doesn’t mean we can’t use existing research to make some rough educated guesses.  One study indicated the threshold where fructose was starting to become primarily converted to fat was around 60 grams per day on average (6).  Another study indicated the threshold to be around 100 grams per day (5).  None of this research was on hard-training bodybuilders, which means the threshold could be higher; however, it is best to be conservative.  Thus, we can roughly estimate that 60-100 grams per day shouldn’t impair your fat loss efforts, assuming you are training regularly and dieting appropriately.  Now, I’m not saying you should go out and get 60-100 grams of fructose per day by drinking soda.  What I am saying is that you shouldn’t be afraid to have some foods with fructose in them if you’re trying to get lean, particularly if that fructose is from whole food sources like fruit.

Overall, it is best to take a big-picture approach when trying to get lean.  Eat enough protein to maintain your muscle mass and control your appetite, watch your overall calorie intake, and get those calories from a predominance of minimally processed foods.  With such an approach, you will never have to worry about every gram of fructose that you ingest, as whole, unprocessed foods aren’t extremely high in fructose anyway.  Combine that with hard training and dieting, and there’s no way that moderate amounts of fructose will slow your fat loss efforts.  Now, when consuming extra calories to build mass, you do want to be more mindful of your overall fructose intake.  However, as long as you avoid processed foods, sugary drinks, or mass-building formulations sweetened with mostly fructose, then you should easily be able to stay in the 60-100 gram range.

It is true that too much fructose can be a problem, but too much of anything can often be a problem.  When consumed in moderate amounts and from whole food sources, fructose is not a nutrient to be feared.


1.            Dantas EM, Pimentel EB, Goncalves CP, Lunz W, Rodrigues SL, and Mill JG. Effects of chronic treadmill training on body mass gain and visceral fat accumulation in overfed rats. Braz J Med Biol Res 43: 515-521, 2010.

2.            Griffiths MA, Baker DH, Novakofski JE, and Ji LL. Effects of exercise training on diet-induced lipogenic enzymes and body composition in rats. J Am Coll Nutr 12: 155-161, 1993.

3.            Havel PJ. Dietary fructose: implications for dysregulation of energy homeostasis and lipid/carbohydrate metabolism. Nutr Rev 63: 133-157, 2005.

4.            Holness MJ, Cook EB, and Sugden MC. Regulation of hepatic fructose 2,6-bisphosphate concentrations and lipogenesis after re-feeding in euthyroid and hyperthyroid rats. A regulatory role for glycogenesis. Biochem J 252: 357-362, 1988.

5.            Livesey G, and Taylor R. Fructose consumption and consequences for glycation, plasma triacylglycerol, and body weight: meta-analyses and meta-regression models of intervention studies. Am J Clin Nutr 88: 1419-1437, 2008.

6.            Sievenpiper JL, Carleton AJ, Chatha S, Jiang HY, de Souza RJ, Beyene J, Kendall CW, and Jenkins DJ. Heterogeneous effects of fructose on blood lipids in individuals with type 2 diabetes: systematic review and meta-analysis of experimental trials in humans. Diabetes Care 32: 1930-1937, 2009.

7.            Stanhope KL, and Havel PJ. Fructose consumption: recent results and their potential implications. Ann N Y Acad Sci 1190: 15-24, 2010.

8.            Tittelbach TJ, Mattes RD, and Gretebeck RJ. Post-exercise substrate utilization after a high glucose vs. high fructose meal during negative energy balance in the obese. Obes Res 8: 496-505, 2000.

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Julian Skinner
Julian Skinner
11 years ago

Yes people lose weight when in calorie deficit, in spite of eating fructose. No one ever said they didn’t. The question has always been how much fructose should people who are not on a calorie deficient regime have per day. The American heart association is saying 6 teaspoons /day for women 9 for men.

David Gillespie Big Fat Lies
Reply to  Julian Skinner

That’s funny that I never see you correct anyone when people like to take the mickey out of professionals who claim that calories do matter in weight loss? Picking your battles or just not wanting to rock the boat amongst your fructose-fearing colleagues? The American Heart Association says that the sugar intake is purely as a percentage of discretionary calories NOT because of fructose (as evidence by the one young active male who’s allowance is 18tsp in the document). See table 5 page 1016 – Despite the fact that you and David Gillespie are corrected on this constantly, you… Read more »

David Gillespie Big Fat Lies

At a recent sugar symposia and number of the experts also spoke about the difference between rats in humans, not just in terms of metabolism but also in terms of addiction models. All of the links to the videos can be found at the end of this article –

It is also interesting to note how poorly Dr Robert Lustig does when grilled by colleagues vs reporters or interviewers on youtube!

Julian Skinner
Julian Skinner
11 years ago
Reply to  James Krieger

Matter of opinion, I thought he did well.

David Gillespie Big Fat Lies
Reply to  Julian Skinner

As a fructophobe who believes the science supports your position, you would say that – despite the fact people shot down his addiction model in humans, figures on de novo lipogenesis in humans and showed that rats didn’t crave fructose vs glucose among other things! The fact that his conclusions are preliminary and aren’t supported by the evidence came through loud and clear – but then again – you are just trying to justify your personal experience and give the impression that it is evidence-based!

13 years ago

Thanks again for dispeling another myth, coz ppl like dr. mercolo goes crazy about fructose telling even to avoid fruits, which is such a fallacy

13 years ago

Great article once again.

My only suggestion for this blog would be to make your links “open in new window” so we can click on them and have them for reference and not have to navigate away from your site. Another nice thing is having your references linkable in the same fashion.

Thanks again for a good article and essentially a free class.

jamie hale
13 years ago

Fructose causes minimal insulin secretion. Even if fructose consumption were high enough to elevate fat synthesis, a lack of insulin would probably result in increased fat oxidation. Assuming a calorie deficit, it all evens itself out at the end of the day.

Josh R.
Josh R.
13 years ago

Excellent article like always, JK. I particularly enjoyed it. I don’t understand why wannabebig wouldn’t want to publish it.

Yew-wei Tan
13 years ago

Very informative. I have a question though, how does this relate to acute fructose consumption. I’m sure that you’re familiar with Martin Berkhan and his LeanGains protocol. In that protocol, despite eating roughly maintenance calories, the way it’s structured means that some large meals would technically put the body into a temporary state of caloric surplus. Looking at the way some people structure their diets using Leangains, I would definitely say that some of them are getting 60g of fructose in a single meal alone. This doesn’t seem unreasonable, especially since 4 apples ≈ 500kcal which would deliver >60g of… Read more »

13 years ago
Reply to  Yew-wei Tan

One of the problems with a lot of the rodent studies on fructose is that de novo lipogenesis is a major pathway for fat storage in the rat, while it is not in humans. According to Jequier’s summary even when DNL rates are increased significantly it doesn’t amount to more than a few grams and these fatty acids are subsequently oxidized. I also came across this very interesting study that demonstrated how carb excesses are “wasted” by lipid futile cycling, I blogged on this twice: Fat futile cycling from carb excess, Fat futile cycling from carb excess – more layperson… Read more »

Luming Zhou
13 years ago

Interesting that the threshold for fructose that would be converted to fat is around 60-100 grams. I don’t think that consuming fructose more than that will also impair fat loss. Here’s a paper which concludes that excess fructose would not cause fat gain: The control of lipogenesis by dietary linoleic acid and its influence on the deposition of fat. Jeffcoat R, Roberts PA, James AT. The replacement of dietary starch by sucrose results in an increase in hepatic lipogenesis in the rat. When corn oil (4% by weight or 9% of the energy content of the diet) was included… Read more »

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