You can almost always find something nice to say about someone if you think hard enough. But - let's be honest - there's a few people that you think are just pure evil. It's the same with some foods. Trans fats, mangled souls of the nutritional underworld, are a prime example: they destroy your lipid profile and wreak havoc throughout the body. Fructose is the same way: the more science researches fructose, the scarier it gets. In this link we're going to look at how fructose sabotages all attempts at weight loss.
Some of you may be thinking that you never eat fructose since you don't really like or buy fruit. Well, it's not that simple: sucrose, or table sugar, is composed of a glucose and fructose molecule bonded together. In other words, when you consume table sugar, your body quickly breaks the table sugar into one half glucose and one half fructose. The glucose can go in its merry way annihilating your testosterone and the fructose can do its job of adding fat and keeping your from any weight loss you might be attempting.
So how does fructose go about its mischief? First of all, fructose has a strange effect on insulin levels or, better said, it has little to no effect on them. We are all used to the idea that eating carbs raises blood sugar levels after a meal, which then triggers the pancreas to pump out insulin in response, right? Fructose does not cooperate with theory, however, and insulin (after consuming fructose) remains relatively unchanged.
NEWS FLASH: Researchers just announced that mercury contamination is extremely common in high fructose corn syrup: 50% of products tested by researchers were contaminated!  The issue apparently is that there are no standards for mercury residue in hydrochloric acid and hydrochloric acid is used to make high fructose corn syrup. This is dangerous for anyone but particularly for children. (NOTE: Mercury can affect testosterone!)
This happens because glucose is pulled into cells through the glut-4 system, which raises insulin as expected. Fructose completely bypasses this system and uses the glut-5 system, which few cells in the body have by the way, thus achieving a greatly muted insulin response. Now this might seem like a godsend to all you insulin resistant middle aged males out there, but it's definitely not. This lack of insulin response actually backends weight loss because it has a negative impact on the appetite-suppression hormones leptin and gherlin. These appetite hormones normally kick in in reaction to insulin, which makes sense because if you've just filled your gut with carbs, you want your body to respond by signalling that you're not as hungry any more. This signal is much, much more muted with fructose and so you stay hungry even though significant calories have been ingested.
The problem doesn't just stop there. Remember how I said that fructose does not produce an insulin response? Well, this is counterintuitive, but fructose also leads to insulin resistance and, as you may have read on my site, insulin resitance has been found to lead to weight gain. By the way, fructose is so powerful at inducing insulin resistance that researchers use it like a drug in lab studies to create insulin resistance.  Of course, insulin resistance is bad for many other reasons besides just assaulting weight loss and you shoud read this link on Metabolic Syndrome if you are unfamilar with the issues involved.
And don't think you can get away with eating table sugar (sucrose) and corn syrup instead of fructose. Table sugar and corn syrup are about one half fructose and so downing a 64 oz. Coke from a convenience store will flood your system with fructose. By the way, researchers actually use sucrose (and sometimes direct fructose as well) to induce insulin resistance!  Think of the food industry, which has known for decades of the evils of fructose and sucrose, and yet still choose to drown us in the stuff!
So what else could this bad boy do besides sabotaging appetite and increasing insulin resistance? Well, how about turn to fat fast - say that three times fast! - so that it even negatively affects how your body processes carbs at the next meal. The reason lies in the fact that fructose is actually metabolized by the liver and thus is rapidly turned into fat signalling the body to store the next meal as fat.
Some of you may be wondering if you should eat fruit, since fructose is often heavily in fruit. My opinion is that fruit is the least of your worries unless you're a juice-a-holic or eat fruit like a monkey at a zoo. Most guys get into trouble from a sweet tooth: table sugar (sucrose) and corn syrup, as I mentioned, are one half glucose and one half fructose. Many guys drink a 64 oz. Mountain Dew and later have a slice of cake and this will deliver a substantial amount of glucose into their system. Let's face it: much of Western Diet is packed with sugar and corn syrup and this can cause trouble with weight loss.
Again, don't forget that table sugar has a glucose molecule bonded to a sucrose molecule. So if you have table sugar, you're also having fructose after the body breaks apart that bond. Corn syrup and high fructose are no better: they are also roughly the same percentage of fructose - not exactly but close enough for discussion purposes. So if you drink a can of coke, you'll ingest 42g of sugars. And here is the key: after your body digests the corn syrup, approximately half of that will be fructose, i.e. a little over 20g. And, of course, nowadays, who has one can of Coke? Most people have bottles or 64 ouncers!
This is how many of you are ingesting large quantities of fructose without even realizing it. It is also in many, many packaged goods as well. Many Western vegans, trying to eat well and live good, eat a lot of sweets and fruit and consume a disproportionately high quantity of fructose thus confounding what may be an otherwise healthy lifestyle.
So, again, let me summarize what some of the key ways - besides needless calories! - that fructose attacks any weight loss plans you might have:
1) Insulin Resistance. Fructose puts your cells' energy burning mechanisms on overdrive. For those of you who can remember your high school biology, fructose exhausts intracellular ATP supplies. Fat cells react by bloating and becoming insulin resistant. Most research shows that, particularly in subjects at or past middle age, fructose leads to Insulin Resistance. And you hopefully know from my page on How to Lose Weight that Insulin Resistance slows down fat loss. (It is also a huge health risk: read my page on Metabolic Syndrome for more details.)
2) Leptin Resistance. Fructose not only makes you insulin resistant but also leptin resistant. Normally, eating sugars and high-glycemic carbs cause a rapid rise in insulin, which affects leptin, which in turn turns off your appetite. Frutctose, though, is unique in that it turns off the pancreas normal output of insulin. The ensuing small rise in insulin is not enough to significantly affect leptin and so your hunger remains. In other words, with fructose you eat a lot of calories and still feel hungry. Even worse, if you continue to consume fructose regularly, your body becomes leptin resistant, which means your hunger never abates.
Stop for a mintue and consider what a daily combination fructose-induced insulin resistance and leptin resistance pose: insulin resistance packs on the pounds more easily and leptin resistance makes it much, much more easy for you to overeat so that you give your insulin resistance a nice, steady supply of calories. This creates an almost impossible environment for a dieter: his hormones and appetite are working against him. No wonder some guys just give up!
Needless to say, if you're on a diet and attempting any sort of weight loss, you need to go easy on the sugars because of the fructose. However, one question you may be asking is, "Should I give up fruit while I'm dieting?" Remember that blueberries are fantastic for the brain and cherries for your heart. Likewise, pomegranate juice is good for the heart and penis. And an apple a day will literally keep the doctor away. So my opinion is that you should eat fruit moderately and pick the heavy hitters that have solid clinical track records. But, above all, leave out all sugars and corn syrup - it's neither necessary nor worth the risk.
In fact, it may be prudent to only eat one of the "heavy hitter" fruits, such as berries and cherries, that are low in fructose. But fruit has many health benefits, so you may have to see how your body responds.
Also, I want to point out again that Jack LaLanne was many decades ahead of his time on this and has consistently preached against sugar. See this link on , pomegranate juice is good for the heart and penis. And an apple a day will literally keep the doctor away. So my opinion is that you should eat fruit moderately and pick the heavy hitters that have solid clinical track records. But, above all, leave out all sugars and corn syrup - it's neither necessary nor worth the risk.
1) J of Pharmacology and Experimental Therapeutics, Accepted for publication 11/12/08;"Increase Endothelial Nitric oxide Synthase Expression Reduces Hyptertension and Hyperinsulinemia in Fructose-Treated Rats; Chun Xia Zhao, et. al.
2) J Nutr, June 2008, 138:1039-1046
3) J. Nutr, Nov 2005, 135:2634-2638
4) J Environmental Health, Jan 2009, 8:2"Mercury from chlor-alkali plants: measured concentrations in food product sugar", Renee Dufault, et al; Institute for Agriculture and Trade Policy, Jan 26 2009, "Much High Fructose Corn Syrup Contaminated with Mercury New Study Finds: Brand Name Food Products Also Discovered to Contain Mercury".