Is high-fructose corn syrup really all that bad for you?

Having just finished the digestive organs unit (covering the gastrointestinal tract from mouth to anus including the liver, pancreas, and gall bladder), we're now learning how those nutrients are metabolized after they're taken up into the body. It's actually very interesting material because it is applicable to everyday life. Our metabolism professor Dr. Besseson says that "Nutrition is just applied biochemistry."

One nutritional controversy that has been in the news for the past few years (and has spawned new marketing campaigns from various food companies) is whether or not high-fructose corn syrup is bad for you - or in more apocalyptic terms, one of the underlying causes of America's obesity epidemic. Here is my attempt at an explanation that is accessible to the lay person.

First, some background. Common table sugar (sucrose) is a disaccharide (two sugar subunits stuck together); specifically, it is one glucose and one fructose molecule stuck together. They are broken apart by enzymes and absorbed in the small intestine before they can be broken down for energy.

There are two key points to consider. The first is how glucose and fructose are absorbed by the small intestine. The absorption mechanism for glucose is tightly regulated by insulin, whereas the absorption mechanism for fructose is not. What that means is that the presence of insulin allows for more glucose uptake, but fructose uptake will keep going regardless of whether insulin is present or not.

The second key point to consider is how glucose and fructose are biochemically broken down to make energy. Glucose enters the glycolysis pathway (i.e. the breakdown of glucose into energy), the activity of which is tightly regulated by.... you guessed it, insulin. In contrast, fructose by and large enters an alternate pathway that allows it to avoid a key regulatory step of glycolysis and produce energy regardless of whether the body is signaling a need for that energy.

As you might imagine, this might be problematic. First, you have fructose absorption even if you don't really need it; second, you have fructose breakdown into energy even if you already have all the energy you need. Then, why don't you get fat when you eat fruit? I asked this question in class, and the answer is simple: quantity. A person eating fruit will feel full and stop eating the fruit, making it less likely that he or she will over-eat.

Here's the crux of the issue: a person eating high-fructose corn syrup is able to eat large quantities of concentrated fructose before feeling full. So, it's not necessarily that high-fructose corn syrup per se is bad for you, just that it's much more likely that a person will eat more of it than he or she really needs, which in turn leads to obesity and all of its adverse health problems.

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I hope that was clear enough for everyone. Also, I'd appreciate hearing from anyone who can add anything to my understanding of the metabolic consequences of high-fructose corn syrup.