The liver is the major site of fructose metabolism. In the liver, fructose can be converted to glucose derivatives and stored as liver glycogen ??? which is good if you're physically active.
However, the liver's ability to do this is limited ??? which isn't so good. With very high single-serving doses of fructose, the fructose that arrives at the liver can easily be converted to fat. This is more prominent in clients with high blood lipids, insulin resistance, or Type II diabetes.
Now, blood levels of fructose aren't directly subject to tight hormonal regulation. This is one of the reasons fructose has a low glycemic response ??? which is often considered a positive.
On the negative side, while high intakes of fructose can lead to the synthesis of fat, it also fails to stimulate the production of leptin.
Since leptin is a hormone involved in the long-term regulation of energy balance, the decrease in leptin production associated with chronic high fructose intake can have harmful effects on the regulation of energy intake and body fat.
In other words, with HFCS, you never get those "I'm full" signals from the brain. So you keep eating.
Source
ProTrainerOnline.com