Why is high fructose corn syrup unhealthy?

Calories: In and of itself its just a kind of sugar, but foods high in fructose tend to have a lot of calories, thus may promote obesity. There is very controversial theories that suggest fructose may affect hormones involved in appetite regulation ( like leptin) and promote obesity in that fashion as well, but direct and definite proof is not yet out there at this time.

Obesity, pain: Humans have a limit to how much fructose the bowel can absorb at any given time. If you max out this limit, the fructose stays in the bowel and causes pain, bloating, and diarrhea. This is not dangerous, however. Just uncomfortable. The real health problem with hfcs is that because of its sugar contents, it is a major contributor to obesity and its consequences such as type ii diabetes.