Both high-fructose corn syrup and regular corn syrup can be bad for a person in excessive amounts because they are types of sugar. High-fructose corn syrup contains a large amount of fructose with glucose, as the name suggests, while regular corn syrup is predominately glucose.
Corn syrup and glucose in general are bad for a person in high quantities because they can raise a person's blood sugar and cause weight gain. High-fructose corn syrup is bad for the same reasons, though the fructose content is more concerning, because when it is consumed without other nutrients, it allows the fructose to go directly to the liver to be processed. The liver creates fats due to this processing, which can result in a fatty liver if the liver is frequently exposed to the fructose.
Diets high in sugars, including corn syrup and high-fructose corn syrup, also lead to weight gain and obesity, Type 2 diabetes and heart disease. In moderation, these syrups, like all sugars, are an acceptable part of a diet; however, since they are added to many foods, it is difficult to moderate these sugars without paying close attention to diet by reading labels and reducing consumption of foods with these sugars added.