The following article is a very comprehensive and informative review of the sugar content of our diets. Bottom line: It is recommended that we consume no more than 13 teaspoons of sugar a day. It would help a lot if the much needed revised nutrition fact labels would include added sugars. For now, the labels have grams of sugar as total sugars in one serving. Divide that number by four to determine how many teaspoons of sugar that product contains.
Most of the studies on sugars and health has concentrated on diabetes and obesity. Check out this previous post on the seldom mentioned effects of carbohydrates on cancer risk. These studies for obvious reasons are cell or animal studies, but it is nevertheless interesting.