Yes, sugar. The University of California and Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology, presents an educational documentary about the bitterness of sugar and "explores the damage caused by sugary foods. He argues that fructose (too much) and fiber (not enough) appear to be cornerstones of the obesity epidemic through their effects on insulin".
High levels of sugar in the blood are a cause of several diseases like Diabetes, a pandemic illness of the 21st century. Just for you to get a picture of how common it is in the population, the American Diabetes Association states that 8.3% of population of US has diabetes. That's a staggering number!
Gary Taubes, a Robert Wood Johnson Foundation independent investigator in health policy and the author of “Why We Get Fat”, who the article "Is Sugar Toxic?" on the NYTimes, shares the same worries of Mr Lusting if not the same conclusion. It's a long read but it's worthwhile.