Yes, sugar is toxic. It was never found in a natural diet for humans in the high quantities it is now ingested by people. The two major effects it has on the body is: 1) simultaneously raising blood sugar levels while promoting insulin resistance when it is eaten, and 2) promoting the glycation process that badly affects metabolic processes. Do a google search on "Advanced Glycation End Products" (also called AGEs) for a better understanding of what exactly sugar does to you.