Excess sugar, both fructose and glucose, activate high glucose sensing neurons in brain hypothalamus that cause you to activate hunger (orexigenic) pathways. This causes you to eat more! And if what you eat is sugar, then you have again activated those neurons! In science and medicine this is called a positive feedback mechanism almost always associated with pathophysiology and disease!
AND if the taste of starches (sugar polymers) and sugars causes dopamine to increase in rewards pathways then you can also add sugar addiction to the mix. This makes it difficult to maintain a low carbohydrate diet in the same manner as alcohol and drug addictions make it difficult to stay alcohol and drug free.
AND that visual, auditory and olfactory (sight, sound, and smell, respectively) cues can override satiety to cause you to eat more explains why advertisement of sugary foods adds a third component to obesity.
All three separate neuronal pathways result in overeating of sugars. The excess sugars cause the phenomen of glycation of proteins. That is, the sugars are chemically bound to the proteins. This changes the shape of the proteins. This changes the function of the proteins. So if that protein was a transporter that was supposed to help nutrients get into a cell, and glycation causes that protein to stop functioning properly, then you just stopped a nutrient from getting to a cell!
That would indeed be an example of toxicicity!