I think with "sugar diets" he specifically refers to diets that contain a lot of Fructose, cooked potatoes contain starch, the liver and muscles convert an excess of that in glycogen and a relatively small amount of fat. Glycogen is actually really useful. Getting a rush afer having eaten an energy rich meal is what is supposed to happen. It's Fructose which is dangerous in large quantities and is linked to type 2 diabetes and metabolic syndrome. Among other things it also blocks Leptin signalling to the brain, so the brain does not properly register how much energy it just gained, leading to overeating and no desire for physical activity.