1) Insulin levels dictate fat storage right? So then a higher blood sugar = more signals for fat storage... How can the body simply just "store more fat" wouldn't it need to slow down the metabolism, or conserve energy somehow?
After immediate energy needs are met and the body has stored all the glycogen it can, the excess glucose gets partially broken down into smaller units. The liver hooks these smaller breakdown products back together to form a different structure, fatty acids. The fatty acids get sent out into the bloodstream and dropped off at the fat tissue stores, also called adipose tissue. The fatty acids then form a larger molecule called triglycerides for storage. The body can store fat in quantities that sometimes seems limitless, unlike glycogen.
2) With regards to leptin, how does eating fat affect leptin levels? From what I understand it makes your cell receptors less sensitive to leptin, and reduces leptin over time meaning high fat diets will lead to overeating eventually and improper fulness signals?
Unfortunately, the relationship between leptin levels and obesity in humans is not straightforward. Rather than being leptin-deficient, many overweight individuals appear to have adequate amounts of leptin. When the body's fat stores increase, leptin levels should decrease. This decrease in leptin tells the brain that the body has enough fat. Surprisingly, leptin levels appear to increase in overweight individuals.
3) Do fat cells grow in size, or quantity when you eat lots of fat? Or both?
Fat cells are storage cells, they can both grow in size and increase in number.