All good answers, one more thing is that insulin in the blood causes the kidneys to retain water, lower the insulin and it is released.
While I would neve say that it is impossible to add fat no matter how much one eats, I know from experimenting on my self that I can consume large amounts of calories and not gain as long as I eat low carb. Insulin is necessary to add body fat, low carb does lower the amount of insulin in the system. Nyah Levi