I was listening to "Scream" by Timbaland in the car today and the first part of the title made me think of dieting. Sometimes our bodies fight our every effort to change our weight. This is a particular problem for people who are trying to lose weight. They feel cold, hungry & listless and they really want to EAT SOMETHING, damn it! It's because they are trying to change their set point weight. For information about set point weight, see Good Science: Dr. Leibel Explains Metabolic Slowdown with Weight Loss.
The problem with getting fat is that adipocytes (fat cells) can only hypertrophy (become larger in size) so much. When they reach a certain size, they secrete something (I don't know how this works so don't ask) that stimulates preadipocytes to become adipocytes. A larger number of something is called hyperplasia. Once adipocyte hyperplasia has occurred, we are stuck with them for life depending on what part of the body they've formed in. Visceral fat (the harmful fat around your internal organs) cannot be removed by liposuction. Only sub-cutaneous fat can. Losing weight & bodyfat results in adipocytes becoming smaller. However, the more of them there are, the smaller they have to become to achieve a given fat mass.
Adipocytes secrete leptin, the amount secreted reducing with reducing size. Leptin works by inhibiting the activity of neurons that contain neuropeptide Y (NPY) and agouti-related peptide (AgRP). Reducing serum leptin level acts on the hypothalamus, increasing our appetites and reducing our energy expenditures. When adipocytes become smaller than normal, leptin secretion reduces considerably and even though there are more of them, net serum leptin decreases and the end result is feeling cold, hungry & listless.
There's another problem that can occur with leptin. In the above example, there's a lack of leptin and the body doesn't work properly. This is analogous to type 1 diabetes. In some people, the hypothalamus becomes insensitive to leptin. Leptin resistance is analogous to type 2 diabetes in that the stuff's present at the correct level but it doesn't have the desired effect and the body doesn't work properly. The end result is also feeling cold, hungry & listless.
It's supposedly possible to restore the sensitivity of the hypothalamus to leptin, but that's for another blog post. So a fat person could say "I'm not fat, I'm hypothalamically-challenged!"