As far as obesity please see about Leptin a chemical produced in our bodies that regulate body fat and also tells our brains when we are full from eating (satiety). People have genetic problems with Leptin and people are Leptin resistant, like with insulin resistance, so their brains do not see themselves as full from food. Cortisol also acts in this way because it detects blood sugar levels and can not work properly. There have been a ton of diet and eating studies with identical twins, fraternal twins, prisoners fasting then eating normal and seeing where their weight goes back to, etc etc and much of obesity is found to be genetic and hormonal.
PBS Nova just had a great science show on fat etc where they show at least one person with some Leptin disease who cannot stop eating because there is no signal to the brain to say you dont need to eat but rather they feel like they are super hungry. Excellent documentary.
NOVA: The Truth About Fat
www.wgbh.org/program/nova/the-truth-about-fat
NOVA: The Truth About Fat Scientists are coming to understand fat as a dynamic organ—one whose size may have more to do with biological processes than personal choices. Explore the mysteries of fat and its role in hormone production, hunger, and even pregnancy.
Below From Wikipedia, various science shows, a book I have by a cardiologist and diet specialist:
Leptin (from
Greek λεπτός
leptos, "thin") is a
hormone predominantly made by
adipose cells and
enterocytes in the small intestine that helps to regulate
energy balance by inhibiting
hunger, which in turn diminishes fat storage in
adipocytes. Leptin acts on
cell receptors in the
arcuate nucleus of the
hypothalamus.
[6]
Although regulation of fat stores is deemed to be the primary function of leptin, it also plays a role in other physiological processes, as evidenced by its many sites of synthesis other than fat cells, and the many cell types beyond hypothalamic cells that have
leptin receptors. Many of these additional functions are yet to be defined.
[7][8][9][10][11][12]
In
obesity, a decreased sensitivity to leptin occurs (similar to
insulin resistance in
type 2 diabetes), resulting in an inability to detect
satiety despite high energy stores and high levels of leptin.
[13]
Although leptin reduces appetite as a circulating signal, obese individuals generally exhibit a higher circulating concentration of leptin than normal weight individuals due to their higher
percentage body fat.
[12] These people show resistance to leptin, similar to
resistance of insulin in
type 2 diabetes, with the elevated levels failing to control hunger and modulate their weight. A number of explanations have been proposed to explain this. An important contributor to leptin resistance is changes to leptin receptor signalling, particularly in the
arcuate nucleus, however, deficiency of, or major changes to, the leptin receptor itself are not thought to be a major cause.
Triglycerides crossing the
blood brain barrier (BBB) can induce leptin and insulin resistance in the hypothalamus.
[18] Triglycerides can also impair leptin transport across the BBB.
[18]