Word: leptin
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...hormones leptin and insulin are longer-term signals. Produced by fat cells, leptin helps manage just how much fat you store around your organs and under your skin through a complex feedback loop. If your fat deposits start to shrink--for example, when you lose weight--the amount of leptin in your body falls, a situation that the brain interprets as a result of starvation. The whole system of chemicals and neurological impulses shifts in an attempt to get the body to burn fewer calories so that it can regain the weight. The greater the weight loss, the stronger...
...even if you could buy PYY in pill form, it might not work. The appetite is a complex system with many subtle triggers and controls. Remember leptin, the wonder hormone that seemed to make mice thin? A drug company paid tens of millions of dollars for the development rights in the mid-1990s only to discover that obese people were immune...
Thus, while we read the paper or carry on a conversation, the hypothalamus--activated by leptin or some other compound--orders cells and tissues to ratchet up energy expenditure. The body responds by idly fidgeting to raise metabolic rates, or by increasing blood flow to the outer layers of the skin in an effort dissipate heat. In this way, we carry out a process known as thermogenesis, which is the body's way of burning excess calories...
...Leptin, which exercises an influence on appetite and thermogenesis, is thought to be key to maintaining this balance. For as we layer on fat, we pump out more leptin, which signals the hypothalamus that it's time to accelerate energy output and brake caloric intake. The problem is, people who gain weight have now been shown to develop a remarkable resistance to leptin's power. The fatter they get, and the more leptin they make, the more impervious the hypothalamus becomes. Eventually the hypothalamus interprets the elevated level of leptin as normal--and forever after misreads the drops in leptin...
...what causes leptin resistance? The answer may turn out to be exceedingly complicated. Not only do the other hormones of the digestive system play a role, but also researchers are learning that they must account for the influence of such mood-altering neurotransmitters as dopamine and serotonin, as well as the stress hormones adrenaline and cortisol. And then there are melanocortins, brain chemicals whose power to affect weight loss and gain is just now coming into focus...