I read this interesting article about leptin resistance similar to insulin resistance. Here's the link http://articles.mercola.com/sites/articles/archive/2010/04/15/shocking-sugar-content-of-common-food-products.aspx
A few points in Dr. Mercola's article:
Sugar consumption has caused people’s appetite regulation system to go awry. Leptin, the hormone responsible for satiety, isn’t working properly anymore in a majority of people.
Leptin appears to reduce cravings for sweet foods by targeting taste receptors on the tongue. Therefore, it is possible that a lack of leptin, or your body's failure to respond to the hormone due to leptin-resistance or defects in your leptin receptors, may contribute to the so-called 'sweet tooth' that affects so many people.
Leptin, which is produced by your fat cells, is an integral part of your weight regulation. When fat cells are “full,” leptin sends signals to your brain to reduce hunger so you can stop eating.
However, once you become leptin-resistant, your brain can no longer hear these signals, and so the sensation of hunger is not shut down. This typically leads to overeating and gaining excessive amounts of weight.