Leptin is hormone produced by the fat cells. It is present in the blood stream and it is in direct proportion to the amount of body fat a person has, i.e. the more fat, the more leptin is produced.
Leptin's primary function is to ac on the hypothalamus, the part of the brain that controls appetite and satiety. It tells the hypothalamus to reduce appetite (because fat stores are high) , which results in decreased food intake.
But when the fat stores are low, like after dieting, leptin levels are reduced. This causes the hypothalamus to increase appetite. An increased appetite obviously results in greater food intake and a corresponding increase in body fat stores. More body fat results in more leptin being produced, which then tells the hypothalamus to decrease appetite, leading to a reduced food intake. This is a classic negative feedback mechanism.
Leptin my be one of the main hormones responsible for rebound weight gain after a diet.
Leptin is accurately described as an 'anti-starvation hormone' because low levels lead to increased hunger. In the past it was described as an 'anti-obesity hormone' but researchers have since discovered that obese people (who produce large amounts of leptin) are resistant to its action. Similar in the way that some people are resistant to insulin.
Leptin resistance can result from over-eating. When someone over-eats as a result of 'emotional eating' or a conditioned response' (a habit that has been created), the receptors in the hypothalamus become 'de-sensitized' to the action of leptin. This means that the hypothalamus can't detect when leptin levels are high, resulting in food cravings and further weight gain. Over time obesity is the result.
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