Leptin is a protein, released primarily from fat cells, although other tissues such as muscle also contribute slightly. Leptin levels primarily associate with body fat percentage; the more fat you have the more leptin you tend to have. At any given body fat percentage, women typically produce 2-3 times as much leptin as men. In addition to being related to the amount of body fat you have, leptin levels are also related to how much you're eating. For example, in response to dieting, leptin levels may drop by 50% within a week (or less) although you obviously haven't lost 50% of your body fat. After that initial rapid drop, there is a slower decrease in leptin related to the loss of body fat that is occurring. In response to overeating, leptin tends to rebound equally quickly (much faster than you're gaining body fat). In contrast to what you might think, it looks like leptin production by fat cells is mainly determined by glucose availability (you'd think it was fat intake). So whenever you start pulling glucose out of the fat cell (dieting), leptin levels go down; when you drive glucose into fat cells, it goes up. Basically, leptin represents two different variables: how much body fat you're carrying and how much you're eating. That is, it acts as a signal to the rest of your body about your energy stores.
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