Interesting read

xscat

http://en.wikipedia.org/wiki/Leptin

Interesting points:

1. This hormone promotes the synthesis of α-MSH, an appetite suppressant.
2. Several studies have shown fasting or following a very-low-calorie diet (VLCD) lowers leptin levels.
3. Leptin levels change more when food intake decreases than when it increases (so if we don't eat enough we are hungry; but if we eat more than usual it doesn't change as much....)

Clinical Significance:

Leptin level is decreased after short-term fasting (24–72 hours), even when changes in fat mass are not observed.[83][84][85]
In obese patients with obstructive sleep apnea, leptin level is increased, but decreased after the administration of continuous positive airway pressure.[86][87] In non-obese individuals, however, restful sleep (i.e., 8–12 hours of unbroken sleep) can increase leptin to normal levels.
Serum level of leptin is reduced by sleep deprivation.[88][89] However, a recent study showed that sleep deprivation was linked with higher levels of leptin.
Leptin level is increased by perceived emotional stress.
Leptin level is decreased or increased by increases in testosterone or estrogen level, respectively.
Leptin level is chronically reduced by physical exercise training (so if we exercise more we tend to get hungrier???)
Leptin level is increased by dexamethasone.[95]
Leptin level is increased by insulin.[96]

Oh and this:

Dieters who lose weight experience a drop in levels of circulating leptin. This drop causes reversible decreases in thyroid activity, sympathetic tone, and energy expenditure in skeletal muscle, and increases in muscle efficiency and parasympathetic tone. The result is that a person who has lost weight has a lower basal metabolic rate than an individual at the same weight who has never lost weight; these changes are leptin-mediated, homeostatic responses meant to reduce energy expenditure and promote weight regain. Many of these changes are reversed by peripheral administration of recombinant leptin to restore pre-diet levels.

In English, it means our metabolism does slow down because we are not "naturally that skinny"..... Not sure how significant it is; just found it very interesting