Circulating leptin levels fall with starvation and rise to prior levels with refeeding. It is possible, but not yet demonstrated, that some lean individuals respond briskly to rising leptin to prevent obesity or respond to another as-yet undiscovered signal to prevent obesity. It is also possible, but not yet demonstrated, that some individuals with obesity with relatively low leptin levels for their degree of obesity might respond to exogenous leptin with weight loss. Most individuals with obesity have high leptin levels to which they are partially resistant, causing them to be unresponsive to exogenous leptin. This “leptin resistance” may be caused by desensitization induced by hyperleptinemia.