Ob/ob Mice, Before and After Leptin Leptin is a hormone produced by body fat. Because of a genetic mutation, these mice, dubbed ob/ob mice, lack the ability to produce leptin. Consequently, ob/ob mice behave as though their brains were telling them that their body fat reserves are completely depleted and that they are starving. Ob/ob mice have voracious appetites and five times as much body fat as normal-weight mice. Yet they display the characteristics of starving animals, including decreased immune system functioning, low body temperatures, and lack of energy. When the ob/ob mouse on the right was given supplemental leptin, it lost the excess fat and began eating normally. Its body temperature, immune system, and metabolism also became normal (Friedman & Halaas, 1998). Unfortunately, what worked for obese mice has not worked as easily for obese people, although researchers remain hopeful (Morrison, 2008).

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