What this suggests is that the primary reason obese animals overeat and become tremendously fat is that their NPY neurons are in constant overdrive because there isn’t any leptin around to keep them in check. Get rid of the NPY neurons, and the animals slim down, even without a trace of leptin. This has a second, even more remarkable implication: The hunger, the obsession with food—many of the physiological and psychological effects that we see in people who are dieting, starving, or born without leptin—could be largely due to a population of hyperactive NPY neurons that is small enough to fit
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