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For someone who is fifty pounds overweight, losing three to five pounds over half a year is a frustrating drop in the bucket. Accordingly, a stock response to these studies has been to declare exercise futile for trimming your waist. Before we entirely dismiss the weight control benefits of walking, the most fundamental type of endurance physical activity, let’s examine the major arguments behind this contention through the lens of evolutionary anthropology. The first is the specter of compensatory mechanisms, notably fatigue and hunger. If I walk ten thousand extra steps, I’ll be more tired and hungry, so I’ll rest and eat more to recoup lost calories. From an evolutionary perspective, these urges make sense. Because natural selection ultimately favors those who can allocate as much energy as possible to reproduction, our physiology has been tuned over millions of generations to hoard energy, especially fat. Further, because almost no one until recently was able to become overweight or obese, our bodies primarily sense if we are gaining or losing weight rather than how much excess fat we have. Whether you are skinny or stout, negative energy balance—including dieting—causes a starvation response that helps us restore energetic equilibrium or, better yet, gain weight so we can shunt more energy toward reproduction.35 It’s unfair, but losing ten pounds elicits food cravings and the desire to be inactive regardless of whether one is skinny or obese.
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Daniel E. Lieberman (Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding)