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For example, you may have heard assertions such as “You should drink water before you get thirsty, because once you feel thirsty, you’re already dehydrated.” But think about it. How could a situation like that have possibly developed? Imagine a creature that didn’t become thirsty until the lack of water was detrimental to its functioning. It seems that an inability to accurately detect something as basic as water needs would be selected against. To suggest that waiting until you are thirsty before drinking is bad for your health should at a minimum be accompanied by an explanation of how such a trait could have survived. Likewise, any theory of obesity that suggests our hunger can’t be trusted to tell us when to stop eating seems at least suspect.

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