For nearly a half century, America has been on a witch hunt to find the ingredient that is making us fat. In the 1980s, the culprit was fat itself. Next it was carbs. Today, sugar is the enemy—unless you’re caught up in the war on gluten.

And none of it has worked. Obesity is now closing in on smoking as our No. 1 preventable cause of death. The U.S. has rarely failed at anything the way it has failed at weight loss.

Perhaps that is because we’re missing a crucial piece of the food puzzle. Oddly enough, all those diet gurus and bureaucrats hardly ever ask the simplest question: How does it taste? We’ve fixated on what food does inside the body, but we’ve almost totally ignored why it gets there in the first place. Even a child knows: We eat because food is delicious.

We have been trained to see this as a bad thing. After all, if food weren’t so appetizing, we wouldn’t eat so much of it. But the human body takes flavor very seriously. Our flavor-sensing equipment occupies more DNA than any other bodily system. If deliciousness is our enemy, why are we programmed to seek it out?

Every other animal depends on taste and smell to identify nutrients crucial to life. Insects use flavor chemicals to distinguish between food and poison. Diabetic lab rats instinctively avoid carbs. Sheep who are deficient in essential minerals, such as calcium or phosphorus, will crave flavors associated with them. And monkeys infected with gut parasites will eat specific leaves that alleviate their conditions. “Flavor,” says Fred Provenza, a behavioral ecologist and professor emeritus at Utah State University, “is the body’s way of identifying important nutrients and remembering what foods they come from.”

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