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The traditional medical view on obesity, as summed up nearly a century ago: “All obese persons are alike in one fundamental respect—they literally overeat.” While this may be true in a technical sense, this is in reference to overeating calories, not food. Our primitive urge to overindulge is selective. People don’t tend to lust for lettuce. We have a natural inborn preference for sweet, starchy, fatty foods, because that’s where the calories are concentrated.

Think about hunting and gathering efficiency. We used to have to work hard for our food. Prehistorically, it doesn’t make sense to spend all day collecting types of food that, on average, don’t provide at least a day’s worth of calories. You would have been better off staying back at the cave. So, we evolved to crave foods with the biggest caloric bang for their buck.

If you were able to steadily forage a pound of food an hour, and it had 250 calories per pound, it might take you 10 hours just to break even on your calories for the day. But if you were gathering something with 500 calories a pound, you could be done in five hours, and spend the next five practicing your wall paintings. So, the greater the energy density—the more calories per pound—the more efficient the foraging. So, we developed an acute ability to discriminate foods based on calorie density and to instinctively desire the densest.

If you study the fruit and vegetable preferences of four-year-old children, their liking correlates with calorie density. They prefer bananas over berries; carrots over cucumbers. Well duh, isn’t that just a preference for sweetness? No, they also prefer potatoes over peaches, and green beans over melon, just like monkeys prefer avocados over bananas. We appear to have an inborn drive to maximize calories per mouthful.

All the foods the researchers tested naturally had less than 500 calories per pound (bananas topped the chart at about 400). Something funny happens when you start going over that. We lose our ability to differentiate. Over the natural range of calorie densities, we have an uncanny aptitude to pick out the subtle distinctions. However, once you start heading towards bacon, cheese, and chocolate territory, which can reach thousands of calories per pound, our perceptions become relatively numb to the differences. No wonder, since these foods were unknown to our prehistoric brains. It’s like the dodo bird failing to evolve a fear response because they had no natural predators (and we all know how that turned out––or sea turtle hatchlings crawling in the wrong direction towards artificial light, rather than the moon). It’s aberrant behavior explained by an evolutionary mismatch.

The food industry exploits our innate biological vulnerabilities by stripping crops down into almost pure calories—straight sugar, oil (which is pretty much pure fat), and white flour (which is mostly refined starch). First, they have to remove the fiber, because it effectively has zero calories. Run brown rice through a mill to make white, and you lose about two-thirds of the fiber. Turn whole wheat flour into white, and lose 75 percent. Or, you can run crops through animals (to make meat, dairy, and eggs), and remove 100 percent of the fiber. What you’re left with is CRAP (an acronym used by one of my favorite dieticians, Jeff Novick): Calorie-Rich And Processed foods.

Calories are condensed in the same way plants are turned into addictive drugs like opiates and cocaine: concentration, crystallization, distillation, and extraction. They even appear to activate the same reward pathways in the brain. Put people with “food addiction” in an MRI scanner and show them a picture of a chocolate milkshake, and the areas that light up in their brains are the same as when cocaine addicts are shown a video of crack smoking.

“Food” addiction is a misnomer. People don’t suffer out-of-control eating behaviors to food in general. We don’t tend to compulsively crave carrots. Milkshakes are packed with sugar and fat: two of the signals to our brain of calorie density. When people are asked to rate different foods in terms of cravings and loss of control, most incriminated was a load of CRAP—highly processed foods, like doughnuts, along with cheese and meat. Those least related to problematic eating behaviors? Fruits and vegetables. Calorie density may be the reason people don’t get up in the middle of the night and binge on broccoli.

Animals don’t tend to get fat when they are eating the foods they were designed to eat. There is a confirmed report of free-living primates becoming obese, but that was a troop of baboons who evidently stumbled across some dumpsters at a tourist lodge. The “garbage-feeding animals” weighed 50 percent more than their wild-feeding counterparts. Sadly, we can suffer the same mismatched fate, and become obese eating garbage too. For millions of years, before we learned how to hunt, our biology evolved largely on leaves, roots, fruits, and nuts. Maybe it would help if we went back to our roots and cut out the CRAP.

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