The Oldest Diet Plan
Eat like your ancestors, author of Waistland advises
By now, we all know that most of America is fat. But Deirdre Barrett, a psychologist with Cambridge Health Alliance and a professor at Harvard Medical School, says the reasons for this—and how to change it—may come as a surprise. In Waistland, she lays out the science behind obesity and shatters some myths that she says are standing in the way of really shaping up.
One of many notions you dispel is that if we just listened to our bodies, we'd naturally crave healthy food. Why isn't that so?
It's counter to biology. That myth is persistent because it sounds good—why wouldn't we be wired with instincts to tell us what's good for us? The problem is we're wired for a much different environment—for a hunter/gatherer society. Before agriculture, essential nutrients like fat, salt, and sugar were rare. So we're wired to crave them. Since then, our environment has gone astray. We've got too much access to simple, refined carbohydrates. Our instincts aren't going to guide us properly unless we're on the savanna, away from fast food.
You also say that contrary to popular belief, our societal ideal of thinness is actually consistent over time—and for a good reason: It's healthiest.
All those E-mails claiming that Miss America has gotten skinnier over the years? It's not true. Most athletes and all but a handful of [too-thin] actresses and models look like what you'd see in a hunter/gatherer tribe. They're all at the very slim end of the recommended BMI [body mass index] range. That is absolutely the healthiest—if it's achieved by eating small servings and getting exercise. Focus on healthy habits, not absolute weight. It's not healthy to get thin by vomiting up meals or taking speed.
What about some studies showing that slightly chubby folks outlive skinny ones?
Those studies failed to control for problems that make people skinny, like smoking or undiagnosed cancer. When you take that out, the healthiest body weight is right at the low end of the recommended BMI range.
So how do we address being overweight? Don't we have to find the emotional reason behind why we binge?
There's an indirect interaction between having some kind of psychological problem and overeating. It's absolutely harder to lose weight if you're depressed or anxious or have post-traumatic stress syndrome. But you can change the habit before you fix emotional problems. In that way, overeating is like smoking.
You say that eating healthfully is largely a matter of habit.
We get into routines. Say you're in the cafeteria line thinking, "There's the ice cream; do I want to stick to my diet today?" If you have no particular routine, the prefrontal cortex, the area of the brain involved with complex thought, is active. If you're in a routine, whether a bad one or a good one, the basal ganglia—the older part of the brain that isn't associated with conscious thought—is active. The upper advanced brain can think about something else. So if you always order the salad and the bowl of berries, you don't even think about the banana split.
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