The Best Diet: Eat Like Our Ancestors
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 the obesity epidemic and shatters some of the myths that she says are standing in the way of really shaping up.
What gave you the idea for the book?
I work in a behavioral medical setting, so I'm very interested in what people need to do in a practical sense. The major impetus was that I didn't like what any of the other books out there were saying. There's so much bad advice, even in otherwise solid books.
And one of those pieces of bad advice is that if we just listened to our bodies, we'd naturally crave healthy food?
It's counter to biology. That myth is so persistent because it sounds so good—why wouldn't we be wired with instincts to tell us what's good for us? The problem is that we're wired for a much different environment—for a hunter/gatherer society. Our instincts aren't going to guide us unless we're on the savanna, away from fast food.
You say we're wired to crave nutrients that were "essential but rare," like fat, salt, and sugar. But those things aren't so rare anymore now that we grow and store things instead of hunting them down daily.
Our environment has gone astray. Agriculture is as old as 10,000 years, but even that is a fraction of all human evolution. It was a huge shift to growing things that simply have a lot of calories, in a few acres, that store well. That shifted us toward simple and even more refined carbohydrates.
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.
It's comforting to think that if your body isn't ideal now, it would have been in an earlier era. But that's not true. All those E-mails claiming that Miss America has gotten skinnier over the years? It's not true. It's been remarkably consistent, except for a small dip in the 1980s. What all but a handful of [too-thin] actresses and models, as well as female athletes, look like is what you'd see in the current hunter/gatherer tribe. They're all at the very slim end of the recommended BMI [body mass index] range. That is what is absolutely the healthiest, if you're achieving that by eating small servings of food and getting exercise. You should focus on healthy habits, not on the absolute weight. That means it's not healthy to achieve thinness by vomiting up meals or taking speed.
What about some studies showing that skinny folks don't live as long as those who are a bit chubbier?
When you really dig into that data, they failed to control for certain things that may specifically cause disease and 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, as long as you're achieving it in a healthy way.
Is BMI the best way to gauge your health?
It's not at all the best measurement, but some of the really best research studies use BMI as an outcome. Waist/hip ratio, waist/height ratio, waist measurements—those all correlate rather well with body fat percentage, which is what you really want to know. But that is very hard to measure accurately. As far as BMI goes, for most of us, it will correlate pretty well. If you're at the low end of the normal BMI range, you're not going to have a frightening amount of body fat.
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. I don't mean that those conditions won't aggravate the problem. But it's not the original, basic cause of most overeating. And, like smoking, you can change the habit before you fix emotional problems.
You say that eating healthfully is largely a matter of habit.
We get into routines. Brain imaging has shown that if you have no routine and are trying to change—say you're in the cafeteria line thinking, "There's the ice cream; do I want to stick to my diet today?"—the prefrontal cortex, the area of the brain involved with complex thought, is active. If you're in a real routine, whether it's a bad one or a good one, the lower basal ganglia—the older part of the brain that isn't associated with conscious thought—is active. The upper advanced brain can be thinking about something else. So if you always order the salad and the bowl of berries, you don't even think about the banana split.
It takes willpower to change those habits.
People like to say that willpower is either a myth—it doesn't exist—or that dieting has nothing to do with willpower. All willpower means is resolutely following through on decisions without getting derailed by short-term temptations. It's very straightforward. The most generous interpretation of why willpower gets bad press as a diet technique is that lots of people have so little of it. But it's a trainable skill.
If most of us could change our bad eating habits through willpower, why do you also offer up public policy prescriptions, such as banning trans fats or taxing unhealthy foods?
In the long term, society has to change. There is no one root of the problem. We've got to change the current food and nonexercise environment if you're talking about getting the population as a whole back to fitness. But most people today want to fix their own health somewhat faster than the rest of the world is changing. So in the longer term, and on a wider scale, I advocate things ranging from outlawing certain things—like advertising junk food to kids—to changing financial incentives so that, say, agricultural subsidies are shifted to healthier foods.
And if I want to start with myself, what should I eat?
We should try to eat as much like our ancestors as possible, with those proportions of protein and fiber. When you look at them, all the standard diets are essentially converging. If you read the last Atkins book, he talks about eating fish, not a ton of saturated fat. And [low-fat guru] Dean Ornish says you should be eating whole-grain carbohydrates, not simple carbs like white bread. Basically, it's fish and lean meats, lots of fruit and veggies, and lots of fiber. And exercise is essential.
Are you optimistic we'll change our ways?
The outlook is quite good for an individual who is very committed to change. In terms of larger society or the world, I'm optimistic in the long run and pessimistic in the short run. Things will get worse before they hit a turning point. At some point it will be clear that our habits are killing us, and that will finally lead to radical change. I'd just like to see that happen sooner rather than later. We waited so long on smoking.