If David Ludwig Is Right, Everything We Thought We Knew About Obesity—and Low-Fat Diets—Is Wrong

But it’s not too late to change our eating habits.

In the late 1990s, David Ludwig brought 12 obese teenage boys into his clinic for the day, and fed them each prescribed meals. It was a simple experiment: The meals all had the same number of calories, but contained different kinds of food.

At the time, it was generally believed that a high-fat diet led to a high-fat body: You were what you ate. Americans had spent decades cutting down on the fat in their foods, in an effort to be leaner and more healthy. And yet studies consistently showed that this didn’t work—people on low-fat diets experienced high rates of hunger, and any weight they lost was soon regained. Ludwig wanted to know what was going wrong. Maybe it had to do with the kinds of calories we ate, not just how many.

Ludwig, who now treats obesity at Boston Children’s Hospital and is a professor of nutrition at the Harvard School of Public Health, fed his teenage subjects meals that all had different ratings on the glycemic index, which measures how rapidly sugar rises in the bloodstream after a meal. For example, instant oatmeal is high-GI: Highly processed and refined, it is quickly digested and dumps sugar into the blood in minutes. But old-fashioned oatmeal made with steel-cut oats, a lower-GI food, gets digested much more slowly, doling out sugar into the bloodstream little by little.

Over the next few hours, Ludwig and his team monitored how hungry the teenagers felt, and how much they consumed in snacks. Teenagers on the high-GI regimen became ravenously hungry and ate a lot of snacks after the meal—80 percent more calories in snacks that day than those who had the low-GI meals. “If just half of that calorie difference occurred day after day,” Ludwig says, “it could explain most of the obesity epidemic in the United States.”

Later, Ludwig followed 21 overweight young adults over a period of three months while feeding them three different diets: low fat with high carbs, low carbs with high fat, and a diet with an equal amount of fat and carbs.

In the end, the low-fat diet had a strongly negative effect: When subjects were on it, they burned 325 fewer calories per day. That is, their metabolism slowed dramatically compared with when they were on the low-carb diet, which did a much better job of burning calories rather than storing them. In effect, it was as if those on the low-carb, low-GI diet put in an extra hour of exercise every day without lifting a finger.

If what Ludwig was seeing was correct, it meant that everything we thought we knew about food was wrong. The body is not just a gas tank. A calorie is not just a calorie. As Ludwig’s colleague Mark Hyman, an author and physician at the Cleveland Clinic, says, “Food is not just food—it is information used by the body.”

It also meant that the past 40 years of food advice had been a terrible, costly mistake.

For decades, the advice about food and obesity from nutritionists and the government had a strong, simple message: Eat less fat. The food industry responded with a massive campaign of substitution to get the fat out of foods. “Fat free” and “lower fat” became almost mandatory marketing points. Hostess offered low-fat Twinkies, and Nabisco offered fat-free SnackWell’s devil’s food cookies. It worked: At the start of the 1960s, our diets were about 42 percent fat; now they are 33 percent fat.

But we didn’t get any healthier. In the early 1960s, 13 percent of adults were obese and only about one percent had type 2 diabetes. Now, 35 percent of adults are obese and 13 percent have type 2 diabetes.

“Despite eating less fat, we are fatter than ever before,” Ludwig says. What was going wrong? And how could it be fixed?

Throughout the history of the species, humans have been used to natural, largely unprocessed foods that take time to digest and deliver energy to the bloodstream. This is the natural pace of eating and energy: digestion over hours, not minutes.

But when we cut the fat out of our diet, a problem emerged: Without the fat, foods didn’t taste as good, so the industry replaced fat with refined carbohydrates. The result: highly processed, refined foods. For the past 20 years, Ludwig has been studying the effects of these new edibles, which he refers to as essentially “pre digested food.”

When these carbohydrate-rich foods rush-deliver sugars to the blood, the body reacts by producing large amounts of insulin.

And insulin is the signal that sends incoming energy off to be stored, rather than burned. “So much insulin is secreted when we eat these rapidly digested carbohydrates that it drives all our nutrients into storage in the few hours after a meal,” Ludwig says. But that leaves the body without anything to burn. It’s like depositing money in a bank but not leaving yourself any cash on hand.

“When calories are cut like that, the brain perceives starvation. It doesn’t register that there are plenty of calories still stored in fat cells. It just sees that there aren’t enough calories in the blood. It thinks that it’s a famine; there’s not enough food. It doesn’t matter how many calories are stored in your fat cells. If your blood sugar is crashing, your brain is at immediate risk.”

Read the full article here: http://www.bostonmagazine.com/

Hat tip to Linda for the link

Graham