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.


David Ludwig is rethinking how we eat. / Photograph by Tony Luong

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.”

A few hours after eating a high-GI meal, the rush of sugar passes, and then the levels of sugar and other fuels, especially fatty acids, in the blood crash. The low level of calories in the blood now causes hunger and sends out stress hormones.

“The body [perceives] this as a crisis…a metabolic crisis,” Ludwig says. “So it’s not what happens at the meal or an hour or two later. It’s what happens four hours later. That’s the time when you either snack or not, or if you’re sitting down to your next meal [you choose] normal-sized or supersized [portions] based on your hunger.”

This is a problem for dieters: When people cut back calories and start to lose weight, “The body begins to fight back,” Ludwig says. And the first defense is to raise the level of hunger. “We can ignore it for a few hours, or a few days. But imagine feeling desperately hungry day after day without relief. And the longer you continue on the diet, the more severe the hunger becomes.”

Ludwig’s research also undermines a lot of our prejudices against those who struggle with their weight. “We as a culture seem to believe that people with this particular medical problem have a more fundamental character defect than people with almost any other medical problem,” he says. “We assume they just lack the willpower to do what they know is right for them. We blame them. It is patently false.”

Instead, Ludwig says, the American diet “puts our hormones and blood sugar through a roller-coaster ride—meal after meal, day after day.… If these theories are right, this explains a substantial amount of weight gain observed over the last 40 years.”


When Ludwig gets peckish after lunch, he turns to the snacks stashed around his office. He opens the top drawer of his desk to show me: “I usually have a bunch of nuts here, and…” There, in a red wrapper, is a big bar of dark chocolate. “I recommend a minimum of 70 percent [cocoa],” he says. High-fat foods. And the jar next to the desk, with a spoon beside it?

Raw coconut butter. Loads of fat. Does he spread it on something? “No, just eat a couple of spoonfuls in the middle of the afternoon.”

As much as he can, Ludwig and his wife, Dawn, a master chef, eat according to his new theory of food—they favor slow-digesting foods without a lot of processed carbohydrates. Their seven-year-old son helps make dessert: dark chocolate with a dollop of peanut butter on it. They serve tea with heavy cream only, not milk.

Together, the Ludwigs are trying to bring this new science to the mainstream, to change the way Americans eat. They’ve recruited 235 people—some at Boston Children’s and some across the U.S.—to go through a 16-week training course in their new concept of healthy eating. Dawn, who has trained chefs and home cooks for years, worked with David to put together a manual and a step-by-step program, the object of which is partly to help subjects lose weight, and, more important, to establish sustainable habits of cooking and eating.

“This is a lifestyle, not a diet,” she says. The people in the program don’t count calories, but instead learn to choose foods that will make them feel satisfied and not hungry, while eliminating a lot of highly refined foods. Their program forms the basis of Ludwig’s new book, Always Hungry? Conquer Cravings, Retrain Your Fat Cells & Lose Weight Permanently. The book is due to be released on January 5.

At the same time, Ludwig and his colleagues have embarked on one of the largest and longest feeding studies ever attempted—following 150 students and faculty at Framingham State University, for an entire academic year, on controlled diets. The entire group will first lose 10 percent or more of their weight on a start-in diet, then be switched for the rest of the year to one of three diets: 20 percent fat with 60 percent carbs; 40 percent fat with 40 percent carbs; or 60 percent fat with 20 percent carbs.

At Framingham State, the study has its own kitchen setup and is careful to offer appealing dishes that are similar for each group. For example, a Mexican meal will have similar flavors but with different proportions or kinds of foods.

They’re looking to provide more evidence that what you eat is as important, if not more, as how much you eat. And there’s evidence that Ludwig’s work is having an effect.

This year, for the first time in more than three decades, the federal Dietary Guidelines Advisory Committee, a group of scientists who set government nutrition policies, removed their limit on fat, and they no longer are recommending low-fat foods to treat obesity. It is a revolutionary shift toward worrying more about processed carbs than fat.

“The limit on total fat is an outdated concept, an obstacle to sensible change that promotes harmful low-fat foods, undermines efforts to limit refined grains and added sugars, and discourages the food industry from developing products higher in healthy fats,” Ludwig and his colleague Dariush Mozaffarian wrote in the New York Times after the committee’s recommendations were published. “Fortunately, the people behind the Dietary Guidelines understand that. Will the government, policy makers and the food industry take notice this time?”

If they do, it will largely be because of the efforts of Ludwig and his colleagues.

“The calorie-balance approach to weight control, enshrined in the low-fat diet, has proven utterly ineffective,” Ludwig says. “But with a focus on food quality rather than calories, we can put biology back on our side to lose weight without the struggle.”