Today’s episode is about what Americans don’t get about food—and the historical origins of our diet delusions. Our guest is Dr. David Ludwig, an endocrinologist who has researched and written on obesity and diet. He explains why scientists still haven’t arrived at a consensus on obesity, why he thinks the conventional wisdom about calories and fat is wrong, what he thinks is really going on, and why the history of diet advice has been so wrong in the last half century. On Friday, we’ll continue the conversation on diet and obesity with an episode on the next generation of weight-loss medication, which could change the way America thinks about self-image and obesity forever.
In the following excerpt, David Ludwig discusses the long-term trends of obesity in the United States.
Derek Thompson: There is so much that I want to talk to you about. I want to talk about the state of obesity. I want to talk to you about drivers of obesity. I want to talk to you about the perfect diet and the ways I think corporations and even the federal government have lied to us over the last 70 years about what comprises the perfect diet. But I want to start with the state of obesity. What is the state of obesity in America today? I know that it increased a lot in the last 40 years. Has it stabilized, or are things still getting worse?
David Ludwig: The state of obesity in the United States is unfortunately bleak. Body weights were remarkably stable in the United States from the end of World War II until the early 1970s. Beginning in the 1970s, something shifted with year after year of increasing body mass indexes, the measure of relative weight to height. Toward the end of the 20th century, it looked like maybe we were beginning to plateau, but that was a statistical fluke. We got too excited about it. We had run it up the flagpole, but it turned out that it was just a temporary pause and rates continued upward. And unfortunately that has extended, that has accelerated during the pandemic. So we’re exceeding four out of 10 people in the United States—more than four out of 10 people would have a weight that characterizes them in the category of obesity. And if you add in overweight to that, it’s more than seven out of 10.
Thompson: And why does obesity matter for health? I mean, there might be some people out there who say it’s irresponsible to assume that just because some people are overweight, they’re automatically unhealthy. What do we know or what do you think we’ve learned from science that speaks to the risk factors of being overweight or being obese?
Ludwig: On an individual basis, the number on a scale is just a number. And it doesn’t tell you a whole lot about health.
We know that somebody could, say, be 30 pounds overweight based on the tables and the charts. And if that is mostly lean because the person’s very physically active—you don’t have to be a bodybuilder, but you’re physically active and you’ve got good muscle mass, good lean mass—then those 30 pounds are not going to be unhealthy. But unfortunately, at the same time that the obesity epidemic has been evolving, we haven’t become marathon runners. And if anything, there’s evidence that lean body mass isn’t keeping up with the expected increase just to keep us even. So that excess fat in the body we know is a serious long-term risk factor for a range of chronic conditions.
Thompson: Let’s talk about why this is happening. I know that you attended a select meeting of top researchers throughout the world at the Royal Society in London to debate, to share ideas on what might be driving the rise of obesity. The New York Times reported on this discussion, that you held this paradigm clash and said that one of the conclusions is that we aren’t really sure what’s driving the rise in obesity. I’d love for you to do two things right now. I’d love for you to try to summarize what you consider the prevailing view on why obesity took off in the U.S. in the last half century. And then after that, I’m going to ask you to provide your own competing view to that paradigm. So first, help me understand what you understand to be the prevailing view. Why is this happening?
Ludwig: Yeah. The prevailing view is based on what has loosely been termed the energy balance model of obesity. It’s really not a scientific model. It’s a collection of thoughts about what affects our body weight. But the general thinking is that we eat too much. We don’t get enough physical activity. Ultimately, according to this view, the answer in one form or another is to eat less and move more. We’ve heard it a thousand times. Now, that exhortation can be couched in many different ways, and there’s certainly an attempt to destigmatize and to recognize that this is a societal problem. It’s not a weakness of character. But ultimately, [in this thinking] the responsibility devolves back to the individual, that we have to ultimately control our energy balance. We have to eat less, move more. During the late part of the 20th century, this notion gave rise to the low-fat diet, the whole low-fat diet craze, beginning in around the 1970s because fat has the most calories per bite. It has more than twice the calories per bite or per gram of protein or carbohydrate. So the thinking was if you want to reduce your calorie intake, cut back on the most energy-dense, calorie-rich foods—fats—and things should work out for themselves. Things didn’t work out so well. In fact, the rates of obesity began going up most rapidly just as we began cutting back on fats and replacing them with carbohydrates in the diet.
This excerpt was edited for clarity.
Host: Derek Thompson
Guest: David Ludwig
Producer: Devon Manze