Nutritionists and researchers generally agree that obesity is rising sharply in the United States, and that it’s a big problem. There’s also broad consensus about why it’s happening: the ever-increasing availability of industrially produced, ultra-processed foods. But there is a debate raging in the academic community about why, exactly, consuming these foods leads to excess bodyfat. One model of obesity emphasizes how much of these foods people tend to eat—how many calories—and the brain’s role in regulating that consumption. The other argues that there’s something particular about unhealthy food that causes the body to go haywire—specifically, that excessive consumption of carbohydrates breaks down the body’s ability to process sugar.
There are many theoretical models of obesity, as Stephan J. Guyenet, an obesity researcher and author of The Hungry Brain, told GQ. The model matters, because it informs our historical understanding of obesity, the biological mechanism at play, seeks to answer why it’s risen so sharply on a global scale, and directs the decisions of doctors, dietitians, and normal people about our food choices.
The first model is the energy balance model. EBM focuses on caloric intake, and attributes the recent growth in obesity to the changing food environment, which increasingly includes convenient, palatable, calorically dense foods, often in large portion sizes. Agnostic of diet, increased calories can result in excessive body fat, what researchers call adiposity. It is important to note that fat isn’t just a bunch of stuff—it’s actually an organ that releases leptin, a hormone responsible for appetite regulation—but in the model it can be thought of as a “passive absorber of energy,” as Guyenet puts it. When in a positive energy balance, body fat is stored, and when in a negative energy balance, body fat is removed.
“A calorie is a calorie” is often used interchangeably with EBM, though that’s not quite correct. It’s indisputable that different macronutrients (fat, carbs, and protein) have different metabolic effects, Guyenent said. But, if you carefully control for overall intake, it doesn’t matter if it’s carbs or fat when it comes down to adiposity—it’s the total number of calories that matter. Another point of confusion is “calories in, calories out,” Guyenet said. This is indisputable if taken to mean body fatness is determined by energy balance, he said. But it absolutely does not mean that calorie counting is the optimal way to regulate weight: “Those two are not the same thing.”
A newer model has gained traction, despite—or because—it contradicts conventions. This is the carbohydrate-insulin model. Instead of focusing on the brain’s role, CIM focuses on carbohydrates. In this model, it is carbohydrate-rich diets that are increasing insulin production, and in turn responsible for the overall rise in obesity.
CIM’s first major elaboration came from health journalist Gary Taubes, who presented the concept in his NYT bestseller Good Calories, Bad Calories in 2007. Guyenet recalls the book blowing his mind—until he became an obesity researcher. Then, he found the narratives to be “incredibly misleading,” though he still thinks that Taubes is a rhetorical genius. (The two debated on this topic on The Joe Rogan Experience in 2019.)
Even though he doesn’t buy into CIM, Guyenet is hesitant to fully toss it out: “I don't think that hypothesis is totally unsupported. I don't think it's coming out of thin air. I don't think it's illogical. And I'm not even necessarily that confident that it's completely wrong, either.”
Taubes found very vocal proponents of the CIM model, including Harvard professor and endocrinologist Dr. David Ludwig, who often debates the merits of CIM versus EBM, including with opponents like Guyenet. Momentum for CIM hit its biggest boost when Taubes, along with Dr. Peter Attia, founded the Nutrition Science Initiative (NuSi) in 2012, raising $40 million in a “Manhattan Project” effort that sought to question the energy balance model, as well as study carbohydrates’ role in obesity.
Attia resigned from NuSi in 2015, and NuSi officially dissolved in 2021, without any definitive support of CIM. One NuSi study meant to show the effects of the keto diet initially placed all participants on a “standard American diet” meant to put them in energy balance, to set a baseline for the study. However, all participants lost weight during this period, which Taubes attributed to the diet not having enough “refined sugary beverages” to “depict average American consumption,” according to Wired.
Attia has since said “experimental data would suggest the energy balance model is easier to explain,” in a June podcast featuring Guyenet. (“I have tons of respect for Peter for changing his mind about something that was so important to his life,” Guyenet told GQ.)
Astute readers may note, correctly, that these two models are not mutually exclusive, even though they are often portrayed as such. Spencer Nadolsky, obesity specialist physician and medical director for weight loss program Sequence, pointed out that a low-carb diet that also controls for overall calories will result in weight loss, but it’s just one tool in the toolbox. The issue with focusing on this one tool—cutting carbs—is that people “may feel like they’re broken” if they can’t adhere to a low-carb diet. The simple and false logic that carbs lead to weight gain is one Nadolsky encounters often. “But that’s absolutely not the case. We can actually reduce adiposity and insulin resistance while eating carbs,” if overall calories are reduced. Instead of limiting carbohydrate intake, Nadolsky works with an individual’s preferences and focuses on foods that are more satiating.
Many of Nadolsky’s patients have obesity, and come in with a predisposition to fear carbs, or a frustration that they’ve tried to lose weight by cutting them. Once they understand the concept of energy balance, he said, helps break the dichotomous thought of carbs equal body fat.
Most science-based clinicians don’t believe in CIM, Nadolsky said, “because if you've actually dealt with patients, you would see that many have lost weight, using all sorts of different methods that don't include going low carb.” Low carb diets can be difficult to stick to for many people, he said, and the bottom line is what matters is adherence.
Both Guyenet and Nadolsky see the success of GLP-1 agonist weight-loss drugs like semaglutide (marketed as Ozempic and Wegovy) as proof of the energy-balance model. Since semaglutide works in the brain to reduce food intake, while initially increasing insulin secretion, that appears to support the energy balance model, Guyenet said. (Attia, discussing semaglutide on his podcast, said the effects were “hard to reconcile” against the CIM model.)
As you can probably tell by now, “it can be really easy to get lost in these arguments,” Guyenet says. The takeaway for the average person is probably to avoid overthinking it and focus on eating “whole” foods that are more difficult to overeat. And as a bonus, they also will probably be naturally lower in refined carbs.
