Kevin Hall’s pioneering research—on ultra-processed foods, weight loss, and more—has made headlines. Hall left the National Institutes of Health in April, charging that political appointees had censored his work. In a new book, Food Intelligence, Hall and co-author Julia Belluz turn nutrition science into a fascinating read. Here’s a glimpse into what it reveals. 

Kevin Hall is a former researcher at the National Institute of Diabetes and Digestive and Kidney Diseases and co-author of Food Intelligence: The Science of How Food Both Nourishes and Harms Us. He spoke with Nutrition Action’s Bonnie Liebman. 

What explains the US obesity epidemic? Is it our genes? 

KH: There’s a huge genetic component to body size. Heritability accounts for 40 to 70 percent of the variability in body size between people in a given environment. 

But genes did not change enough to give rise to the explosion of people with obesity in many westernized countries over the past half-century. 

Something else had to generate the weight gain in the most genetically susceptible people. 

What might that be? 

KH: We incentivized our food system to produce a calorie glut because of a longstanding fear that population growth would outpace our ability to grow enough food. 

As late as the 1980s, when obesity rates were starting to climb, we had biologists like Paul Ehrlich portending mass starvation in America. Fortunately, the science and policies of the Green Revolution averted this disaster by driving up the calories and protein we were producing. 

Far more than we needed? 

KH: Yes. If you look at the four major US food commodity crops—corn, soy, wheat, and rice—we produce 15,000 calories per person per day. 

How have the commodity crops changed our food supply?

KH: They’re not directly eaten by people. Rather, they’re exported or used to produce biofuels or feed animals, and some are used as cheap inputs for ultra-processed foods. 

When you walk around a supermarket, you have the illusion of diversity, but you’re mostly looking at commodity crops in various guises. 

The rows of soda and sweet drinks, the aisles stocked full of snacks and candy, the freezers brimming with ready-to-heat meals. Most are derived from corn, rice, wheat, or soy, mixed with artificial colors and flavors. 

Meanwhile, we tell people to eat more vegetables, but we don’t grow enough to feed them, and we don’t subsidize their production at the scale required to make them affordable.  

And our food system is harming the planet? 

KH: Yes. Almost half of the Earth’s habitable land is used to grow food. And nearly 80 percent of that is devoted to producing livestock, both for grazing and producing feed. 

Worldwide, agriculture is responsible for about 70 percent of our freshwater use, and it’s responsible for a quarter of greenhouse gas emissions. The vast quantities of fertilizer, pesticides, and herbicides used in agriculture have poisoned our land and waterways. 

What did your study on ultra-processed foods, or UPFs, find? 

KH: Our first study showed that a diet that was high in UPFs led people to overconsume calories—an extra 500 per day—compared to a diet with no UPFs that had the same amounts of nutrients like sodium, sugar, carbs, and fat. 

As a result, people gained weight on the UPF diet and lost weight on the diet without UPFs. 

So the question was: What are the main drivers of this effect? 

Which drivers were the most likely? 

KH: One was the calorie density—the calories per gram—of the meals on the plate, not counting the beverages. Second was the number of so-called hyperpalatable foods—that is, foods that were high in fat and sugar, or high in fat and salt, or high in carbs and salt. Our new study—which is still ongoing—is using four diets to test those two factors. 

What are the four test diets? 

KH: The first diet is minimally processed, so it has no UPFs, and it’s also low in both calorie density and hyperpalatable foods. The second diet is high in UPFs, with meals that are high in calorie density and hyperpalatable foods. 

The third diet is also high in UPFs, but it’s low in both calorie density and hyperpalatable foods. The fourth diet is high in UPFs and calorie density, but low in hyperpalatable foods. 

All four diets have the same amount of sodium, sugar, fat, carbs, fiber, and protein. 

What have you seen so far? 

KH: When we looked at the first half of the participants, people consumed about 1,000 calories a day more on the UPF diet that was high in calorie density and hyperpalatability compared to the minimally processed diet. And they gained about two pounds in a week. 

However, people didn’t overconsume calories on the UPF diet that didn’t have a high calorie density or as many hyperpalatable foods. 

So the additives in UPFs may not matter? 

KH: We didn’t test that directly, but it doesn’t seem that additives are very important for overconsuming calories. The UPF diet that was low in both calorie density and hyperpalatable foods still had many food dyes, preservatives, emulsifiers, and other additives. Yet the diet didn’t lead to overconsumption.

And the fourth diet? 

KH: On the UPF diet with a high calorie density but few hyperpalatable foods, people still overconsumed calories, though perhaps not as much as when the diet had hyperpalatable foods. 

What about the new British study that you co-authored? 

KH: It provided 55 people with either ultra-processed or minimally processed foods to eat at home for eight weeks each. Both diets met British guidelines for eating healthy foods like vegetables, fruits, whole grains, and low-fat dairy. 

The participants lost about two pounds on average after the ultra-processed diet. But they lost about four pounds on average after the minimally processed diet, which was lower in calorie density. So even if an ultra-processed diet meets nutritional guidelines, people lose more weight eating a minimally processed diet. 

What myths about weight loss have you debunked? 

KH: When I first arrived at NIH, the 3,500-calorie rule was popular with dietitians, the American Society for Nutrition, and the government. 

The idea was that a pound of body fat stores 3,500 calories. So if I cut 500 calories a day from my diet, I’d lose a pound of fat tissue after a week. 

And if I did that for a year, I’d lose about 50 pounds. And if I then went back to eating what I was eating before, I’d maintain that weight loss because the fat would be gone from my body.

But it’s not so simple? 

KH: No. When you lose weight, you don’t just lose body fat, like the 3,500-calories-per-pound rule suggested. You also lose fat-free mass, some of which is muscle. 

More importantly, as you lose weight, the number of calories your body burns—your metabolic rate—decreases and your appetite grows. 

Smaller bodies require less energy to move around than larger bodies. We’ve estimated how those changes play out over time. 

Does your Body Weight Planner take that into account? 

KH: Only partially. It tells you the calories and activity needed to reach your goal weight and maintain it over time. You can find it at niddk.nih.gov/bwp. But it doesn’t include the appetite changes that tend to gradually drive intake higher as weight is lost. 

What did you learn testing low-fat versus low-carb diets? 

KH: While people can find success using both dietary approaches, the differences in body-fat loss are minuscule once you account for the calories consumed. We need to move beyond the idea that some ratio of carbs and fat can magically give rise to a lot of body-fat loss. 

And what did you find by studying the contestants on ‘The Biggest Loser’ TV show? 

KH: This was a controversial show where the contestant who lost the greatest percentage of their original body weight after 30 weeks went home with a $250,000 cash prize. 

The participants drastically cut their calorie intake, by about 65 percent. And they burned about 4,500 calories a day doing three or more hours of vigorous exercise on the ranch. 

After 13 weeks, the contestants went home, where they burned around 3,000 calories a day. By 30 weeks, they had lost, on average, 130 pounds. 

What had the largest impact on weight loss? 

KH: It wasn’t just exercise, as implied on TV. Those who cut the most calories lost the most weight. 

Did their metabolic rate change? 

KH: By the show’s finale, their metabolic rates had dropped spectacularly. They were burning several hundred fewer calories per day. 

That contradicted the idea—in countless fitness books and magazines—that working out to build and maintain muscle “boosts” metabolism during weight loss. 

What could explain that? 

KH: We’re not sure. Maybe when you have physical activity demands and not enough calories coming in, the body turns down some functions, like a cell phone that goes into power-saving mode when the battery is running low. 

Did the contestants’ metabolic rates return to normal? 

KH: No. Six years later, when they had regained, on average, about two-thirds of the weight that they’d lost, their metabolic rates hadn’t changed. 

But the people who were the most active sustained the most weight loss, even though they’d also had the greatest slowing of metabolism. 

People think a higher metabolic rate is always better. In fact, both during the weight-loss competition and six years later, the Biggest Losers who had the greatest slowing of metabolism had the most weight loss.