Shocking Findings From ‘The Biggest Loser’ Contestants

The following is an excerpt from Food Intelligence: The Science of How Food Both Nourishes and Harms Us by Julia Belluz and Kevin Hall.

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Food Intelligence: The Science of How Food Both Nourishes and Harms Us

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It was an unusual laboratory for a metabolism and weight loss study: a sprawling ranch, the former home of the Gillette razor-blade tycoon, nestled in the dusty green Santa Monica Mountains near Malibu, California. A group of people milled about a hallway in the ranch’s white, Spanish Colonial-style mansion early one morning before breakfast—­ and they were not exactly chipper.

Six weeks into season eight of the reality TV program The Biggest Loser, the contestants were tired. Their bodies ached. Days on the ranch had been spent mainly in the gym and what amounted to a sports rehab facility, where health professionals attended to blisters, stress fractures, and sore muscles from working out, sometimes plunging the contestants into a plus-­ size ice bath designed for racehorses. Despite all that exercise, they’d also drastically cut their calorie intake. The contestants were motivated to carry on anyway because of the strange, weight loss Olympics premise of the show they were starring in. Whoever lost the greatest percentage of their original body weight after thirty weeks, the “biggest loser,” went home with a $250,000 cash prize.

In addition to all those hours training and recovering, and stints in front of the cameras, the sixteen men and women had agreed to submit to the demands of the scientists. That day, starting around five a.m., Kevin and his colleagues welcomed the contestants, one by one, into a dimly lit room in the mansion—­ a makeshift metabolism laboratory. For forty-­ five minutes, each Biggest Loser took a spin in a “metabolic cart.” The person would lie down on a bed, and the cart’s domelike bubble would be placed over their head to read their resting metabolic rate—­ or how many calories their bodies burned while they were doing next to nothing.

When the day’s data gathering was done, the contestants went back to working out. The show was an extreme televised version of a “fat camp,” and exercising was pretty much all they were allowed to do. Their phones had been taken by the show’s producers and they had no access to TV or the internet. Kevin, meanwhile, walked around the ranch, touring the Biggest Loser gym, which was filled with state-­of-­the-­art exercise equipment and banners featuring motivational quotes from the show’s celebrity trainers.

The conference room, where contestants deliberated over who would be sent home each week, was run-­ down. There was no personal chef working behind the scenes, just a simple kitchen where the show’s participants made their own small meals, often using food from Biggest Loser partners such as Jennie-­O low-­fat turkey. Everything looked a lot less glitzy than it did on TV, Kevin thought.

Mostly, Kevin felt a little out of place. Not only was this a strange research setting, but the study was among the first in humans he’d ever run. The higher-­ups at his agency, where Kevin had his own lab, didn’t want the reputation of their prestigious institution tarnished by association with a reality TV weight loss contest. But he’d found a way to make the research happen anyway because he was sure of one thing: Peering inside the contestants’ bodies as they crash-­dieted and exercised their way to thinness would help unlock mysteries of metabolism and obesity.

Weight loss of the magnitude the contestants were experiencing rarely happened outside of bariatric surgery. Trying to induce it for the purposes of a study today would never get the approval of a research ethics board. The bodies of the Biggest Losers, unbeknownst to them, were a “natural” experiment, brimming with original data, and the scientists were capturing it all—­before, during, and long after the competition was over.

In the end, the show contestants delivered new insights, as Kevin expected. What he didn’t anticipate: The findings would run entirely contrary to the way many of us think about metabolism and weight loss. The discoveries even challenged the most long-standing and persistent assumption of all: that the speed of one’s metabolism determines their body size, and a faster metabolism means a thinner body.

If you read just about any lifestyle magazine or newsletter, or follow the shenanigans of self-­ proclaimed wellness experts on social media, you might believe metabolism has something to do with how much you can eat without gaining weight, a knob inside the body that can be finely tuned with dedicated “metabolism boosters.” Some people are blessed with a fast metabolism. The rest of us can try chili peppers, cold exposure, green tea extracts and other supplements, or muscle-­building exercise routines.

The implicit message in all the products and protocols is that you can do certain things to speed up your metabolic rate, or calorie burn—­ and that the increased speed is desirable because it helps with weight loss. On the flip side, the influencers tell us, weight loss can cause the metabolic rate to decline perilously, making it even more difficult to keep fat off.

This metabolism boosting subsection of the weight loss industrial complex has a surprisingly rich intellectual history. One of the most effective weight loss drugs ever invented sped up metabolism. Factory workers in France during World War I used a chemical called 2,4-­ dinitrophenol, or DNP, to manufacture explosives, when they started to report fever, sweating, nausea, and vomiting—­as well as rapid, unintentional weight loss.

When researchers at Stanford University learned about the chemical’s slimming effects, they saw an opportunity. DNP could be used as a “metabolic stimulant” for weight loss. After testing in animals, then in humans with obesity, the scientists boasted about the drug’s “power to increase metabolism to very high levels” and promoted DNP as a safe and effective tool for treating obesity. Within a year after they shared their initial findings, in 1934, at least 100,000 Americans had reportedly taken DNP. One brand’s packaging featured testimonials about “literally burning the fat away,” also warning consumers “DO NOT BECOME ALARMED” if rashes, eye and skin discoloration, or burning developed. That the drug originated with toxic side effects during explosives manufacturing didn’t seem to deter consumers; such is the desire for fast fat loss.

By World War II, a foundational study in nutrition science bolstered the related idea that a metabolic slowdown—­ meaning a person burns calories more slowly than expected for their body size—­kicked in with weight loss. The legendary American physiologist Ancel Keys gathered thirty-­ six lean and healthy young men at the University of Minnesota and slashed their dietary calories in half for six months. The study’s participants—­ all conscientious objectors eager to serve their country—­ had volunteered to go hungry with a compassionate goal: figuring out how best to rehabilitate famine survivors and malnourished people after the war. Over twenty-four weeks, the men wandered the university campus and town, often sitting in restaurants watching other people eat, while mostly abstaining from food themselves. As they withered away, dropping a quarter of their body weight, they grew tired, feeble, cranky, and cold. They also saw their metabolic rate cut by almost 40 percent.

The findings of the “Minnesota starvation experiment” built on fasting research, which already demonstrated that as we cut calories, the body senses the new low energy state and, much like a smartphone or laptop running short on battery, goes into power-­saving mode. The decrease in calories burned was often greater than what could be accounted for by a person’s new, smaller body size. (Smaller bodies, contrary to popular wisdom, generally have lower metabolic rates than larger bodies.)

Putting these findings together with the DNP example suggested metabolism was important not only to body weight; it could accelerate weight loss if properly manipulated.

It didn’t take long before cases of blindness caused by “dinitrophenol cataracts’’ and deaths linked to DNP turned up in humans. By 1938, the drug attracted the scrutiny of newly empowered federal food and drug regulators in the United States and was banned after being deemed “extremely dangerous and not fit for human consumption.” Using a drug to speed up one’s metabolism suddenly seemed like a very bad idea.

Yet the link between metabolism and body fat had been established in the marketplace of weight loss ideas. Products continue to trickle out to this day (including illegal DNP-­laced supplements, which are still sold for weight loss and bodybuilding over the internet; as of 2011, over sixty related deaths were reported in the medical literature). In addition to the metabolism-­boosting supplements to buy, health and fitness influencers promote exercise and muscle building as natural metabolism enhancers during weight loss—­claims that Kevin often wondered about.

Kevin had studied the details of the Minnesota experiment. The brains of the conscientious objectors focused almost exclusively on food. Eating was all they could think about. If they couldn’t eat, they wanted to be around or see pictures of food. “I was one of the many,” one participant later recalled, “that mentally was transfixed on cookbooks. And I collected probably a hundred cookbooks . . . I would read cookbooks like you would read Reader’s Digest.” As they ate less and lost weight, they became so weak that they could barely muster the strength to open a door, let alone work out.

Something quite different happened on every TV episode of The Biggest Loser. For two years before Kevin’s study trip to the ranch, the show had become a guilty obsession but not because of its absurd spectacle. Kevin’s research focused on figuring out how what people eat and how much they exercise affect metabolism and body weight. He had studied these phenomena using data from hundreds of patients, collected by other researchers over many decades. Never, ever had he seen people lose weight at anything close to the rate of the Biggest Losers—­ not even during the Minnesota study.

The contestants screeched, cried, vomited, and panted their way through grueling workouts in front of toned and tanned celebrity trainers. One was hospitalized with rhabdomyolysis, a sometimes fatal condition caused by overexertion. Others starved and dehydrated by spending lots of time in the sauna and subsisting only on low-­ calorie foods—­ vegetables and sugar-­ free Jell-­O. In one case, a contestant reported that he’d been urinating blood, a potential sign of kidney damage. The people on the show seemed to be eating far less, and moving far more, than the Keys research participants. How was this possible?

From FOOD INTELLIGENCE: The Science of How Food Both Nourishes and Harms Us by Julia Belluz and Kevin Hall, PhD, published by Avery, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2025 Julia Belluz and Kevin Hall