Research shows the battle to keep weight off isn’t about willpower—it’s biology. Here’s how scientists say we can work with our bodies, not against them.

Our bodies evolved to survive famine, not dieting. Researchers are decoding how metabolism, hunger, and hormones conspire to make keeping weight off so difficult.

Big Cheese Photo, Getty Images

ByAshwin Rodrigues

November 5, 2025

Losing weight is hard. More than half of Americans want to do so, but keeping it off is even harder. Research shows that 80 to 95 percent of people who lose weight regain it within three to five years, according to data from the National Institutes of Health and the Obesity Society.

Scientists say that’s not a failure of willpower. Hormones, genetics, and even evolution push the body to fight back—driving hunger up, slowing metabolism, and urging the pounds to return. It’s a biological tug-of-war that popular culture rarely acknowledges, says Kimberly Gudzune, medical director for the American Board of Obesity Medicine.

“There’s this presumption that we have that once you’re there, it just magically stays there. But unfortunately, that is really not the case,” she says.

For decades, dieting has been framed as a battle of discipline. Researchers are now beginning to understand why the body resists weight loss so fiercely—and how hormones, brain chemistry, and even early life experiences set the stage for lifelong weight regulation.

What reality TV taught scientists about weight loss

Pop culture has long glorified dramatic weight loss, rarely showing what comes after. Few examples made that clearer than The Biggest Loser, a reality series that aired from 2002 to 2016. Contestants were rewarded for shedding as much bodyweight as possible, supported by teams of trainers, nutritionists, and medical staff—but for scientists, it also revealed how the body resists transformation.

A 2016 study published in Obesity focused on 14 contestants—all of whom had class III obesity, previously known as morbid obesity—and compared their body composition and resting metabolic rate from the time they finished the competition to six years later.

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The study found that the contestants regained a significant amount of the weight they’d lost during The Biggest Loser. Even those contestants who’d experienced long-term weight loss also had a slowed metabolic rate. This metabolic slowdown makes maintaining weight far more challenging. “As you lose weight, your body doesn’t burn as many calories,” Gudzune says, meaning that someone who lost weight must eat fewer calories than someone of the same size who never did to maintain that weight.

That difference isn’t just metabolic—it’s hormonal. After weight loss achieved through diet alone, the hormone ghrelin (which signals hunger) increases, as peptide YY and leptin (which signal satiety) decrease. Even one year after losing weight, these hormonal changes have been observed.

These are just a few of the many “metabolic adaptations” that our bodies use to “fight back” when we lose weight, says Andres Acosta, a gastroenterologist and obesity researcher at Mayo Clinic. Treatments such as bariatric surgery and GLP-1 medications can help offset those changes by improving communication between the brain and gut regarding hunger and satiety. However, some patients are apprehensive about using these methods.

The science behind your body’s “set point”

One idea that helps explain these biological defenses is the set-point theory of obesity. It suggests that the body has many mechanisms to maintain a weight set early in life, and that most people have several different set points over the course of their lives.

(Fat cell number is set in childhood and stays constant in adulthood)

This theory is often discussed but is not unanimously accepted by obesity experts. Gudzune finds that it can oversimplify several processes, but might help people understand why maintenance can be so difficult. There are mechanisms with stronger evidence, Gudzune says, such as hormonal shifts and changes in energy expenditure, both of which support the concept of set-point theory, though they also stand on their own.

Why stigma makes obesity harder to treat

Weight stigma doesn’t just shape how people are treated—it can directly affect their health.

“There is this perception about what ‘normal’ weight folks do to achieve their weight stability,” says Andrew Kraftson, director of the Weight Navigation Program and the Post Bariatric Endocrinology Clinic at the University of Michigan Health.

People who are struggling with weight might imagine that smaller-bodied individuals weigh their food and track every calorie they consume. But “that’s totally not true,” says Kraftson. Comparing the two, Kraftson adds, is “fallacious thinking”—their experiences aren’t the same.

Making such comparisons can lead to internalized shame—and notions that effective treatments, including bariatric surgery or long-term medications like GLP-1s, are cheating, or an easy way out.

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The treatment for obesity has changed radically in a short period of time, Gudzune says. When she first started practicing obesity medicine in 2010, Gudzune would regularly see patients who did not tell their family or friends they were seeking help.

“The shame and that stigma around obesity was so strong that they didn’t want to disclose to anyone that they were seeking treatment,” Gudzune says.

Social media has made these tensions more visible. While some platforms have helped normalize conversations about obesity treatment, others amplify unrealistic body ideals and anti-fat bias.

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A recent Reuters-led investigation found Instagram regularly promoted “eating disorder adjacent” content to vulnerable teens. But Gudzune also acknowledges that some people on social media have also normalized treating obesity. “It’s still a hotly contested issue,” she admits, but patients are much more open to sharing their willingness to seek out professional help than they used to be.

Judgment about willpower remains a significant barrier, both socially and in healthcare. But people with obesity can also experience “internalized weight bias,” Gudzune says, which is a type of self-shame and stigmatization linked to several poor health outcomes, including body image issues, depression, and disordered eating.

Gudzune has also heard countless stories of patients going to see a doctor for an issue like a sore throat, only to be told that they need to lose weight. “These are things that people’s doctors and other health professionals have told them,” she says, “which doesn’t inspire anybody to want to seek out healthcare and treatment if you’re constantly berated.”

Awareness of weight bias in healthcare is growing, Gudzune says—and she hopes that makes seeking treatment more approachable.

How doctors are personalizing obesity treatment

Before Acosta talks to his patients about weight maintenance, he explains that the weight-loss journey—if they choose to embark on it—will involve metabolic adaptations. As their physician and obesity expert, Acosta’s role is to walk with them on the journey and help them counteract the specific challenges they face along the way.

People respond differently to weight loss, Acosta says, which is why he developed a genetic test to help identify the unique factors most prevalent in a patient with obesity who is struggling to lose weight, or that might prevent them from losing weight in the future. The four groups are classified as “hungry brain,” “hungry gut,” “emotional hunger,” and “slow burn.”

The “hungry brain” group requires more calories to feel full, “hungry gut” includes people who feel hungry shortly after eating, “emotional hunger” refers to a drive to eat as a way to cope with emotions (both positive and negative), and “slow burn” refers to those with a decreased metabolic rate.

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By addressing a patient’s specific phenotype, Acosta says, patients can receive personalized treatment and medication that target their underlying disease of obesity, leading to long-term success not only in weight loss but also in weight maintenance.

Engaging with a healthcare professional can help identify which interventions or medications best match a person’s goals and preferences. There is still an “underlying perception,” Gudzune says, that obesity shouldn’t necessarily be treated by a healthcare professional, though she’s working to change that.

“We don’t ask people who have high blood pressure, hypertension or diabetes, ‘Go figure it out yourself,’” Gudzune says.

The future of weight loss care

The broader discussion, Kraftson says, is not about a number on the scale. It’s about health. Even stating your goals can be clarifying, as some might be self-derived, while others might’ve been subconsciously imposed on you.

Ideally, the proper structure for helping a person with their weight is through comprehensive care, Kraftson says. Doctors don’t always have the time and expertise to address a patient’s needs—and they would benefit from a team that includes dieticians and mental health professionals. “It’s not just about a prescription,” he says. Obesity is a complex, chronic condition without one simple explanation. Biology, environment, and mental health all play a role.

“Our whole built environment is counter to our health,” Kraftson says, “it would take significant investment to change that.”