Managing obesity isn’t just about losing weight—it’s also about keeping it off. That’s a difficult prospect for many, but not because of a lack of discipline. The body is biologically designed to resist sustained weight loss.

Obesity is a complex, chronic disease. Once viewed simply as a matter of willpower, researchers now know obesity is influenced by complex biological processes that regulate hunger, energy use, and body weight. One of the most important concepts behind this understanding is set point theory, which is the idea that the body works to maintain a certain weight range, even after weight loss.1

“Set point theory is this idea that the body has systems that are going to help regulate and defend a certain body weight range,” explains Dr. Danna Breen, Vice President and Head of Obesity Discovery at Pfizer. “When you lose weight, the body can respond by increasing hunger and reducing energy use.”

These biological responses help explain why maintaining weight loss can be so challenging. It’s also why treating obesity increasingly focuses on long-term weight health rather than short-term weight reduction.

Why your body may resist weight loss

When body weight decreases, several biological changes occur at the same time.

In many cases, hunger signals increase, metabolism slows, and the body becomes more energy efficient, meaning they require fewer calories to perform the same work.2

Throughout evolution, these changes helped humans survive periods of famine.1 But in today’s environment, they can make long-term weight loss feel like a never-ending challenge.

“This makes it really difficult to keep weight off over time,” Breen says. “It's one reason people think obesity should be understood as a chronic disease influenced by biology—not as simply a matter of willpower.”

Research suggests these biological adaptations can begin after relatively modest weight loss—around 5 to 10% of body weight, helping explain why weight regain is common regardless of how weight was initially lost.3

Solving the set point puzzle

Even though researchers have made enormous advances in obesity science, they still don’t fully understand what establishes an individual’s body weight set point. It’s also not clear how that set point changes over time.

Current obesity medications can modify biological pathways involved in appetite and metabolism while treatment continues. But clinical studies have shown that many patients regain weight if they discontinue treatment, although the degree of weight gain varies among individuals.1,2

Understanding exactly how the body sets and defends a certain weight remains one of obesity research’s biggest unanswered questions. It’s also one that could shape future therapies. But weight regain can happen regardless of whether medications are taken to help with weight loss.4

You can still regain weight even if you’ve lost it through old-fashioned diet and exercise,” Breen explains. “Weight regain is driven by biology that’s largely outside our control. Helping people understand the biology of obesity can free us from that label of failure.”

This perspective is changing how we think about obesity treatment. Rather than viewing regained weight as personal failure, doctors and researchers increasingly recognize it as part of the disease process itself.

The long-term nature of treatment

It’s a common misconception that a person only needs to use obesity therapies briefly to achieve a goal weight, rather than over the long-term. In that sense, current evidence suggests obesity is more like other chronic diseases than previously thought.

Breen compares obesity treatment to managing hypothyroidism: “If you stop your thyroid hormone replacement, your underlying [thyroid hormone] deficiency is still there,” she explains. “The treatment is managing the condition. It isn’t curing it.”

Likewise, many newer obesity medications continue working only while they’re being taken. Clinical studies indicate that discontinuing treatment frequently results in substantial weight regain, reflecting the return of the body’s biological drive toward its previous weight.5,6

But that doesn’t mean lifestyle changes aren’t important. Healthy eating and physical activity are still essential for people living with obesity, but achieving and maintaining weight loss often requires medication to address underlying biology as well.

Looking beyond the scale

The definition of success in obesity treatment is also changing. In the past, the number on the scale served as the primary measure of progress—it offered a simple, inexpensive way to estimate changes in body fat. Now researchers are thinking more holistically.

“I think the conversation in this next era of obesity treatment is more focused on overall health and body composition,” Breen says. “For the patient, success means feeling healthier and doing things that they enjoy.” Future measures of success may include:

  • Higher quality of life
  • Improved physical function
  • Better sleep

Scientists are also working to build on the progress they’ve made with today’s therapies. Instead of targeting a single biological pathway, many experimental medicines target multiple pathways that influence appetite, metabolism, body composition, and obesity-related conditions.7

Researchers hope these new combination therapies will help people lose weight while also maintaining healthy muscle mass and function, improving cardiovascular health, addressing problems such as liver disease, and reducing sleep apnea. Researchers are also exploring ways to help increase weight loss, preserve muscle mass, and improve cardiovascular health, among other benefits.3

Letting science lead the conversation

Obesity has long been plagued by stigma, misinformation, and unrealistic expectations. Now, scientific advances are steadily changing the conversation.  

Instead of asking why people “can’t keep the weight off,” researchers want to know how biology shapes long-term weight regulation. This could give insight into how future therapies might better support patients over an entire lifetime. For people living with obesity, that shift may be just as important as any new medication. 

“People just need to be kind to themselves,” Breen says. “It's not a one-and-done scenario, [even though many people are looking] for a quick fix. It’s been a century of wild ways to manage body weight with very little science behind it. So it’ll take a while to undo that and let science lead the conversation.”  

In other words, obesity management is a marathon, not a sprint. Continued scientific progress is helping researchers better understand the disease over time. When scientists better understand why we gain and lose weight in the way we do, including the mystery of the body’s set point, obesity care will become more personalized and rooted in a deeper understanding of how the body works.