About the Expert:
Dr. Rachel Batterham
What contributes to weight gain?
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RACHEL BATTERHAM: The systems that regulate body weight developed in a very different environment. So throughout the course of human evolution, having enough food has been a problem. So all the systems that drive us to eat are set up to make us eat when food is available, because that's how we survived. So when we are surrounded by excess food, food cues, energy, and dense food, then the majority of people will gain weight.
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Weight gain rarely has a single cause. Research shows it results from the interplay of four broad categories:
- Biological factors include genetics, epigenetics, and hormonal influences on metabolism. Some people are simply predisposed at a genetic level to store energy more efficiently — a trait shaped by hundreds of thousands of years of human evolution.
- Psychological factors are often underappreciated. Weight bias and stigma, mental health challenges, and chronic stress all play a meaningful role. Cortisol, the body's primary stress hormone, actively encourages fat storage, particularly around the abdomen.
- Lifestyle and behavioral factors such as physical activity level and sleep quality also matter. Poor sleep disrupts the hormones that regulate hunger and fullness, leaving you feeling hungrier and less satisfied after meals.
- Environmental and social factors round out the picture. Where you live, your access to healthy food, your socioeconomic status, your support network, and even your access to treatment all shape your weight — often in ways entirely outside your personal control.1,2
How does our evolution work against us?
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RACHEL BATTERHAM: There's also something called a set point, which is this idea that a person's body weight tends to be within a certain range, and if they lose weight, then the body tries to go back up to that set weight. Part of that is because when a person goes on a diet, that's the modern-day equivalent of a famine, and we've developed really protective mechanisms to stop us dying out during a famine. So when a person restricts how much they eat, the body compensates and drives the feeling of hunger and tries to do everything to stop them losing weight and to go back up to the highest weight they've ever been.
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Our bodies were built for a world that no longer exists. Homo sapiens have been around for roughly 300,000 years — and for most of that time, food was scarce and unpredictable. To survive, our ancestors evolved genes that encouraged large food intake when it was available, and genes that promoted low activity and energy conservation when it wasn’t.3,4
The problem? Those same survival genes are still running the show today. We now live in an environment of food abundance, but our biology is still protecting against famine. This means our bodies are wired to defend much more strongly against weight loss than against weight gain — making it harder to tip the scale in your favor.3,4,5
How can your body work against you?
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RACHEL BATTERHAM: So our modern diets are based on really energy dense, but nutrient poor foods, for example, ultra processed junk food. And these foods also contain many chemicals that have an adverse effect on the body's appetite regulating systems and also on the microbiome. So the bugs within our GI tract have been changed by these foods, and this changes how the signals that control appetite signal to the brain and predispose people to put on weight.
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Even with the best intentions, your own physiology can become an obstacle. Over time, the biology of a person living with obesity actually changes in ways that predispose them to further weight gain — creating a self-reinforcing cycle that has nothing to do with willpower.6
Here's how it happens:
- Altered gut-brain communication: Signals from the GI tract to the brain that normally regulate appetite and fullness become disrupted, making it harder to recognize when you've had enough.
- Impaired fat-storage signals: The signals that fat cells send to the brain to indicate "we have enough stored energy" become less effective, leaving the brain in a state of perceived scarcity.
- Changes in brain structure: Prolonged obesity is associated with structural changes in areas of the brain involved in self-regulation and reward.
- Heightened response to food cues: The brain becomes more reactive to the sight, smell, and thought of food — amplifying cravings and making it harder to resist eating in response to environmental triggers.
Together, these changes drive increases in both energy intake and fat tissue — not because of a lack of discipline, but because of genuine neurological and hormonal shifts.6
Why is it so hard to keep the weight off?
This is where biology can feel especially unfair. When you diet, your body interprets the calorie reduction as famine — and responds with a powerful set of changes designed to restore your previous weight.7,8
- Ghrelin rises: The appetite-stimulating hormone increases, making you feel genuinely hungrier than before you started dieting.7
- Satiety hormones fall: Hormones that signal fullness and satisfaction decrease, meaning it takes more food to feel the same level of satisfaction.7
- Metabolism slows: Your body reduces energy expenditure to conserve resources, making it easier to gain and harder to lose.7,8
- Food becomes more compelling: The brain's response to food cues — in both the reward and hunger-regulation regions — actually increases after weight loss, making food feel more appealing, not less.7
The result is that long after the diet ends, your body is still fighting to regain the weight. Hunger is up. Metabolism is down. Interest in food is heightened. This isn't a personal failing — it's a well-documented biological response.7,8
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[Overhead drone footage shows a man and woman hiking in the woods with their dog. Video alternates between close-ups of the couple and overhead drone footage.]
SUZANNE POTTS: My husband Josh and I are pretty active people. We're pretty avid hikers. We also have a puppy who likes to keep us active. He is a bundle of energy. And so every day we've got to be able to get out and take him to do things and run him around a little bit.
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[A woman opening a door to a coffee shop called Stouthaus Coffee. Inside the coffee shop, the woman is working on her laptop at a table.]
SUZANNE POTTS: I feel like I'm getting better as I'm getting older. I have more opportunities now.
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[Woman in a blue shirt sitting on a tan couch speaking to camera.]
[Title: Suzanne Potts, Clinical Trial Participant.]
SUZANNE POTTS: So having a clinical trial experience that was positive, encouraging, compassionate, it made me want to tell people about it. It made me want to advocate for others who might be really struggling in this space.
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SUZANNE POTTS: I was never a person who struggled with my weight growing up. I was active. I played soccer.
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[Photos of a mother holding a baby; a mom and dad with their son; a couple smiling; a woman exercising; a mom on vacation with her two sons.]
SUZANNE POTTS: Having kids, and then as time went on, I had a hysterectomy, which really changed my hormonal makeup. And so it just became frustrating. I tried anything you can do to try to manage your weight. And nothing was working, and it just like kept ticking up. It's embarrassing, and you know, as a mom, you don't want to be in pictures with your kids, and that just wasn't who I am or who I was.
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SUZANNE POTTS: For me, at that point, I was not hesitant to start the clinical trial because I was desperate.
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SUZANNE POTTS: The risk of either having a placebo paired with really great supports through the doctor's office or taking something that might actually help my health was really worth the risk.
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And I met with the first researcher who did weight and blood work and circumference and things like that. And she explained to me that it was this 7-to-8-month research project where I'd be meeting with them, yes, to talk about nutrition, but that there was a medicine that was involved in it.
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Then I was all in at that point. I was really tired of struggling with my weight and with health issues. So, I was excited to try something new.
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[A woman in blue medical scrubs walks into the waiting room at a doctor’s office.]
NURSE: Suzanne?
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SUZANNE POTTS: Yes.
NURSE: You ready?
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SUZANNE POTTS: I honestly think it was a clinical trial winner. I crushed it. I really did appreciate the process.
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SUZANNE POTTS: It was kind of exciting to feel like I was part of science.
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[Scenes of a woman playing tennis, filling a bird feeder in the backyard and preparing vegetables in the kitchen with her husband.]
SUZANNE POTTS: I feel really good about taking risks and trying new things. It's been humbling to try a new sport at 50. Oh. This is my year to, like, take charge of my health and figure out what I need to do to live doing the things that keep us happy and healthy.
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SUZANNE POTTS: So it's kind of exciting.
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And now you know why. Your body isn't broken, and you are not weak. You're navigating real, measurable biological headwinds that science is only beginning to fully understand.
The good news is that understanding these barriers is genuinely empowering. And with growing advances in obesity medicine, there are more tools than ever to help you work with your biology rather than against it.
Talk to a health care provider to explore strategies tailored to your biology, lifestyle, and goals.
References
- Bays H, et al. Obesity Pillars. 2022;3:100034.
- Safaei M, et al. Comput Biol Med. 2021;136:e104754.
- Yu YH, et al. Obes Rev. 2015;16(3):234–247.
- Blüher M. Nat Rev Endocrinol. 2019;15(5):288–298.
- MacLean PS. Am J Physiol Regul Integr Comp Physiol. 2011;301(3):R581–R600.
- Nota MH, et al. Neuroscience and Biobehavioral Reviews. 2020;108:646–57.
- Sumithran P, Proietto J. Clin Sci (Lond). 2013;124:231–41.
- Lau DCW, Wharton S. Canadian Adult Obesity Clinical Practice Guidelines: The Science of Obesity.