Metabolic Adaptation: The Science Slimming World Hides from You
posted on: February 20, 2026. posted in: Weight loss, Dieting
"I have lost weight following Slimming World and regained but I regained because I stopped eating healthily and ate junk food instead and stopped exercising."
Hello and welcome to Rebelfit 😊
My name is Liam.
I am a personal trainer and nutrition coach.
I've worked in nutrition, fitness and weight management for over 20 years.
Right now, you're perfectly at peace with the explanation to your weight regain...
"I regained because I stopped eating healthily and ate junk food instead and stopped exercising."
But why did you stop eating healthily? And why did you stop exercising? Why didn't you just sustain these behaviours permanently, and keep the weight off permanently?
Now diet culture will answer...
"You stopped eating healthily because you're greedy. And you stopped exercising because you're lazy. And you regained the weight because you lacked the willpower to sustain the results."
And this is an answer that many slimmers are satisfied with.
"I'm greedy, I'm lazy, I lack willpower, and therefore my weight regain is my fault."
No.
That's not a reasonable and scientific explanation. There is, however, a reasonable and scientific explanation as to why you might think it's some kind of flaw in yourself, your fault, and it's called Internalised Weight Bias (IWB).
Let's go deeper.
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My First Doubts
Back when I first started observing my clients regain the weight after a diet, I'd hear them tell the "I'm greedy, I'm lazy, I lack willpower, and therefore my weight regain is my fault" story.
What made me doubt this story was a lack of evidence.
There was no evidence that they were greedy. In fact, they had demonstrated an incredible level of self-control to restrict their calories for a very prolonged period of time on diets, sometimes for many months, even years.
There was no evidence that they were lazy. In fact, they had exercised vigorously and consistently for a long period of time, and I'd personally seen them put themselves through some gruelling workouts.
There was no evidence that they lacked will power. In fact, many of my clients were doctors who had the willpower to complete a Medicine degree, business owners who had the willpower to run a successful business, and mums who had the willpower to hold down a job, look after the kids, and look after everyone else, except themselves.
My clients were regaining the weight after their diets...
But they were not greedy.
They were not lazy.
They were not lacking willpower.
So I kept wanting to know...
What's the real reason here?
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Metabolic Adaptation
To help understand why my clients regained the weight after diets, I spent my 20s and 30s immersing myself in the science and research around nutrition, obesity, and in particular a phenomenon called "metabolic adaptation", and here's what I found.
What if, at the end of your diet, you didn't stop eating healthily and start eating junk because you were greedy, but because your body and brain were adapting to raise your calorie intake?
What if, at the end of your diet, you didn't stop exercising regularly and start avoiding exercise because you were lazy, but because your body and brain were adapting to conserve calories?
What if, at the end of your diet, you didn't suddenly stop trying because you lacked willpower, but because your body and brain were adapting to force you into a calorie surplus and force you to regain the weight?
My wasted youth, researching this subject for two decades, enabled me to fill the gaps, and swap...
"My clients regained the weight after their diets because they were lazy, greedy and lacking willpower."
For the evidence-based truth...
"My clients regained the weight after their diet because their body and brain adapted to the diet, producing an increase in their appetite, cravings and food noise, and a decrease in their energy, mood and movement. A phenomenon known as metabolic adaptation."
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Understanding Adaptations
We adapt to survive. All animals adapt to survive. It's adapt, or die.
You know when you go out in the sun, and you get a tan? That's adaptation. Your skin adapting to the UV light. Similarly, you know when you get callouses on your hands? That's adaptation. Your skin adapting to repeated pressure.
Imagine a scenario where millions of people were explaining their tanning skin, or their thickening skin, as...
"I'm just lazy. I'm just not washing enough. That's why my skin is darkening and thickening."
But it's not true is it?
You could wash three times a day and still get a tan. You can wash three times a day and still get callouses. So no amount of washing can prevent tanning and callouses from happening, because they're nothing to do with hygiene, and everything to do with adaptation.
They are adaptations to UV light on the skin, and pressure on the skin. So for as long as their skin is exposed to that UV light and that pressure, they're going to sustain those adaptations - that darker skin, and thickening of the skin. Spend more time in the shade, or spend less time applying pressure to the skin, and those adaptations reverse, right?
But washing? Does nothing.
Now apply this to obesity...
Sunlight triggers an adaptation, darker skin. Pressure triggers another adaptation, thicker skin. Calorie restriction triggers the adaptation we're learning about, increased fat stores. The very act of restricting your calories causes your body and brain to adapt to defend fat stores. This explains why dieters are more prone to weight and fat gain than non dieters.
People shouldn't see tanned skin or thicker skin as poor hygiene, but rather, adaptations to light stress and pressure. Equally people shouldn't see obesity as a lack of willpower, but rather, adaptation to calorie restriction. And this particular type of adaptation is known as "metabolic adaptation".
The way the body and brain achieves that adaptation, that higher level of fat, is to...
1) Increase appetite, cravings and food noise, to raise calorie intake, and
2) Decrease energy, mood and movement, to lower calorie output
Resulting in a calorie surplus and weight gain.
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The Science
For those wanting to see a small selection of the science and evidence to support this...
• Kramer et al. (1989) followed obese adults after a structured very low calorie diet and found that most regained weight within a year, with many returning to or above their starting weight as metabolic adaptations reversed initial losses. Learn more »
• Keesey & Hirvonen (1997) argued that body weight is physiologically regulated around a defended “set point", helping explain why dieting often triggers biological mechanisms that drive regain rather than sustain weight loss. Learn more »
• Curioni & Lourenço (2005) found in a systematic review that while diet interventions produce short term weight loss, long term maintenance is uncommon and regain is frequent. Learn more »
• Mann et al. (2007) reviewed long term diet studies and found that while people typically lose a modest amount of weight initially, most regain it within a few years, and a significant proportion end up heavier than when they started. Learn more »
• Rosenbaum & Leibel (2010) reviewed evidence showing that weight loss triggers coordinated reductions in energy expenditure and increases in appetite that actively defend prior body weight, promoting regain. Learn more »
• Sumithran et al. (2011) found that after significant diet induced weight loss, hormonal changes persist, meaning people feel hungrier and less satisfied long after the weight comes off, promoting regain. Learn more »
• Dulloo et al. (2012) reviewed adaptive thermogenesis and concluded that reductions in energy expenditure during weight loss contribute to resistance to further loss and long term regain. Learn more »
• Fildes et al. (2015) used nearly a decade of UK primary care records and found that the annual probability of an obese adult reaching a normal BMI was extremely low, and that most modest losses were followed by regain within a few years. Learn more »
• MacLean et al. (2015) showed that weight loss activates persistent biological signals that increase hunger and improve metabolic efficiency, creating strong physiological pressure toward weight regain. Learn more »
• Robinson et al. (2015) found that adults in the US and UK who perceive themselves as “overweight” are more likely to gain weight over time, and that stress related overeating partly explains this counterintuitive association. Learn more »
• Fothergill et al. (2016) found that contestants from The Biggest Loser experienced significant and persistent metabolic adaptation six years after weight loss, with resting metabolic rate remaining suppressed, promoting weight regain. Learn more »
• Benton (2017) reviewed evidence on dieting and bodyweight regulation and argued that dieting often doesn’t lead to sustained weight loss because the body’s physiological systems adapt to counteract weight loss, meaning many people regain. Learn more »
• Haynes et al. (2018) systematically reviewed evidence on weight status perception and found that perceiving oneself as overweight increases the likelihood of disordered eating and greater weight gain over time. Learn more »
• Fan et al. (2024) found that engaging in any dieting behaviours in the past 12 months was associated with greater eating disorder psychopathology, including unhealthy attitudes and behaviours around food and body image. Learn more »
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Must Watch
• Neuroscientist Sandra Aamodt explores the science behind why dieting not only doesn't work, but is likely to do more harm than good. She suggests ideas for how to live a less diet-obsessed life, intuitively. Learn more »
• Author and psychologist Traci Mann speaks about her book Secrets from the Eating Lab: The Science of Weight Loss, the Myth of Willpower, and Why You Should Never Diet Again. Learn more »
• A clip from the documentary Super Slimmers, where endocrinologist Thomas Barber explains the mechanics of metabolic adaptation, with specific reference to the role of the hormone leptin. There is also reference to how the slimming companies know about it, but have declined to comment. Learn more »
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And so...
You're reading this nodding your head.
The penny has finally dropped that you didn't regain the weight because "you stopped eating healthily and ate junk food and stopped exercising", but rather "you experienced the full force of metabolic adaptation, which was no fault of your own".
The blame here lies with the slimming companies that know about metabolic adaptation, and have known about metabolic adaptation for decades, but hide the science from you because metabolic adaptation is great for business.
A customer who knows about metabolic adaptation would never diet, ever, which would put these slimming companies out of business. A bit like how with the rise of awareness around the risks of sun beds has, rightly, started to put many of the tanning salons out of business too.
Liam x
Rebelfit
p.s. I suppose the next question is what next, if you're no longer dieting? And if you want the answer to that question, drop me an email at liam@rebelfit.co.uk with your history of dieting (and the inevitable weight gain it's caused via metabolic adaptation) and I'll answer it for you 🙏
