After the ordeal of trying to lose weight, the last thing you want is for it all the extra pounds to come creeping back a few months later.
But that scenario is very common. The data suggests the odds of keeping lost weight off are against you. Studies of thousands of people over decades show that in 80–95 per cent of cases, a person will regain any weight they’ve lost over a number of years.
It’s tempting to blame the weight’s return on overindulgence, but as we delve deeper into our genetics, we’re learning that our body chemistry – the choreography of hormones that influence our moods and behaviour – is constantly trying to get us to eat more.
“This is how the body works – its goal is to bring you back [to your original weight],” says Dr Fatima Cody Stanford a clinician and researcher from Massachusetts General Hospital and Harvard Medical School who treats and studies obesity.
“It’s not that you, the person, did something wrong,” she says.
For some, this leads to another problem: ‘yo-yo dieting’ – or weight cycling, as scientists prefer to call it. This is a vicious cycle of weight loss and gain that ultimately leaves people worse off than when they started.
For those trying to fit into their favourite summer outfit, yo-yo dieting can be vexing. But for the 16 per cent of the world whose obesity puts them at higher risk of developing a disease, not being able to keep the weight off can be a matter of life and death.
So how do we rewrite this story? Can we get our body on board with our weight-loss goals?
How to win at losing... temporarily
The Biggest Loser was a hit TV show of the mid-2000s. Broadcast on the NBC network in the US, it rewarded participants who lost the most weight while following drastic diets and intense regimes of physical exercise.
Danny Cahill was one of those who lost a staggering amount of weight. Over the seven month run of the show, he lost 108kg (239lbs) to reach a weight of 86kg (191lbs), a feat that made him the winner of season eight.
Interviewed by The New York Times six years later, Cahill said he still followed a strict diet, eating 800 calories less than a typical man his size. Despite this, he had put 45kg (100lbs) back on since the show ended.
Most of the contestants from season eight had also gained a substantial amount of weight.
A study published in the journal Obesity in 2016 looking at 14 of the show’s contestants, provided some insight into what was going on.
As they were spending huge amounts of energy with these gruelling workouts while feeding their bodies less, the contestants’ resting metabolism rate slowed down substantially to compensate, meaning their bodies were burning fewer calories at rest than before they lost the weight.
The problem is, six years later, the contestants’ metabolisms hadn’t bounced back so, as The New York Times noted, despite some of the contestants keeping up the intense diets, “their bodies were not burning enough calories to maintain their thinner sizes.”
Worse still, their levels of leptin, a hormone that helps us feel full, were still substantially lower than before. Because of this, they were still battling intense cravings and hunger pains. In essence, their weight loss didn’t mean their obesity was gone.
This and other studies strongly suggest that the human body is often actively working to draw its weight back up.
“What’s clear is that it’s much easier to increase body weight,” says Prof Ferdinand von Meyenn, an obesity researcher at ETH Zurich who studies the genetics of weight gain. People are more likely to return to their original body weight than to stay at their new weight after a diet, he adds.
We still don't understand weight change
Unfortunately, researchers don’t know what’s behind the body’s obsession with weight gain. Weight regulation relies on an interplay of signals, often mediated by hormones, that modify how our fat, guts, brains and muscles react to food intake. And the more we learn about it, the more complicated it seems to become.
“New hormones are discovered almost every month and we don’t really understand how they interplay. That’s something to study in the future,” says Prof Paul Pfluger, an obesity researcher from Helmholtz Zentrum München who studies the genetics of yo-yo dieting.
At least part of the answer seems to lie in the heart of our fat cells. Von Meyenn was the lead author on a recent study, published in Nature, which suggests that fat cells can keep a memory of weight cycling in their epigenome – chemical signatures that stick to the cells’ DNA that highlight or silence parts of the genome.
Every cell in our bodies starts from the same basic instructions – the DNA. But epigenetic changes tell each cell what part of the instruction manual it should use to fit its specific purpose.
In this study, mice’s fat cells developed a characteristic epigenetic trace after exposure to a high-fat diet. The study found that mice carrying these epigentic markers put weight back on faster after weight loss.
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Fat cells hold a grudge
What’s more important is that, even after the mice lost the weight, the fat cells, which are very long-lived, seemed to hold on to memories of their days bathing in a high calorie environment. This suggests that the weight loss doesn't erase that memory, which persists with the fat cells.
When researchers looked at fat cells taken from human tissues from formerly overweight people who had undergone bariatric surgery (an operation to make the stomach smaller), they saw similar epigenetic traces, even two years after the procedure.
“It’s a proof of concept that this memory exists,” says von Meyenn. “How is it exactly regulated? We don’t know.”
The study stopped short of proving that these epigenetic changes played a part in the weight’s return, but it’s plausible. After all, this lingering memory of fat makes sense with what we know about yo-yo dieting.
It could contribute to tipping the scales, for instance, by changing the amount of hormones the fat cells secrete or priming fat tissue to pack on the pounds quickly as soon as high-calorie foods are available.
Further research will likely find that other cells in the body hold a similar memory, say both Pfluger and von Meyenn.
Another recent study in Nature Communications, also carried out on mice, suggests immune cells in fat tissue retained a memory of weight cycling. That memory built up with each weight regain, generally increasing inflammation and ultimately leading to some immune cells getting exhausted.
If this mechanism also exists in humans, it may be part of why some people get sicker after weight cycling, according to the study’s authors.
Pfluger and von Meyenn also suspect brain cells are likely to keep a similar trace, which could influence how well we control food-seeking behaviours over time, for instance. This needs to be studied, though.
It’s still not clear how long that memory can last and how much it contributes to dragging someone’s weight back up. One clue comes from the people using the weight-loss drug semaglutide (which activates the body’s GLP-1 hormone receptors) for therapeutic weight loss, however.
A 2022 study in the pharmacological journal Diabetes, Obesity and Metabolism found that although the people kept their weight off while they were on the drug, they started regaining it after they stopped the treatment. This suggests that it didn’t cure their bodies’ programming towards a higher weight point.
“I would put money on the fact that a large number of the people who regain weight very quickly the moment GLP-1 drugs are gone are going to be those that tend to yo-yo diet,” says Prof Giles Yeo, a geneticist at the University of Cambridge who studies obesity.
Part of the problem is that food was scarce for the majority of the time humans have existed. Sourcing calories meant having to run and fight to meet daily energy needs. Your body is still working on rules established thousands of years ago, when a sudden drop in weight could mean death.
“Today, we don’t have to run around anymore to get our food. We have plenty in front of us,” says von Meyenn.
But while our bodies once had a very good reason to code in mechanisms that would store as much fat as possible or encourage us to seek out as much food as we could, that’s no longer the case.
As such, it’s possible that this weight-gain mechanism doesn’t have an ‘off switch’. It’s something we can study in more detail, now that scientists have pinpointed this fat memory.
Conditioned from birth to stay at a certain weight
Chances are the weight-loss dice were cast, to some extent, before we were born. Evidence suggests that the genes we get from our parents could predispose us to having a higher body mass index.
“When you meet two parents that have severe obesity, unfortunately, the likelihood of having a child with severe obesity is high,” says Stanford.
For a rare few with severe, early-onset obesity, the genetic link is clear. Their obesity can be traced back to a variation on a single gene.
Most people, however, will have a polygenic form of obesity, meaning their proclivity towards putting on weight may have, in part, been passed on through hundreds of gene variants.
In fact, as per the World Obesity Federation, that can account for somewhere between 40–70 per cent of the likelihood of having obesity.
One 1990 study in the New England Journal of Medicine supports this idea. Scientists overfed 12 pairs of identical twins by 1,000kcal a day, six days a week, for 100 days. After this period, no two pairs of twins looked alike.
Some put on more weight than others; some carried the weight on their hips, others around their waists. Within the pairs of twins, however, there were striking similarities in both the weight gain and its distribution, suggesting, as the study reported, that “genetic factors are involved”.
Of course, drawing firm conclusions from these kinds of observations is challenging and without pinpointing the genes involved, we’re still in the dark about how much influence this inheritance truly has.
All kinds of other factors could influence these results – for instance, siblings are typically raised in a similar environment to their parents, which means they may have similar food habits, be exposed to similar quality food and so on.
Still, “I'm sure there are genes involved. We don’t know what they are yet,” Yeo says.
Of course, carrying these versions of the genes “doesn’t necessarily mean” that the person who inherits them will become obese, says von Meyenn.
Environment, access to food and exercise, stress levels, sleep and so on still likely play a role in the weight gain. But what it may do, is make avoiding obesity “very hard,” he says.
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You're a yo-yo dieter, so what?
Experts tend to say that yo-yo dieting isn't good for our health, but as with anything that revolved around delicately balanced whole-body systems, it's unlikely to be that simple.
Studies suggest that yo-yo dieting can drive weight up with each weight cycle. Other works suggest that going through cycles of weight loss and gain can promote inflammation, which is generally thought to worsen diseases that accompany obesity.
Those results, however, have drawn controversy, as there’s a lot of person-to-person variability behind the broad statistics.
“There are people going on a yo-yo diet who stay perfectly healthy. And some people are going to be adversely affected by the yo-yo dieting because it stresses the body,” says Yeo.
Pfluger recently published a study in the Journal of Translational Medicine that suggests yo-yo dieting worsens the health of mice.
But he agrees that we also need to zoom in to look at what happens individually. Even in his study, which looked at genetically similar mice, he says he saw very different reactions to weight cycling.
“Some [of their health got] worse, but many of them didn’t, really,” says Pfluger, but adds: “Weight loss is important and even if you have the risk of regaining the weight at some point, it’s still worth trying to lose weight.”
Avoid extreme weight-loss methods
That said, if the reason you’re yo-yo dieting is that you enjoy dabbling in the latest ‘crash diet’ fad, that’s probably a bad idea. Unless you think you can keep it up in the long term, that crash diet and daily 5am workout regime might not be the best approach.
Doing so is more likely to send your body into a panic and could also cause nutrient deficiencies. Extreme weight loss typically also means extreme weight gain, which puts the body under unnecessary stress, says Yeo.
"Anything extreme, by its very nature, is unsustainable,” he says. Pfluger points, once again, to the study looking at the contestants of The Biggest Loser. The author of the study later revisited his earlier results in light of other work that didn’t find the same persistent slow resting metabolic rate after weight loss.
In the journal Obesity, he pondered: could the lasting memory of the extreme weight loss be linked to their substantial increases in physical activity during and after the show?
“That’s also something that people need to spend more time studying: is super-intense physical exercise actually something you should do, or you should maybe avoid?” asks Pfluger. “How you lose weight might be an important factor in how you regain it.”
Some people might benefit from extra help
All of this is to say: don’t be too hard on yourself. A rebound effect after dieting is natural and some forces driving weight up are beyond your control.
“For some people, a diet is just not going to help,” says Yeo. Depending on how far along you are in your weight gain, how ill you are from the weight or whether you’ve had success with dieting in the past, you may benefit from weight-loss pills or bariatric surgery.
Some have more urgent needs than others. While one person might have a higher weight, but show no discomfort or symptoms of disease, another person’s test results might show they’re developing dangerous weight-related conditions such as diabetes, heart disease and liver disease.
Equally, someone else’s excess weight might keep them from getting a good night’s sleep, put pressure on their joints or prevent them from carrying out day-to-day activities, which could limit their independence, especially as they get older.
Your doctors can run tests to figure out how the weight is affecting you and help you assess the best approach for you.
How to lose weight and keep (some of) it off
Still, that shouldn’t put you off experimenting with ways to lose excess weight and keep at least some of it off.
Spending time at a lower weight is generally seen as a good thing by experts and research suggests even a moderate amount of weight loss, as little as 5 10 per cent of your total body weight, can substantially improve your health.
But that starts with understanding what’s within your control, how you can help your body adapt to the changes and what works for your unique body chemistry.
“The journey to better health begins with small, mindful steps. By noticing how each change affects us, we can cultivate a lifestyle that truly supports our wellbeing,” says Stanford.
Diet
When it comes to diet, however, bear in mind that we live in an obesogenic environment geared towards selling us more calorie-rich, nutrient-poor, highly processed foods.
These are generally cheaper, more available and more aggressively marketed than better-quality food. That limits the control we have over what we eat.
Consider why you’re reaching for that highly processed food: Are you tired? Did you see it in an ad? Are you craving sugar? Then, think about ways you can change your habits to reach for the least-processed, highest-fibre, lowest-sugar version of that food that's available to you.
The upside? These healthier foods will, in the long run, generally make you feel fuller and more satisfied than their junk-food equivalent.
“I think you have to really understand your own stuff. Be honest with yourself and structure your intervention around this ‘weakness,’” says Yeo.
Exercise
Before you consider exercise, understand that it can only do so much. As outlined by the World Obesity Federation, 60–80 per cent of the energy we burn daily happens at rest and we have little control over that resting metabolic rate.
But doing 150–300 minutes of moderate-intensity aerobic physical activity a week, such as walking, dancing or mowing the lawn, can reduce your risk of cancer, heart disease, stroke and diabetes by 20–30 per cent. The trouble is that most of us fail to do even that.
Sleep and other habits
Then there’s sleep. An average adult should get 7–9 hours of sleep a night and research suggests sleep deprivation might change appetite hormones, increase fatigue and also give us more time to reach for a snack during waking hours. Stress also makes us crave sugary, fatty and salty foods.
So, getting better sleep and limiting stress can help you have more energy, eat less and generally feel better. Simply improving your sleep hygiene might be the thing that helps you maintain a weight that’s healthier for you.
Still, if you’re not managing alone, don’t hesitate to find a doctor who will act as a trusted ally and help you assess your situation objectively, without blame. After all, being overweight can increase the risk of serious conditions and it deserves medical attention.
“It’s about using the right tool for the size of the problem” and “working with a trusted clinician [who’s] able to help you through those times,” says Stanford. “The goal is to get people to the happiest, healthiest weight for them in the safest way possible.”
About our experts
Dr Fatima Cody Stanford is a clinician and researcher from Massachusetts General Hospital and Harvard Medical School, in the US, who treats and studies obesity. She is published in the likes of Journal of General Internal Medicine, Annals of Internal Medicine and Clinical Obesity.
Prof Ferdinand von Meyenn is an obesity researcher and assistant professor at ETH Zurich, in Switzerland, who studies the genetics of weight gain. His work is published in various scientific journals including Stem Cell Reports, Proceedings of the National Academy of Sciences and Nature Structural & Molecular Biology.
Prof Paul Pfluger is an associate professor of neurobiology of diabetes and obesity researcher from Helmholtz Zentrum München, in Germany, who studies the genetics of yo-yo dieting. He's published in Journal of Pharmaceutical and Biomedical Analysis, Molecular Metabolism and Nature Metabolism (to name a few journals).
Prof Giles Yeo is a geneticist at the University of Cambridge, in the UK, who studies obesity. He is also a broadcaster and author (one book by him is Why Calories Don't Count). He is published in many places, including right here with us on BBC Science Focus.
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