Why ‘eat less, move more’ won’t work for weight loss, according to experts

It will be no surprise that weight loss is one of the most common exercise goals on the planet, especially at this time of year. But if losing belly fat is one of your resolutions and you think that exercise is the key, then it’s time to think again.

“The single best thing we can do for ourselves is exercising. It works on almost all diseases we can measure - but not for weight loss,” says Professor Christoffer Clemmensen of the University of Copenhagen’s Novo Nordisk Foundation Centre for Basic Metabolic Research.

In fact, studies have repeatedly shown the “eat less; move more” mantra parroted to those looking to lose body fat is ineffective. And nor is willpower the vital key that many people believe it to be.

“Weight loss is not a matter of willpower,” adds his colleague, PhD fellow Valdemar Brimnes Ingemann Johansen. “It’s critical to acknowledge the powerful biological mechanisms that interact with your environment.

“Genetic predisposition is a big determinant of our body size and how we look – and this powerful biology that underlies our body size is often beyond an individual’s control.”

The pair are responsible for a recent literature review, published in the Cell journal, which dives deep into this topic. Here, they reveal more about the lesser-known mechanisms behind weight loss that might just help you achieve your goals this year.

A calorie is a unit of energy – more specifically, one kilocalorie is the estimated amount of energy needed to heat 1kg of water by 1C. Since the 19th century, this has been the most common unit of measurement used for listing energy in food and drink.

A calorie deficit is the common denominator behind weight loss, and so it is widely prescribed to people looking to lose body fat, presenting the idea that if you expend more calories than you consume, you will create a negative energy balance and lose weight. This fact is not disputed by our experts.

“Anyone can lose weight if you put them in a restrictive environment,” says Professor Clemmensen. “It’s the laws of thermodynamics and it will work on everyone; if you have a negative energy balance, you will lose weight.

“But you can’t tell people to stay in a negative energy balance in the free world, because you have these strong biological forces that are telling people to eat. Unless you can restrict your meal size, or you have someone to put a lock on your cabinets, it’s just impossible.

“Calorie deficit comments disregard the power of biology underlying weight control. It’s like telling people who are stressed to relax, or telling people who have psychological illnesses to just get their act together.”

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Obesity is classified as a chronic, relapsing disease by the World Health Organisation, with the institution reporting: “In 2021, higher-than-optimal BMI caused an estimated 3.7 million deaths from noncommunicable diseases such as cardiovascular diseases, diabetes, cancers, neurological disorders, chronic respiratory diseases and digestive disorders.”

So why would our bodies contain “strong biological forces” to promote or protect a condition linked to ill health? The answer, according to Johansen, lies with our predecessors.

Stored fat would have helped our human ancestors stave off starvation when food was scarce, so the body “remembers” it and tries to replenish these stores when food becomes available, he says. As part of this, the body treats weight loss like a threat, fighting it by releasing a flood of hunger hormones and heightening food cravings while reducing our energy expenditure.

Historically, this might have been helpful; in the 21st century, where energy-dense foods are more accessible than ever and movement is optional, these impulses can be harmful.

Professor Clemmensen led a 2025 literature review on this topic, which found evidence to support the presence of what he describes as an “obesity memory”.

“This idea about memory of obesity is a little bit elusive," he says. “We don’t know where that memory is stored, but you can see it’s there. If you look at studies where people have undergone extreme weight loss, most of them will find their way back quite accurately to their original weight.

“There seems to be a very specific memory, for want of a better term, of what your weight was. That’s what we’re working on in our lab to try to understand.”

The role of this memory will, as with most things in the human body, vary from person to person depending on factors such as genetics.

“How much people are able to sustain lost body fat and how they will respond to different types of interventions – both behavioural and pharmacological, like weight loss drugs – will depend on the person,” Johansen explains. “We see quite some variability among people when we look at big clinical trials.

“It would be interesting, and very valuable, if we could understand more about what makes one person regain all their lost bodyweight in just a couple of months, while another person is able to sustain their weight loss for years. This is probably, if I am allowed to speculate, down to the interaction between the environment that person is living in and their genetic make-up.”

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Anyone who has tried to lose weight will be aware of a disconnect between wanting to reduce body fat and succeeding in doing so. Professor Clemmensen’s idea of “obesity memory” provides a possible explanation for this.

However, if “eat less; move more” does not work as an approach, what does? This is a tricky question to answer, Professor Clemmensen says, as research into the topic is ongoing.

“Do we actually have enough knowledge to guide people on how to live, in terms of losing weight? I don’t think so. I think that’s part of the hard truth we, as researchers, sometimes have to deliver,” he says.

This does not mean diet and exercise are unimportant – wider health-promoting behaviours, regardless of their impact on body weight, should always be encouraged. Eating a nourishing diet rich in a large variety of vegetables combined with protein and some kind of fish is more likely to have a favourable impact on the body – compositionally and hormonally – than less nutrient-dense alternatives like junk food, helping you feel and function better.

“Macronutrient composition will have different effects on the various hormonal systems in the body,” Professor Clemmensen explains. “If you get all your calories from liquid sugar versus from chicken breast, it will have very different impacts on the hormonal environment in the body, and subsequent feelings of hunger.”

The same applies for exercise. Moving more will have a transformative effect on health markers such as blood pressure and your risk of chronic diseases, but Professor Clemmensen says, “There is very little evidence to suggest it can override that memory of prior bodyweight.”

There is no single intervention that provides an all-encompassing solution, and people’s varied genetic make-ups render blanket prescriptions impossible. But a combination of manageable interventions appears to be the most productive course of action.

Developing sustainable habits such as sufficient sleep, a nutritious diet and regular exercise will form part of the puzzle. These are things you can implement to improve overall health – and possibly assist in appetite regulation, energy balance and weight management too.

But Professor Clemmensen and Johansen suggest that environmental and societal changes, long-term planning and pharmacological interventions are also necessary in reversing wider obesity rates in the long run.

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“To reduce obesity rates, we have to look at a more long-term perspective, thinking about generations beyond our little existence,” says Professor Clemmensen. “I think we need to funnel a lot of research into preventative strategies and learning how to create societies where we will have fewer gene-environment interactions that lead to obesity in the first place.”

In other words, the world needs to change to encourage healthier behaviours – for example, greater prominence and availability of nourishing foods over ultra-processed energy-dense alternatives, and neighbourhoods that prioritise walking and cycling over travelling by car.

What about weight loss drugs like Ozempic, Wegovy and Mounjaro, which fall under the banner of “pharmacological interventions”?

“I think weight loss drugs have actually helped people acknowledge how difficult it is to sustain weight loss through willpower," Professor Clemmensen continues. “Now we have pharmacological tools that we know work, and they will be relevant for people who are made sick by their excess body fat.”

Yet, as Johansen points out, many of these weight loss drugs “don’t seem to be good at maintaining weight loss when people stop the treatment”.

“People also experience comorbidities like diabetes, high blood pressure and systemic inflammation when they stop the treatment,” he adds.

“Some evidence suggests that a combination of exercise and pharmacological weight loss can be very beneficial for supporting weight loss maintenance.”

Professor Clemmensen also emphasises the importance of promoting healthy behaviours in children – particularly those aged seven and below, due to their more “malleable weight regulation system”.

“There are different critical periods of your life,” Professor Clemmensen says. “There’s your time in the womb, then post-birth, then there’s this growth spurt between four and seven – then again, as a teenager.

“There is some genetic predisposition [to changes in weight], but if you put that person or those genes in an environment that is really unsuitable, you risk developing excess fat mass in those critical periods.

“It’s not about restricting these children or individuals, but creating an environment in which they don’t have free access to a ton of calories 24/7, or exposure to a lot of screens, poor sleep and poor psychological health.”

This could mean reducing the marketing of junk food to children, investing in healthier school meals, standardising restaurant portion sizes and encouraging regular exercise.

Developing favourable traits in childhood to suit the genetic hand someone has been dealt could then set them on a trajectory to having less body fat as an adult, Professor Clemmensen ventures.

But his ultimate goal, and the intervention he believes has the greatest scope for success, is researching, identifying and altering an individual’s “obesity memory”.

“We still think if we can understand where and how this memory resides, maybe we can erase it and help people reset their body weight,” he says. “If we can learn to understand the physiology, we can learn how to intervene.”

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