Scientist suggests a reason why fat loss is so difficult – and a possible solution

Many people want to lose weight; few successfully do. Research also shows that those who are able to lose fat often gain this weight back over time. Yet weight loss is frequently presented as a simple and easy concept: to see the number on the scale drop, create a calorie deficit by consuming fewer calories (eating less) and burning more calories (moving more).

These are the laws of thermodynamics and not something I’m here to debate. However, given people’s difficulty with losing weight, something is clearly complicating matters.

Experts I’ve interviewed have provided a few possible explanations. There is the concept of obesity memory, in which the body tries to retain fat for energy as an archaic survival practice. There is also the idea that humans have a subconscious snacking habit, conditioned by an obesogenic environment in which we are constantly shown, and given access to, more-ish foods.

Professor Lewis Halsey, a professor of environmental physiology at the University of Roehampton, proposes another factor: energy compensation. In layman’s terms, this means that when we move more and increase our calorie expenditure, our body finds other ways to save energy in order to compensate.

For this reason, classic calorie calculations and the data from your favourite fitness tracker may miss the mark, Professor Halsey says. But he also puts forward a possible method for counteracting these unwanted compensation effects.

The plateau problem

“If somebody wants to lose weight, they might start going to the gym, cycling to work or going for a daily walk around the park,” Professor Halsey says. “If their fitness tracker tells them they’ve burned 300 calories, and they keep eating their normal meals, they might assume they are now in a 300-calorie deficit and will lose weight.”

This “additive model of energy expenditure” is, to some extent, true, he continues. After introducing a new form of regular exercise, most people will lose some weight. But research has repeatedly shown that this early progress soon stalls.

People who are trying to lose weight by exercising more often find that their weight loss plateaus after two or three kilos. There are several reasons for this, says Professor Halsey.

“Number one is that the extra exercise makes them hungrier, so they start eating more. Other times, without realising, they might gradually start exercising less, or with less vigour.

“But there are also some pretty well-run studies that suggest when someone starts an exercise regime, even if their food intake doesn’t seem to change and they stay consistent with their workouts, their weight loss isn’t what you would expect it to be given the discrepancy between energy in and energy out.”

His explanation is that the body reacts to additional calorie burn by saving the energy elsewhere – for example, dampening down the immune system (although Professor Halsey makes clear, “that doesn’t mean people are at risk of falling ill because they do exercise”). Another way the body might compensate for the extra energy spent doing exercise is by reducing activity levels elsewhere in the day.

“Somebody might go for a run on Sunday morning then zonk out for the rest of the day,” says Professor Halsey. “That’s another way you could save energy; spending less on fidgeting, pottering and just being upright.

“Metabolic rate while standing is about twice what it is when lying down. It doesn’t quite compensate for a 10-mile run, but if you spend hours lying down rather than standing up, it does have quite an effect.”

No rest for the active: One example of energy compensation is exercising in the morning, then reducing your activity levels for the rest of the day (Getty/iStock)

The surprising evidence for energy compensation

In 2015, evolutionary anthropologist Professor Herman Pontzer released research on Hadza hunter-gatherers in Tanzania, measuring daily walking distance, speed and total energy expenditure. He disovered that, compared with developed, industrial populations, Hadza adults had similar total energy expenditures.

In other words, despite incredibly active lifestyles, the Hadza people had relatively similar daily energy expenditures to people leading sedentary lifestyles, such as desk workers.

Researchers concluded that “adults with higher levels of habitual physical activity may adapt by reducing energy allocation to other physiological activity”.

“If you translate that to someone starting to do their walk around the park each night or go to the gym three times a week, to start with, the body probably doesn’t compensate for expending those extra 300 kilocalories and you are in a calorie debt,” says Professor Halsey.

“But the argument is that if you are regularly burning lots of additional calories by doing exercise, over a period of months, the body will clock what’s going on and start to compensate.”

The exact compensatory mechanisms are unclear, he adds, but the process makes sense evolutionarily. Energy is the human currency for survival, and it was far harder to come by for our ancient ancestors, so preserving as much as possible would have been logical.

“These days, most of us can go down to the Co-op and buy more food if we are feeling hungry,” Professor Halsey explains. “But back in the day, that wasn’t possible.”

Energy compensation could be a hangover from this.

Alternating exercise methods every couple of weeks, between higher calorie burn activities and lower calorie burn activities, could reduce energy compensation, Professor Halsey suggests (Getty/iStock)

Why it pays to mix things up

The question is how do we counter energy compensation?

“This is always the toughest question for people in my position because it’s so open-ended – I tend to undertake pure research rather than applied research,” says Professor Halsey. “In theory, if you upregulated your activity for a week or two, backed off for a week or two, then repeated this sequence, the body might never learn what is going on and never start to compensate.”

But this doesn’t necessarily mean exercising intensely for a fortnight then kicking back and relaxing in the two weeks that follow. Instead, he mulls the merits of alternating between two contrasting types of exercise.“ For example, two weeks of aerobic activities such as running, swimming, cycling or using exercise machines with relatively high calorie burn, followed by two weeks of strength training with relatively low calorie burn, then repeat.”

“There’s some data to suggest that energy compensation kicks in after a number of weeks,” Professor Halsey continues. “Therefore, I would say this two weeks on, two weeks off model should fall within that window. But no one has yet run studies to look at this.” Perhaps variety is, indeed, the spice of life – particularly where exercise is concerned. And the perks of mixing up your routine don’t end there.

A 2026 study from the Harvard T.H. Chan School of Public Health concluded that “long-term engagement in multiple types of physical activity may help extend the lifespan” – even when compared to those doing the same amount of exercise but focusing on a single activity.

Some research suggests that exercise might play a smaller role in weight loss than previously thought (Getty/iStock)

Is exercise worthless for weight loss?

Professor Halsey’s argument and Professor Pontzer’s research suggest that the “move more” element of the “eat less, move more” approach to fat loss might not hold much weight. This claim tends to set the cat among the proverbial pigeons.

Why? Because it goes against much of what we have been taught about weight loss, and contradicts many people’s observations too: people who exercise a lot are often a healthy weight, and people who have lost weight have often increased their activity levels. One reason for that might be the significant identity and behavioural shifts that often accompany this process – a dietary overhaul, new hobbies, early mornings.

Other people try myriad methods to lose fat and still struggle to do so. Energy compensation is one compelling explanation behind people’s expectations rarely meeting reality.

Experts tend to agree that changing your diet is the most effective thread you can pull to lose weight. But their other recurrent advice for effective weight loss is avoiding putting all of your eggs in one basket. Rather, make positive and sustainable changes in several areas – eating more fibre to feel fuller for longer, eschewing calorie-laden drinks, building muscle to raise basal metabolic rate, improving sleep quality to curb appetite and, in this case, introducing structured exercise variance to counter energy compensation.

The benefits of exercise also extend far beyond weight management. It contributes to improved longevity, combats chronic diseases, maintains physical function and reduces injury risk – it helps us live better for longer.

“The literature also suggests exercise can help with not putting weight back on, for those people that are able to lose it in the first place,” Professor Halsey adds. “So whether energy compensation is a thing or not, it should not put people off doing exercise.”

Scientist suggests a reason why fat loss is so difficult – and a possible solution

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