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The amount of exercise required to burn off more calories than we eat is beyond what most people can achieve in a day, adds Ian Macdonald, a professor of metabolic physiology at the University of Nottingham.
In fact, the body's use of calories appears to plateau no matter how active you may be.
In one of the breakthrough studies in this area, US researchers studied the Hadza tribe, hunter-gatherers in northern Tanzania who live an active lifestyle, spending their days foraging for wild food and regularly covering long distances on foot.
The assumption was their active lifestyle meant they burned more calories than those on a Western diet. Their average calorie intake is comparable to that found in the developed world.
But to their surprise, the researchers found that the Hadza's overall energy expenditure was no higher than the average American or European, according to the study published in the journal PLoS One in 2012.
These findings were backed up by another study last year by the same researchers, from the City University of New York. They measured the daily energy expenditure (the total calories burnt a day) and activity levels of 332 people during a week.
They found that while people with moderate activity levels burnt around 200 more calories than those who were sedentary, people who exercised more than this moderate level didn't burn any more calories.
Energy expenditure through exercise seemed to plateau at around 2600 calories a day.
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Or as the researchers put it: "Individuals tend to adapt metabolically to increased physical activity, muting the expected increase in daily energy [expenditure]."
The traditional theory is that there is a linear relationship between exercise and energy expenditure, that is, the more you exercise, the more calories you burn, says David Stensel, a professor of exercise metabolism at Loughborough University.
"These researchers suggest that rather than being linear, beyond certain limits energy expenditure is constrained."
Not all studies that have been conducted into this have reached the same conclusion, but it's compelling research and there are plausible reasons to explain what's going on.
One of the suggestions is so-called compensatory mechanisms; our mind and body's reactions to exercise which offset the benefits of activity.
For example, as we increase exercise, our expenditure through other physical activity that isn't formal exercise may decline, says Professor Stensel.
This other activity includes fidgeting, standing and generally moving around, this is known as non-exercise activity thermogenesis, or NEAT.
The theory is if you do more exercise, you're less likely to fidget and burn calories this way.
"There has been an increased appreciation in recent years that NEAT may be an important part of weight control and weight loss," says Professor Stensel.
"The implication is if you do more formal structured exercise, it's possible that activity levels at other times of the day may go down, as you have less energy to maintain both effectively."
The theory is that people may feel more tired after formal exercise and this makes them less active during the remainder of the day, so they tend to lie down more, fidget less or take the lift rather than the stairs.
Increased exercise may also not lead to weight loss because people can overestimate the number of calories they've burnt off. Exercise makes people hungrier, too.
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"Our studies show one reason people don't lose as much weight with exercise is because of a compensatory increase in energy intake," says Professor Dylan Thompson, a physiologist at the University of Bath.
"This is related to a fall in levels of the hormone leptin, one of the body's satiation hormones, meaning people feel hungrier after exercising than they usually would."
Ploughing away at the gym won't necessarily speed up weight loss as the extra calories we burn off account for only a tiny part of our total energy expenditure, too.
Most factors that contribute to the overall amount are out of our control. For example, our basal metabolic rate, the calories needed to keep your body functioning while at rest, accounts for around 80 per cent of energy expenditure.
It is predominantly our body size that determines energy expenditure, says Professor Macdonald.
"Even tables that suggest you get through 900 calories in a two-hour run may be inaccurate for a given individual as their body weights will vary."
As we lose weight we get physically smaller and our metabolic rate then declines, as less energy is required to support a smaller amount of tissue.
The key to weight loss is cutting calories. When Professor Thijssen and his colleagues recently compared a restricted diet with regular endurance exercise during a three- to five-month period, they found that the diet "leads to much larger weight loss".
But there is no doubting that exercise is good for health. Many studies have shown that exercise improves heart and brain health and reduces the risk of chronic conditions such as type 2 diabetes and high blood pressure.
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Exercise also changes our body composition, such as a large reduction in visceral fat, the fat stored around our organs.
"Losing visceral fat is more strongly related to prevention of diabetes than just losing weight," says Professor Thijssen. When you cut calories, you lose a bit of everything; muscle, body fat and visceral fat.
Even if you don't lose weight after exercise, you will lose visceral fat mass, so don't be disheartened and give up if the scales aren't budging, he adds.
The most effective way to improve your body composition and lose weight is to cut calories first, then add regular bouts of exercise into your routine.
The message, as researchers wrote in the British Journal of Sports Medicine in 2015, is simple: You cannot outrun a bad diet.
Worldwide childhood obesity rates to rocket
Global childhood obesity rates are set to skyrocket in the next few years.
By 2025, around 268 million children aged between 5 and 17 years old will be overweight, according to estimates by the World Obesity Federation.
More than 90 million of those children will be registered obese, the data suggests.
It is a significant revision up from previous estimates by the World Health Organisation, which had envisioned 70 million children would be overweight in 2025.
