Increasing long-term exercise reduces the amount of calories we burn, research reveals

The weight loss plateau is a common complaint among people who embark on a new exercise regime in order to lose weight.

This means that while a person might find themselves losing weight at a steady rate in the first few weeks or months of following a new diet or fitness regime, it stops declining even if they continue working out.

The stall can often be frustrating and discouraging for people trying to reach a particular weight for health reasons. According to the NHS, it happens when the body adapts to the current programme of exercise and “learns to cope with the same energy demands while burning fewer calories”.

But new research by the University of Roehampton suggests that this might not be the only reason for a plateau, and that it could depend on how individual bodies compensate for energy spent doing physical activity.

After analysing human energy expenditure in more than 1,750 adults globally, the researchers found that people who take part in regular exercise burn fewer calories on body maintenance compared to people who don’t do any strenuous activity.

In order to understand how this is possible, we must first take a look at how our bodies use energy to burn calories.

The human body doesn’t just burn calories during exercise. It is constantly burning them while we use energy to go about our daily lives through two types of metabolism; basal (resting) metabolism and digestive metabolism.

The basal metabolic rate accounts for 60 to 70 per cent of the body’s overall metabolism, and burns calories even when we are sleeping. Digestive metabolism typically burns 10 to 15 per cent of daily calories just by digesting food.

But according to the study, the amount of calories the body burns through basal metabolism decreases by 28 per cent during periods when daily exercise levels are consistently high.

“This means the more we exercise over the long-term, the less calories our bodies burn for the most rudimentary activities, therefore reducing the overall calories we burn per day,” wrote the study’s authors.

Body mass index may also affect the number of calories the body burns during exercise and throughout the day, the study suggests.

Adults with the highest body mass index (BMI) burned fewer calories in the background following an increase in activity, with 54 per cent of calories burned by exercise or activity actually translating into calories burned at the end of the day.

This reduction in calories burned through basal metabolism happens because the body reduces the amount of energy it spends carrying out basic functions in order to compensate, said the researchers.

In contrast, 70 per cent of calories burned during exercise by those with the lowest BMI translated into more calories burned at the end of the day.

This could mean that people with higher BMI, and therefore greater fat levels, may burn fewer calories per day when they increase the amount of exercise they do over longer periods of time.

Why this happens is unclear. One possible reason could be energy intake; if someone with a higher BMI does not eat more in response to an increase in activity, the body then has fewer resources for other functions and may reduce energy expenditure to compensate.

The researchers say that if there is an underlying genetic reason for the way different bodies handle energy compensation, it may be possible to “screen” individuals in the future to determine if exercise would be a “valuable fat loss intervention” or not.

The study also disregards the widely-held belief that calories burned during exercise easily translates into extra calories burned overall for the day. The researchers called for current weight loss programmes – which use additive models to calculate how many calories need to be burned in order to lose fat – to be personalised according to a person’s BMI.

Lead author Professor Lewis Halsey, of the University of Roehampton, said: “Around the world, national guidelines tend to recommend a 500 to 600 calorie deficit through exercising and dieting to lose weight.

“However, they do not take into account the reduction of calories being burned in the most basic of human functions as the body compensates for the calories burned on the exercise as shown in our research, and the variation in this compensation between people with different levels of body fat.

“Not only should these guidelines be revised, but there is also a need for greater personalisation of exercise plans depending upon body mass.”

The study’s authors also recommended that public health strategies for fat loss be revised to recognise energy compensation, and urged further research into the “potentially substantial diversity of energy compensation between sub-populations”.

“In the future, personalised exercise plans targeting fat loss might be developed partly based on an individual’s genetic propensity for energy compensation,” it concluded.