Exercise and weight loss: Why working out may not help as much as you think

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A new study of how physical activity impacts the metabolism of humans suggests that increasing levels of activity may bring diminishing returns in energy expenditure.

This is due to "compensatory responses in non-activity energy expenditures," international researchers wrote in the study, published in August in the journal Current Biology.

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For every 100 calories burned as a result of working out, the study says most people will net fewer than 72 calories burned. 

"This suggestion has profound implications for both the evolution of metabolism and human health. It implies that a long-term increase in activity does not directly translate into an increase in total energy expenditure (TEE) because other components of TEE may decrease in response – energy compensation," the study summary notes.

The group found that for 1,754 adults living "normal lives," energy compensation averages 28% due to reduced basal energy expenditure (BEE) and that "this suggests that only 72% of the extra calories we burn from additional activity translates into extra calories burned that day."

A young woman runs on a bridge. (Credit: iStock)

BEE is how many calories are burned simply by being alive.

The researchers subtracted numbers from TEE to understand energy expenditure from exercise and other movement and used statistical modeling to draw these conclusions.

In addition, the degree of energy compensation for different body compositions varied considerably. 

The reason, the study suggests, could be due to among-individual differences in calorie compensation, with people who compensate more being more likely to accumulate body fat.

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Alternatively, the researchers said, the process might be within individuals – bodies compensating more strongly for calories burned during activity and making weight loss more difficult.

"Determining the causality of the relationship between energy compensation and adiposity will be key to improving public health strategies regarding obesity," the study states.

Notably, the study did not examine food intake.

The U.S. Centers for Disease Control and Prevention (CDC) notes that adult obesity prevalence in the U.S. was 42.4% in 2017 to 2018. 

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Obesity also impacts some groups more than others, with non-Hispanic Black adults with the highest age-adjusted prevalence of obesity followed by Hispanic adults and non-Hispanic White adults.

Another study published this month in iScience points to increased physical activity and improvements in fitness levels as key to reducing the risk of obesity-related health conditions and mortality – even with the absence of weight loss.

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"We propose a weight-neutral strategy for obesity treatment on the following grounds: (1) the mortality risk associated with obesity is largely attenuated or eliminated by moderate-to-high levels of cardiorespiratory fitness (CRF) or physical activity (PA), (2) most cardiometabolic risk markers associated with obesity can be improved with exercise training independent of weight loss and by a magnitude similar to that observed with weight-loss programs, (3) weight loss, even if intentional, is not consistently associated with lower mortality risk, (4) increases in CRF or PA are consistently associated with greater reductions in mortality risk than is intentional weight loss, and (5) weight cycling is associated with numerous adverse health outcomes including increased mortality," the University of Arizona and University of Virginia authors wrote.

"Adherence to PA may improve if health care professionals consider PA and CRF as essential vital signs and consistently emphasize to their patients the myriad benefits of PA and CRF in the absence of weight loss," they said.