While physical activity offers substantial health benefits, its effectiveness in promoting weight loss has long been debated. Recent analyses provide compelling evidence, explaining why increased exercise often results in less weight reduction than anticipated based on calories burned.
Individuals who engage in more exercise do indeed burn additional calories. However, they typically do not lose as much weight as would be predicted from this increased energy expenditure. A thorough examination of 14 human trials now reveals that our bodies actively compensate for this extra calorie burn by conserving energy in other physiological functions.
This compensatory effect is notably amplified when individuals also reduce their caloric intake alongside increasing their exercise. In such cases, the body’s energy conservation can entirely offset the calories expended during workouts. Essentially, while dietary changes alone reliably lead to weight loss, combining dieting with exercise may not yield any additional reduction in weight.
“The critical factor is that when you combine exercise with dieting, your body significantly increases its compensatory mechanisms,” explains Herman Pontzer from Duke University in North Carolina. “Exercise remains beneficial for overall health, but its impact on weight loss is diminished by this response.”
Pontzer’s earlier research with the Hadza hunter-gatherers in Tanzania yielded surprising results. Despite their highly active lifestyles, their overall daily energy expenditure was comparable to individuals with sedentary jobs. This observation led Pontzer to propose in 2015 that human physiology may have evolved to regulate energy expenditure, compensating for increased physical activity by reducing energy use in other bodily processes.
While supporting studies exist, the scientific community has not universally accepted this concept. Pontzer and Eric Trexler, also at Duke University, have revisited existing studies conducted for unrelated purposes. By analyzing their findings, they sought independent evidence of this energy compensation mechanism. Pontzer highlights that these studies were not designed with compensation in mind, thus minimizing potential bias.
Their analysis focused on 14 trials involving approximately 450 participants. The relatively small sample size is attributed to the complex and costly methods required to accurately measure total daily energy expenditure. The researchers discovered that, on average, participants’ total energy expenditure increased by only about one-third of the amount theoretically expected from their increased exercise levels.
“For example,” Pontzer illustrates, “if individuals engaged in enough exercise to burn an extra 200 kilocalories per day, their total energy expenditure in these studies typically rose by only around 60 kilocalories.”
However, significant variability was observed. For participants who maintained their usual eating habits, total energy expenditure increased by approximately half of the expected amount. In contrast, individuals who reduced their food intake while simultaneously increasing their exercise often showed no increase in total energy expenditure. “They might be performing,” Pontzer notes, “200 kilocalories of exercise daily, but this activity level is not reflected in their overall energy output.”
The type of exercise also appeared to influence the compensation effect. Aerobic activities, such as running, were associated with energy conservation. However, resistance training, like weightlifting, actually led to an energy expenditure exceeding expectations. For instance, individuals burning an extra 200 kilocalories through weightlifting showed an increase of 250 kilocalories in their total daily energy expenditure.
The Impact of Different Exercise Types
Pontzer cautions that accurately measuring the energy expenditure associated with weightlifting presents challenges, requiring a degree of circumspection in interpreting these results. He speculates that the additional energy expenditure might be related to the physiological processes involved in muscle repair and growth following resistance training.
“Previously, I believed the type of exercise was inconsequential,” Pontzer admits. “This finding is highly surprising and opens up new avenues of understanding that were previously unknown.” He also observes that participants engaging in weightlifting gained muscle mass but showed minimal fat loss, reinforcing that it is not an optimal strategy for weight reduction.
Understanding the Mechanisms of Compensation
The question of why total energy expenditure doesn’t proportionally increase with aerobic exercise remains a key point of inquiry. The analysis suggests that the body compensates by reallocating energy, reducing the resources allocated to other background metabolic functions. This could include a decrease in resting metabolic rate, particularly during sleep, in response to sustained aerobic activity.
“We are altering the functioning of our various organ systems [after exercising],” explains Pontzer. “By identifying precisely which systems are affected, we can gain a more profound understanding of how exercise impacts our bodies and why some individuals appear to derive greater benefits from it than others.”
Alternative Perspectives and Future Research
While Pontzer views these findings as strong evidence of compensation, some researchers remain unconvinced. Dylan Thompson from the University of Bath in the UK points to meta-analyses suggesting that aerobic respiration does not significantly alter resting metabolic rates.
Javier Gonzalez, also from the University of Bath, highlights significant limitations within the analyzed studies. He suggests that the additional exercise individuals were asked to perform might have supplanted other forms of physical activity, such as gardening. This substitution, he posits, could account for the observed lower-than-expected increases in overall energy expenditure.
Pontzer counters that these alternative explanations can be ruled out in certain included studies. Furthermore, he notes that similar compensatory effects have been observed in animal research, lending further support to the human findings. Nevertheless, Thompson and Gonzalez advocate for more rigorously designed studies to confirm these results.
“We genuinely require meticulously planned randomized controlled trials in human subjects,” Thompson emphasizes.
Journal Reference: Current Biology DOI: 10.1016/j.cub.2026.01.025