Your body may be cancelling out your workout, study finds
Baku, May 19, AZERTAC
Many people spend hours exercising in the hope of losing weight, only to step on the scale and see little change, according to TPS-IL. A new Israeli-U.S. study has uncovered why: the body adapts to increased physical activity by reducing energy use in other systems, effectively “offsetting” the calories burned during exercise, Tel Aviv University scientists announced on Monday.
In a study led by Dr. Tzachi Knaan of Tel Aviv University in collaboration with researchers from the University of Colorado, participants who were overweight took part in a supervised walking program four to five times a week for 12 weeks. Researchers closely tracked energy expenditure, diet, and body composition using advanced measurement tools.
Although participants improved their fitness and burned additional calories through exercise, they did not lose significant weight. However, they did show clear improvements in body composition, including reduced fat and increased muscle mass. According to the researchers, the body responded to the exercise by becoming more energy efficient. Moreover, resting metabolism dropped, meaning participants burned fewer calories while at rest, and they also used less energy during everyday activities.
But the most surprising finding was that key internal organs also changed. The liver and kidneys—both major consumers of energy in the body—decreased in size by about five percent, while the brain remained unchanged. Importantly, the study found no significant increase in food intake, suggesting that participants did not compensate by eating more. Instead, the “offset” came from internal physiological changes.
“The central message is that physical activity alone is not designed to reduce weight,” said Professor Yftach Gepner, in whose laboratory the research was conducted.
“For years, science has shown that weight loss resulting from exercise is more limited than commonly expected, and in this study, for the first time, the mechanism of organ shrinkage that explains this phenomenon was observed. It is important to emphasize that physical activity has unique and substantial health benefits, but it is not intended on its own to lead to weight loss,” he explained.
The findings suggest that weight-loss strategies should rely on a combined approach that includes both physical activity and careful attention to nutrition. If the body compensates for increased activity by reducing overall energy expenditure, dietary intake and calorie control are likely to play a more decisive role in achieving meaningful and sustained weight loss than exercise by itself.
The study also highlights the importance of shifting how success is measured in exercise and obesity treatment programs. Although participants did not lose significant weight, they did improve body composition, gaining muscle and reducing fat. This suggests that clinicians and trainers should place less emphasis on scale weight and more on indicators such as fat percentage, muscle mass, and metabolic health.
Additionally, the study’s findings on metabolic adaptation—such as lower resting energy expenditure and increased movement efficiency—help explain why some individuals experience plateaus during weight-loss efforts and may benefit from periodically adjusting their training or nutrition plans.
“The human body is extremely sophisticated—it knows how to adapt itself to maintain balance. Physical activity is very important for good health, but when it comes to weight loss, it is not always sufficient on its own,” said Knaan. “The implication for the public is clear: exercise is a vital component of a healthy lifestyle. It improves fitness, reduces fat, and improves health indicators, but to see significant weight loss, it must be combined with the appropriate nutrition.”
The study was published in the peer-reviewed Communications Medicine.
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