• New international study led by Tel Aviv University explains why exercise alone often leads to limited weight loss.
• Researchers found that as physical activity increases, the body adapts by reducing energy expenditure elsewhere.
• Participants improved fitness, reduced fat, and gained muscle mass — but did not significantly lose weight.
• The body responded by lowering resting metabolism and shrinking energy-consuming organs such as the liver and kidneys.
• Researchers emphasize: physical activity is essential for health, but meaningful weight loss typically also requires proper nutrition.
Many of us invest considerable time and effort in physical exercise with the goal of losing weight, yet the results often fall short of expectations. A new and groundbreaking international study, led by researchers from Tel Aviv University in collaboration with colleagues in the United States, provides a comprehensive scientific explanation for a question that troubles many: Why doesn't physical activity alone lead to significant weight loss, despite sustained training efforts?
The answer: As exercise increases, the body becomes more energy-efficient. Key energy-consuming organs, such as the liver and kidneys, shrink in size, while energy expenditure in other daily activities declines.
The study was led by Dr. Tzachi Knaan as part of his doctoral research in the laboratory of Prof. Yftach Gepner of the School of Public Health and the Sylvan Adams Sports Institute at Tel Aviv University. It was conducted in collaboration with Prof. Ed Melanson of the University of Colorado, Prof. Gal Dubnov-Raz, Prof. Carmit Levy, Prof. Chen Luxenburg, and additional students from the participating laboratories. The findings were published in the journal Communications Medicine.
The Body Burns Less Energy Elsewhere
Participants who were overweight were recruited for the study and completed a supervised aerobic walking program (4-5 times per week) over a 12-week period. Throughout the study, energy expenditure, activity levels, dietary intake, and even changes in body composition were precisely measured using advanced technologies. Despite burning hundreds of calories per week through physical activity and improving their overall fitness, participants did not lose weight. However, they did show improvements in body composition, with reductions in fat and increases in muscle mass.
So why didn't their body weight change? According to the researchers, the body simply adapted to the new situation. Among other findings, they observed a decline in resting metabolic rate, indicating that the body offset the increased activity by burning fewer calories at rest. In addition, participants became more efficient in their movements, expending less energy while performing the same daily activities.
Another surprising finding concerns the body's internal organs: the researchers found a decrease of about 5 percent in the volume of the liver and kidneys - major energy consumers - while the brain remained unchanged. The implication is that the body reduces its internal energy expenditure in order to compensate for the increased demands of physical activity.
However, contrary to common assumptions, the researchers found no significant increase in food intake. In other words, the observed compensation does not stem from eating more, but from physiological and metabolic adaptations that reduce total energy expenditure.
"The Human Body Is Extremely Sophisticated"
Dr. Tzachi Knaan explains: "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. 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 researchers note that the new study sheds light on one of the major questions in health and fitness and may even change the way we think about weight loss. Instead of relying on exercise alone, the most effective approach is likely to involve a proper combination of physical activity, nutrition, and an understanding of the body's mechanisms.
Prof. Yftach Gepner concludes: "This study offers a window into an evolutionarily rooted mechanism of energy compensation. In the past, when food was not available near the tribe, hunters and gatherers had to travel considerable distances, necessitating a reduction in energy expenditure in order to successfully return to the tribe with food. The central message is that physical activity alone is not designed to reduce weight. 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."
