A new Tel Aviv University-led study has found that exercise alone may fail to produce major weight loss because the body compensates for added physical activity by lowering energy use elsewhere, including through changes in organ size and resting metabolism.
The study, published in Communications Medicine, was led by Dr. Tzachi Knaan as part of his doctoral work in the laboratory of Prof. Yftach Gepner at Tel Aviv University’s School of Public Health and the Sylvan Adams Sports Institute. Researchers from the University of Colorado and other Israeli laboratories also took part.
The findings address a common frustration among people trying to lose weight: regular exercise improves health and fitness, but often does not produce the expected drop on the scale.
Research team members (L-R) Prof. Yftach Gepner, Lior Friedmann and Dr. Tzachi Knaan. (Tel Aviv University)
Researchers recruited overweight participants for a 12-week supervised aerobic walking program, with sessions held four to five times a week. During the study, they tracked energy use, physical activity, food intake, body composition, and metabolic changes using advanced measurement tools.
Participants burned hundreds of additional calories each week and improved their fitness. Their fat mass decreased, and their muscle mass increased, but their overall body weight did not fall.
The researchers found that the body adjusted to the added exercise by becoming more efficient. Resting metabolic rate declined, meaning participants burned fewer calories while at rest. They also used less energy during ordinary daily movement.
One of the study’s more striking findings was a roughly 5% decrease in the volume of the liver and kidneys, organs that consume large amounts of energy. The brain did not show a similar change.
The researchers also found no meaningful rise in food intake, suggesting the stalled weight loss was caused not by eating more but by metabolic adaptation.
Dr. Tzachi Knaan said: “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 study reinforces the view that exercise remains essential for health but is most effective for weight loss when paired with dietary changes.
