The New Challenge After Surgery: More Than Just Losing Weight

In recent years, bariatric surgery has become one of the most effective tools for treating obesity, with significant weight loss, improvements in diabetes, reduced cardiovascular risk and even longer life expectancy. But alongside the impressive metabolic success, there is a known and less-discussed side effect, loss of muscle mass and decreased bone density during rapid weight loss. Dr. Osnat Raziel, a specialist in surgery and director of the Multidisciplinary Center for Obesity Treatment at Assia Medical, Assuta Ramat HaHayal, explains.

A new and important study published in 2026 in the prestigious Journal of Cachexia Sarcopenia and Muscle examined for the first time in a comprehensive way which type of physical activity is most effective for preserving muscles and bones after bariatric surgery. The study provides, for the first time, controlled data on the effect of different types of training on body composition, muscle mass, bone density and physical function after bariatric surgery. The findings emphasize that not every type of physical activity affects the metabolic and musculoskeletal adaptation processes that occur after rapid weight loss in the same way. The study, conducted as part of the POWER BARIATRIC project, was led by a broad Israeli and international group of researchers that included doctors, dietitians and exercise physiologists, among them Ariela Goldenshluger, Lior Friedman, Tamir Turgeman, Roy Yavetz, Osnat Raziel, David Guitayen, Irit Marcus, Hassan Kis, Ron Sternfeld, Estherina Trechtenberg, Gal Dubnov Raz, Ilinat Meler, Ilan Youngster, David Feld, Roy Amadi, Andrei Kedar, Edward Livingston, Nasser Sakran, Dror Dicker and Yiftach Gepner.

Bariatric surgeries are now considered the most effective treatment for severe obesity, with significant weight loss and dramatic improvement in metabolic disease. However, in recent years, literature has increasingly shown that a significant share of the weight loss is not only fat loss but also loss of lean body mass, reduced muscle strength and harm to bone density. The clinical significance is broad. Loss of muscle mass may lead to a lower resting energy expenditure, impaired function, increased frailty and a higher risk of regaining weight. At the same time, bone loss may increase the risk of osteopenia, osteoporosis and fractures in the years after surgery.

Study design. The researchers recruited 58 patients aged 18 to 65 after bariatric surgery. About 70% of the participants were women, and the mean BMI at the start of the study was 41.7 ± 4.4 kg/m². Of 443 candidates screened for eligibility, 58 were randomized into four groups:

• Aerobic training • Resistance training • Combined aerobic and strength training • Control group

The training program lasted 26 weeks and included three supervised workouts per week, with intensity gradually increasing over the intervention period. The completion rate was especially high, with 91.3% of the participants included in the final analysis. The primary endpoint was change in fat free mass, measured by DXA. Secondary endpoints included fat mass, bone density in the hip, femoral neck and lumbar spine, blood markers of bone breakdown and physical function.

Why is strength training so important? Muscle is a living, dynamic tissue. When the body is in a significant calorie deficit, as happens after bariatric surgery, it may break down muscle tissue as part of the weight loss process. Strength training sends a clear biological signal to the brain and the muscles, the muscle is still needed. As a result, muscle protein-building mechanisms are activated, and the body preserves muscle mass better. In addition, mechanical load on the bones helps reduce the decline in bone density.

And what about aerobic activity? Aerobic exercise such as walking, cycling or swimming contributes to cardiovascular health, improves endurance and helps continue fat burning. However, when done alone, its ability to prevent muscle and bone loss is more limited than resistance training. Therefore, the combination of the two types of training appears to be the optimal solution.

Combined training preserved muscle mass best. One of the study's most striking findings was the advantage of combined training. In the combined training group, the average decrease in fat free mass was 5.1 ± 2.8 kg, compared with 7.7 ± 2.8 kg in the control group. The effect size was 0.60, with a confidence interval that did not cross zero, a finding indicating a significant benefit of the intervention. In addition, the combined training group also achieved a greater reduction in fat mass. The relative decrease in fat mass reached 11.9% ± 5.7%, compared with only 10.1% ± 4.9% in the control group. This finding is especially important because it refutes the concern sometimes held by patients and medical teams that strength training “slows weight loss.” In practice, the combination of aerobic and resistance exercise not only preserved muscle mass but also improved overall body composition.

Aerobic training best protected the bones. As for bone health, aerobic training delivered the most impressive results. Loss of total hip bone density was only 3.3% in the aerobic training group, compared with 5.4% in the control group. In absolute terms, this was a decline of 0.037 g/cm² versus 0.058 g/cm², respectively. The effect size was 0.69. A moderate protective effect was also shown at the femoral neck, with an effect size of 0.30. By contrast, no significant differences were found in lumbar spine bone density.

A reduction in bone breakdown markers. The researchers also measured the bone turnover marker CTX, a biochemical marker of bone breakdown. In the control group, CTX levels increased by 196.4% after surgery. By contrast, the increase was lower in the aerobic training group, 157.8%, and in the combined training group, 161.8%. This finding reinforces the hypothesis that physical activity can moderate the accelerated rate of bone breakdown that occurs after bariatric surgery.

Strength training best preserved grip strength. When grip strength, one of the standard measures for assessing muscle strength and function, was examined, the resistance training group showed the best results. The effect size for preserving grip strength reached 0.77, the strongest finding observed in the study in the area of muscular function. In addition, the researchers found a positive association between protein intake and preservation of grip strength, reinforcing the importance of high-quality nutritional follow-up after surgery.

Clinical implications for doctors treating obesity. The study's central message is clear, the success of bariatric surgery is not measured only by the number of kilograms lost. The real challenge is to preserve muscle mass, protect the skeleton and improve functional capacity over time. The new data indicate that combined aerobic and resistance training is the most effective strategy for preserving body composition, while aerobic training provides especially significant protection for bone density. In an era in which obesity treatment is shifting from a focus on weight alone to a focus on body composition, quality of life and overall metabolic health, structured physical activity appears to be an integral part of bariatric treatment, not just an accompanying recommendation. The current study provides a significant scientific basis for building personalized rehabilitation programs after surgery that combine strength training, aerobic activity and nutritional support, in order to achieve not only weight loss but also long-term health.