BackgroundMuscle mass loss after gastrectomy is associated with a negative impact on quality of life (QOL) and long-term prognosis following gastric cancer treatment, especially in elderly patients. We conducted a prospective study to examine short-term changes in body composition and QOL after gastrectomy in elderly patients with gastric cancer who received exercise and nutritional therapies.MethodsPatients over aged 65 years of age who underwent gastrectomies for gastric cancer were enrolled in our study. Patients received exercise and nutritional therapies with branched-chain amino acid (BCAA)-rich supplements during 1 month after surgery. Body composition was evaluated using InBody S10 before surgery, and at 1 week and 1 month postoperatively. Other variables including QOL status (EQ-5D-5 L), serum albumin level, hand grip strength, and gait speed were evaluated at the same time.ResultsEighteen patients were analyzed. The mean loss of skeletal muscle mass index (SMI) was 4.6% (1 week) and 2.1% (1 month) compared to the preoperative period. QOL scores showed almost the same degree of recovery at 1 month after gastrectomy as preoperative scores. Serum albumin levels, hand grip strength, and gait speed decreased at 1 week and then increased at 1 month after surgery, similar to the changes seen in SMI.ConclusionsMultidisciplinary approaches play key role in the surgical treatment of elderly patients. Postoperative exercise and nutritional therapies with BCAA-rich supplements may benefit elderly patients after gastrectomy by reducing loss of SMI and decreases in QOL.Trial registrationUMIN Clinical Trials Registry; UMIN000034374 (registration date: 10/10/2018).Supplementary InformationThe online version contains supplementary material available at 10.1186/s12893-023-02086-4.