Background Laparoscopy-assisted distal gastrectomy (LADG) for gastric cancer may prevent the loss of body weight and lean body mass resulting from reduced surgical stress in comparison to open distal gastrectomy (ODG). A multicenter phase III trial conducted by the Japan Clinical Oncology Group (JCOG0912 trial) was performed to confirm the non-inferiority of LADG to ODG for stage I gastric cancer in terms of relapse-free survival. Methods This study was performed as a single-institution exploratory analysis using the data of the patients from our hospital who were enrolled in the JCOG0912 phase III trial. Body weight and lean body mass were evaluated using a bioelectrical impedance analyzer within 1 week before and at 1 week, 1 month, and 3 months after surgery. Results One-hundred six patients were randomized to undergo ODG (54 patients) or LADG (51 patients). Body weight loss at 1 week, 1 month, and 3 months was -3.0%, -4.9%, and -5.4%, respectively, in the ODG group and -2.7%, -4.3%, and -5.7%, respectively, in the LADG group; the differences were not statistically significant (p = 0.330, 0.166, and 0.656, respectively). Lean body mass loss at 1 week, 1 month, and 3 months was -2.8%, -4.1%, and -2.3%, respectively, in the ODG group and -2.7%, -2.9%, and -3.0%, respectively, in the LADG group; the differences were not statistically significant (p = 0.610, 0.413, and 0.925, respectively). Conclusions The laparoscopic approach did not attenuate the loss of body weight and lean body mass in comparison to patients who underwent open distal gastrectomy for gastric cancer.