Increased weight-bearing load reduces biological body weight while sodium and water balances are unaffected

Increased weight-bearing load has previously been shown to reduce body weight in obese rodents, primarily by lowering food intake. However, it remains unclear whether increased loading also affects body weight through acute changes in extracellular water. This study aimed to determine whether increased weight-bearing load acutely produces negative sodium and water balances, as indicators of changes in whole-body extracellular sodium and water content. Diet-induced obese (DIO) rats were housed in metabolic cages with free access to hypotonic 0.5% NaCl solution and a low-sodium, high-fat diet. A novel, less traumatic loading method was used, where intra-abdominal capsules implanted two weeks earlier were either filled in vivo with wolfram granulate (Load group) or sham-filled (Control group), resulting in 15% and 2% increases in body weight, respectively. Compared to controls, increased weight-bearing load decreased body weight by 4.2% (95% CI -6.3, -2.0; P = 0.001) and reduced food intake by 1.6 percentage points (95% CI -2.6, -0.6; P = 0.003). No significant differences in sodium or water balance were observed. These findings suggest that load-induced reductions in body weight are not mediated by changes in whole-body extracellular sodium or water content, indicating that fluid homeostasis does not contribute to the homeostatic regulation of body weight.