The effect of bariatric surgery on gravitational loading and its impact on bone mass

Abstract

Mechanical unloading associated with weight loss might be one of the main causes for bariatric surgery (BS) induced bone loss. However, no study has tested this hypothesis through objectively measured accelerometry-derived gravitational loading. We aimed to assess how gravitational loading changes following BS and how this correlates with bone mass losses. Twenty-one patients submitted to gastric bypass were assessed before, 1, 6 and 12months after BS for areal bone mineral density (BMD), calciotropic hormones, sclerostin, body composition and daily physical activity. Gravitational loading was determined as the sum of ground reaction forces assessed by accelerometer which considered the interaction between weight and daily ambulation. Mechanical stimuli promoted through the significant increase in steps number counterbalanced the gravitational loading decreases derived from the significant weight loss after BS. Gravitational loading volume decreased between pre-BS and 1month post-BS (-2215kN·d-1; p=.023), but remained stable between 6 and 12months post-BS, despite decreases on hip (-7.0%; p<.001), femoral neck (-8.8%; p<.001) and lumbar spine (-5.2%; p<.001) BMD. Serum sclerostin increased from pre-BS to 1month post-BS (+0.118ng·mL-1; p=.021), returning to pre-BS levels 6months after surgery. Neither vitamin D nor parathyroid hormone were affected by BS. Weight variation was a predictor of BMD decreases at total hip (R2=0.06; p=.026) and femoral neck (R2=0.12; p=.022), whereas daily gravitational loading volume was not. Fat and lean mass changes were also predictors of BMD decrease at total hip (R2=0.05; p=.031) and femoral neck (R2=0.14; p=.010), respectively. Our findings suggest that gravitational loading only decreased during the first month after surgery remaining stable thereafter, and these changes do not seem to explain BS-induced bone loss. The association between weight and bone loss seems to result from other physiological aspects, fat and lean mass loss, rather than from gravitational loading decrease.