The constituents of weight loss following bariatric surgery are poorly known. There is an expectation of a limited loss of lean body mass (LBM), and a significant loss of fat mass (FM) as well as muscle mass (MM), which could lead to functional loss and metabolic impairment. This prospective study analysed the determinants of MM changes after Roux-en-Y gastric bypass and sleeve gastrectomy.

The study cohort comprised 114 consecutive candidates for bariatric surgery referred to a bariatric surgery centre. Using DEXA, the subjects’ body composition was assessed before, and three and 12 months (n=92) after, the surgery, along with their biological status. The main study outcome was changes in MM.

At three months, patients had lost 20.3kg, made up of 41% LBM and 59% FM. The contribution of MM to weight loss was 16.4%. Cluster analysis showed that 52 patients lost<15% of their weight as MM, while 62 patients lost>15% as MM. At 12 months, patients had lost 37kg, made up of 70% FM and 30% LBM. At this time, only 27 patients lost>15% of their weight as MM. The determinants that were negatively and independently associated with MM changes at three months were FM loss and changes in glycaemia and thyroid-stimulating hormone ([TSH]; thyrotropin) before surgery, whereas change in glycaemia was the only 12-month determinant associated with MM changes.

Two phenotypes – one with muscle wasting and the other with acceptable muscle loss – with a threshold of 15% and very few predictive factors were identified by this study.