Hanvold SE, et al. – This study investigated the association between selected plasma amino acids (AAs) and weight regain from 2 to 4 years following Roux–en–Y gastric bypass (RYGB). At 2 years post–operation, the following outcomes were observed: plasma aromatic AAs (AAAs), branched chain AAs (BCAAs), and total cysteine (tCys) were associated with BMI; plasma AAAs and BCAAs with weight loss from 0 to 2 years; tCys with weight regain from 2 to 4 years. These findings indicate that high tCys at 2 years may be used as a prognostic marker for future weight regain.