The Roux-en-Y gastric bypass (RYGB) procedure is one of the most efficient bariatric surgeries, inducing rapid weight loss and improvements in comorbidities of obesity. Barataud et al. performed duodenal-jejunal bypass (DJB), which is equivalent to RYGB without size restriction of the stomach, in mice and found that DJB induces dramatic loss of fecal matter in obese mice fed a high fat, high sugar diet. In obese mice, this could be sufficient to explain the lasting weight loss and associated improvements in glucose control associated with DJB. Fecal energy loss should no longer be underestimated in bypass surgery studies in the mouse.