Background: The strict long-term weight maintenance in good nutritional conditions observed after biliopancreatic diversion (BPD) needs to be explained. Materials and Methods: 15 operated subjects were maintained at an isoenergetic and isonitrogenic diet as similar as possible to their usual diet. Apparent absorption (AA) of energy, fat, nitrogen and calcium was calculated subtracting the fecal content, measured directly, from the oral intake, derived from tables. The alimentary protein absorption was directly determined by I-125 albumin oral administration. Results: Mean AA for energy and fat was 57% and 32%, respectively; AAs were unrelated as absolute value and negatively associated as percent of the intake with the energy and fat intake. I-125 intestinal absorption was 73%, while nitrogen percent AA was 57%, indicating higher than normal loss of endogenous nitrogen. Calcium AA was 551 mEq/day, 26% of the intake. A positive correlation between nitrogen and calcium AA as absolute values and alimentary intake was observed, while there was no correlation when AA were considered as per cent of the intake. Conclusions: For energy and fat, an increase in intake corresponds to an increase in percent malabsorption, so that the absolute amount absorbed tends to remain constant, accounting for the excellent weight maintenance observed following BPD. This was confirmed by a long-term hypernutrition study after BPD. On the contrary, for nitrogen and calcium, the percent absorption tends to remain constant when intake varies, so that an increase in alimentary intake results in an increased absolute amount absorbed.
