Amino acid tolerance tests were performed before and after jejunoileal bypass surgery for morbid obesity to determine whether an enteric factor(s) originating in the bypassed jejunum and/or ileum potentiates the insulin response to oral nitrogen loading.
Preoperati vely a 30-gm. mixture of amino acids given orally evoked a larger peak insulin response than an intravenous load yielding comparable plasma amino acid elevations (82 ± 17 μU./ml. versus 38 ± 8 μU./rnl., p < 0.05). Four months after operation, basal insulin concentrations were 46 per cent (p < 0.001) of preoperative values. After surgery the response to intravenous amino acids was preserved when expressed as percentage increase above basal. In contrast, the peak increment and the percentage increase in insulin secretion after 30-gm. oral amino acid loading was significantly blunted (p < 0.005). A smaller amino acid load (16.5 gm.) was given preoperatively to duplicate the plasma amino acid elevations seen postoperatively with the 30-gm. mixture given by mouth. The insulin response postoperatively was still significantly lower (167 ± 33 per cent versus 98 ± 16 per cent, p < 0.05). After various explanations for the diminished postoperative insulin release following oral amino acid ingestion are considered, the results are best explained by the loss of an enteric insulinotrophic factor(s) normally released by the bypassed portions of jejunum or ileum in response to ingested protein.