The exact mechanism for the dramatic effect of surgical procedures for obesity on type 2 diabetes remains unknown.

Five diabetic morbidly obese patients and 5 nondiabetic morbidly obese patients undergoing biliopancreatic diversion were compared retrospectively. A 75-g trans-gastrostomy glucose tolerance test was administered on the fifth day postoperatively and a standard 75-g oral glucose tolerance test was performed on the seventh day postoperatively, with blood sampling for measuring plasma glucose and insulin levels at 0, 30, 60, 90, 120, and 180 minutes.

All 5 diabetic patients were shown, at the same time, still to have diabetes or an impaired glucose tolerance test when tested through the biliopancreatic limb but patients were normal when tested through the new alimentary channel. No significant difference was seen in the nondiabetic patients.

Biliopancreatic diversion can completely normalize the glycemic cycle in type 2 diabetes patients in the week after the intervention, even before any significant weight loss has occurred. The surgical procedure itself, designed to exclude most of the stomach, duodenum, and part of the jejunum, directly affects carbohydrate homeostasis.