BACKGROUND: Gastric bypass surgery seems to have an effect on glucose metabolism beyond what is mediated through weight reduction. The magnitude of this effect on fasting and post-challenge glucose levels remains unknown.

RESULTS: Morbidly obese subjects without known diabetes performed a 75 g oral glucose tolerance test before and after either gastric bypass surgery (n = 64) or an intensive lifestyle intervention programme (n = 55), ClinicalTrials.gov identifier NCT00273104. The age-adjusted effects of the therapeutic procedures and percentage weight change on fasting and 2-h glucose levels at 1 year were explored using multiple linear regression analysis. Mean (SD) serum fasting and 2-h glucose levels at baseline did not differ between the surgery and lifestyle groups. Weight-loss after surgical treatment and lifestyle intervention was 30 (8) and 9 (10) % (p < 0.001). At 1 year, fasting and 2-h glucose levels were significantly lower in the surgery group than in the lifestyle group, 4.7 (0.4) versus 5.4 (0.7) mmol/l and 3.4 (0.8) versus 6.0 (2.4) mmol/l, respectively (both p < 0.001). Gastric bypass and weight-loss had both independent glucose-lowering effects on 2-h glucose levels [B (95 % CI) 1.4 (0.6-2.3) mmol/l and 0.4 (0.1-0.7) mmol/l per 10 % weight-loss, respectively]. Fasting glucose levels were determined by weight change [0.2 (0.1-0.3) mmol/l per 10 % weight-loss] and not by type of treatment.

CONCLUSIONS: Gastric bypass surgery has a clinically relevant glucose-lowering effect on post-challenge glucose levels which is seemingly not mediated through weight-loss alone.