Volunteers will be recruited among the candidates to a gastric bypass, and will be studied before and 3 months after surgery. They will have on each occasion 2 random meals, corresponding to 30g glucose, one made of starch the other made of glucose, both naturally labeled with 13C. The digestion of starch will be assessed with the increase in the plasma 13C-glucose tracer. Plasma samples will be collected for 3 hours.