The jejunum of eight obese (mean weight 139 kg [306 lb 8 oz]) and four lean subjects (mean weight, 71 kg [156 lb 8 oz]) was perfused with isotonic glucose-sodium chloride solutions containing a nonabsorbable marker polyethylene glycol and, in four subjects, also folic acid, before and after a fi ve\x=req-\ day total caloric fast. Absorption of glucose, water, sodium, chloride, and folic acid and secretion of urea and potassium were determined. The absorptive capacity of the sub-stances measured was identical in normal\x=req-\