OBJECTIVE: To evaluate the effects of a completely soluble ®ber on fasting and postprandial hormone levels, respiratory quotient (RQ) and subjective ratings of satiety during a controlled weight-loss program. DESIGN: In a ®ve-week prospective, randomized, double-blind study, a 3.3 MJ (800 kcal)ad diet was provided during a two-week wash-in period. Then, during the intervention weeks, separated by a one-week wash-out period, a 3.3 MJ (800 kcal) formula containing either 20 g ®ber or placebo daily, was given in a cross-over design and on days 1, 3 and 7 of the intervention weeks (weeks 3 and 5) measurements were taken after an overnight fast. SUBJECTS: 25 obese but otherwise healthy females (age: 46 AE 6 y, body mass index (BMI): 35 AE 6 kgam 2 ) were studied. MEASUREMENTS: Body weight; hungerasatiety ratings; glucose, insulin, cholecystokinin (CCK) and leptin concentrations; RQ during the intervention weeks. RESULTS: In the fasting state, the supplement had no effect on any of the measured parameters, including blood concentrations of glucose, insulin, CCK, and leptin, RQ and satiety ratings. In the 2 h postprandial period following the test meal, none of the measured parameters differed signi®cantly from that following the non-®ber-supplemented meal, except for the CCK response. CCK demonstrated an overall higher concentration after the ®ber-supplemented meal (P 0.007), even after adjustment for age, weight, height and treatment sequence. The postprandial peak in CCK also occurred earlier (at 15 min vs 30 min) after completion of the ®ber-supplemented meal. CONCLUSIONS: The results indicated that a hydrolyzed guar gum ®ber supplement produced a heightened postprandial CCK response, but did not alter other satiety hormones or increase satiety ratings, in either the fasting or the postprandial state.