The effects of fiber, added to a high-starch diet, on glucose and insulin metabolism were studied in seven healthy men in a controlled environment. Diets rich in starch (carbohydrate provided 62% energy) contained either 16 g or 100 g of fiber from several sources of food and were given for 10-day periods. Two parameters of glucose control were measured: glucose metabolism during insulin-glucose infusions (seven subjects) and glucose production measured by infusing tritiated glucose tracer (five subjects). Three sets of studies were carried out in the sequence: low-fiber, high-fiber, low-fiber. The respective mean values (+/- SE) for glucose utilization were 6.70 +/- 1.4, 7.01 +/- 1.02, and 6.77 +/- 1.34 mg/kg X min. Analysis of variance failed to show a significant effect of dietary change. Values for basal glucose production were 2.0 +/- 0.1, 1.9 +/- 0.2, and 2.4 +/- 0.3 mg/kg X min with the low-fiber, high-fiber, and low-fiber diets, respectively, which were not significantly different. The one significant response to the high-fiber diet was a lowering in plasma cholesterol, the mean values for the seven subjects during the three periods being 154 +/- 12, 138 +/- 10, and 156 +/- 13 mg/dl (P less than 0.05).