An extruded, directly expanded, high fiber reduced calorie food product, such as a ready-to-eat (RTE) cereal or sweet or savory snack, is produced at high production rates without substantial loss of extrusion functionality and extrudability by replacing a substantial portion of at least one flour with a gelatinized, enzyme-resistant starch type III ingredient or bulking agent as a reduced-calorie, high fiber flour replacer. The resistant starch type III ingredient or bulking agent contains an enzyme-resistant starch type III having a melting point with an endothermic peak temperature of at least about 140° C., and may have a water-holding capacity of less than 3 grams water per gram of the starch-based bulking agent. The total dietary fiber retention of the gelatinized, starch-based bulking agent may be at least about 90% by weight after the extrusion using a die temperature of least about 100° C., and a die pressure of at least about 150 psig.