In vivo digestibility experiments using the cannulated pig model were used to study the digestion of fiber from diets formulated with high concentrations of corn co-products and fed to growing pigs. Experiment 1 was conducted to measure the effect of increasing levels of insoluble-low fermentable fiber from corn in the diet, using corn bran with solubles from the corn-ethanol distillation industry (CB-S), on digestibility of energy, fiber, and AA, and hindgut fermentation of fiber in diets fed to growing pigs. Results indicated that increasing fiber from corn lowered (P < 0.01) the apparent ileal digestibility (AID) of GE, DM, CP, and all indispensable amino acids except Arg, but did not affect (P > 0.05) the AID of neutral detergent fiber (NDF) or total dietary fiber (TDF). Increased fiber from corn also reduced the apparent total tract digestibility (ATTD) of GE, DM, CP, NDF, and TDF (P < 0.01). A decrease (P < 0.01) in hindgut fermentability of NDF (19.6 to 6.4%) and TDF (21.9 to 9.7%) was observed with the dietary inclusion of CB-S. Two subsequent 28-d growth trials were conducted in Exp. 2 to measure the effects of increasing dietary fiber from CB-S in 2 sets of 7 diets formulated either with declining (growing phase: 2,387 to 2,133 kcal NE/kg; finishing phase: 2,499 to 2,209 kcal NE/kg) or constant dietary NE (growing phase: approximately 2,390 kcal NE/kg; finishing phase: approximately 2,500 kcal NE/kg), on growth performance and apparent total tract ATTD of energy in 70 growing (BW = 48.9 kg; n = 10) and 70 finishing (BW = 102.0 kg; n = 10) pigs. Results showed that increasing fiber with declining diet NE lowered BW, ADG, and G:F (P < 0.05) in growing and in finishing pigs. When NE was held constant, as fiber increased, BW and ADG were unaffected (P > 0.05) in growing and finishing pigs, and G:F was unaffected in finishing pigs but improved in growing pigs (P < 0.05) with increasing dietary fiber. In both growing and finishing pigs, ADFI was not affected (P > 0.05) by the increased fiber from corn, regardless of the NE content of diets. Experiment 3 was conducted to determine the effects of addition of reduced oil distillers dried grains with solubles (DDGS-RO) and soybean oil (SBO) on dietary Lys, acid hydrolyzed ether extract (AEE), and NDF digestibility in corn-based diets fed to growing pigs. Results showed that the AID of Lys was not affected by SBO concentration (P > 0.05), but DDGS-RO inclusion showed a quadratic effect (P < 0.001). An interaction between DDGS-RO and SBO on the AID (P = 0.003; R2 = 0.68) and ATTD (P = 0.004; R2 = 0.79) of AEE, as well as on the AID (P = 0.037; R2 = 0.53) and ATTD (P = 0.004; R2 = 0.36) of NDF was observed. It was concluded that DDGS-RO increased the digestibility of AEE, and decreased the digestibility of NDF, but the effect was modulated by SBO. Soybean oil increased the digestibility of AEE but the effect was modulated by DDGS-RO, and increased the AID of NDF in diets without DDGS-RO. The AID of Lys decreased with DDGS-RO and was not affected by addition of SBO. Experiment 4 was conducted to determine a best fitting dietary fiber component to estimate the effect of dietary fiber concentration on the digestibility of energy, fiber, and AA, and energy value of 9 corn co-products. It was observed that the arabinoxylan and NSP xylose residue were the dietary fiber components that best explained variation due to dietary fiber concentration and, with the exception of AID of Lys, can be used to predict the digestibility of energy and dietary fiber, and the DE and ME values in corn co-products. In conclusion, dietary fiber from corn co-products has an intermediate digestibility and does not affect digestibility of the other nutrients in the diet. The ability of pigs to digest fiber from corn origin is modulated by the fat concentration of the diet. The xylose and arabinoxylan concentrations in corn co-products better explain the variation in digestibility of dietary fiber and energy than most of the commonly used fiber procedures.