Enteral nutrition (EN) or enteral tube feeding is an effective means of nutritional support for individuals who do not or cannot eat adequately. Patients restricted to EN frequently suffer from abnormal bowel function which affects their intestinal bacteria and impacts quality of life. Dietary fibers have a variety of physiological benefits; fibers that provide fecal bulk promote regular bowel movements while fermentable fibers are utilized by gut bacteria to produce short chain fatty acids (SCFA) and gas in the process. SCFAs positively influence gut health, gut transit time, and fecal moisture while gas may reduce the tolerance of fibers. Fortification of EN with a blend of fibers with various physicochemical properties more closely represents a normal diet, and may maximize physiological benefits. Moreover, using a blend of fibers with different rates of fermentation may minimize gas and bloating commonly associated with highly fermentable fibers. Batch in vitro systems allow fiber fermentation modeling without absorption and may help to estimate potential health benefits and gastrointestinal tolerance of fiber in vivo. Well controlled, blinded and randomized intervention human studies are the "gold standard" for human nutrition research. The primary aim of this project was to conduct a human clinical trial utilizing a fiber blend fortified EN product. The secondary aim was to relate the human study findings to the in vitro fermentation profiles of the fiber blend and its individual components. The objective of the human study was to compare the effects of a fiber blend fortified enteral formula (FB, 15 g/L), a fiber-free formula (FF) and habitual diet on bowel function, fecal bacteria and quality of life. The fiber blend consisted of a 50:50 insoluble:soluble ratio of fructo-oligosaccharides (FOS), inulin, gum acacia and outer pea hull fiber. In a randomized, double-blind, crossover design, 20 healthy subjects consumed both FF and FB for 14 days with a 4 week washout. Fecal samples were collected the last 5 days of each period and assessed for fecal output, whole gut transit time (WGTT), and major bacterial groups. Subject gastrointestinal quality of life index (GIQLI) and tolerance were also measured. On formula diets, 5-day fecal output decreased by more than 55% from habitual diet, but was 38% higher on FB than FF (p=0.0321). WGTT was approximately 1.5 times longer on formula diets than habitual diet (p<0.0004). Total bacteria declined from habitual diet on FF (p<0.004), but not on FB. Numbers of bifidobacteria and lactobacilli declined from habitual diet on both formula diets, but bifidobacteria was higher on FB compared to FF (p<0.0001). Bacteroides and clostridia numbers did not change between diets. GIQLI and incidence of gas symptoms did not differ between formulas. The objectives of the second project were to compare the in vitro fermentation profiles of FOS, inulin, gum acacia, and pea fiber alone or blended using a 24 h batch model and relate these finding to the human study results. For the in vivo measurements, stool samples were collected to measure pH and SCFA. Tolerance was also measured. The in vitro fermentation of the fiber blend resulted in a delayed pH decrease and gas and SCFA production compared to the FOS and inulin. Human samples had higher total SCFA on the fiber formula compared to the fiber free formula (p=0.029), and both formulas yielded lower SCFA than habitual diet (both p<0.0001). Mean fecal pH for both formulas was 7.5; higher than habitual diet pH 6.5 (p<0.0001). No differences in gas/bloating were found between any diet. By blending fibers, a slower fermentation was observed in vitro and was well tolerated in human subjects. Fiber addition to enteral formula increases fecal short chain fatty acids which may reflect increased fermentation.