The purpose of this study is to understand the potential of protein and psyllium fiber to manipulate gut microbial densities, intestinal permeability, and inflammation in humans.

Inclusion Criteria:

• Adults ≥ 18 years old
• On a baseline diet characterized by: Fiber intake of </= 20g/day, > 18% of daily calories from protein

Exclusion Criteria:

• Prior history of gastrointestinal surgeries (except appendectomy and cholecystectomy)
• Known diagnosis of inflammatory bowel disease (Crohn's or ulcerative colitis), microscopic colitis, celiac disease, or other inflammatory GI condition
• Use of tobacco products within the past 6 months (since nicotine may affect intestinal permeability)
• Use of NSAIDs or aspirin and unable or unwilling to stop taking two weeks prior to permeability test and for the duration of the study
• Use of osmotic laxatives and unable or unwilling to stop taking one week prior to permeability test and for the duration of the study
• Use of oral corticosteroids and unable or unwilling to stop use of oral corticosteroids within the previous 6 weeks and for the duration of the study
• Multiple dietary restrictions or unable/unwilling to alter dietary protein or dietary fiber.
• Unwilling to stop ingestion of alcohol and artificial sweeteners such as Splenda™ (sucralose), Nutrasweet™ (aspartame), sorbitol, xylitol, lactulose, or mannitol 2 days before and during the permeability testing days, e.g. foods to be avoided are sugarless gums or mints and diet beverages
• Unwilling to stop stenuous exercise (running >5 miles or equivalent) one week prior to the permeability tests
• Bowel preparation for colonoscopy less than one week prior to completion of the first stool kit and permeability test and flexible sigmoidoscopy and upper endoscopy
• Pregnancy or plan to become pregnant during the study time frame
• Vulnerable adult
• Known bleeding disorders or takes medications that increase risk of bleeding from mucosal biopsies
• Use of oral antibiotic(s) within the past 4 weeks (can be enrolled after 4-week washout period)
• Use of commercial probiotic formulations and unwilling to stop for the duration of the study
• Diagnosis of diabetes
• Any other disease(s), condition(s) or habits(s) that would interfere with completion of study, or in the judgment of the investigator would potentially interfere with compliance to this study or would adversely affect study outcomes

Principal investigator and analysts will be blinded. The participant will not be told to which diet they are randomized.