This prospective, double-blind, randomized, placebo-controlled clinical trial will investigate the effects of 4-week B. lactis HN019 supplementation on whole gut transit time and gastrointestinal symptoms in adults with functional constipation. Participants who meet all study entry criteria will enter a 2-week run-in period. Following successful completion of the run-in period, subjects will be randomized to 4-week supplementation with B. lactis HN019 (10 billion cfu), B. lactis HN019 (1 billion cfu), or placebo. The primary outcome of whole gut transit time will be evaluated with abdominal x-rays on days 0 and 28. Secondary outcomes include IBS-specific questionnaires (administered on days 0 and 28); abdominal pain severity, bowel movement frequency, stool consistency, and adverse events (evaluated daily throughout the study); and overall product satisfaction (evaluated at day 28). Physical activity questionnaires and 24-hour food recalls will be completed periodically during the trial.