BACKGROUND: Lacticaseibacillus paracasei strain Shirota (LcS) has shown benefits for stool consistency in populations with certain bowel complaints (e.g., constipation), but data in a United States population are limited. OBJECTIVES: This study evaluated the effects of LcS on hard or lumpy stools (HLS) in an otherwise healthy population that generally represents the United States adult demographic. METHODS: In this randomized, controlled, open-label study, 50 participants (41 females, 9 males) with HLS in ≥25% of bowel movements (BMs) during a 14-d run-in period were randomly assigned to receive either 80 mL fermented milk containing 8.0 × 10 RESULTS: The active group showed a significantly lower proportion of participants with HLS in ≥25% of BMs [odds ratio: 0.34
95% confidence interval (CI): 0.14, 0.80
P = 0.014] along with a higher proportion of participants with a reduced frequency of BMs with HLS from baseline (odds ratio: 2.86
95% CI: 1.03, 7.92
P = 0.043). The active group also demonstrated positive improvements in total PAC-QOL scores (P = 0.003). CONCLUSIONS: Daily consumption of LcS-containing fermented milk significantly reduced constipation symptoms and improved quality of life in an otherwise generally healthy United States adult population with a history of intermittent BMs producing HLS. The intervention was well tolerated, with no serious adverse events related to the product, suggesting its safety and potential as a dietary strategy for managing constipation. TRIAL REGISTRATION NUMBER: This trial was registered at clinicaltrials.gov as NCT06014008 (https://clinicaltrials.gov/study/NCT06014008).