The aim of this study is to determine the effect of daily consumption of Bifidobacterium animalis ssp lactis B94 (B. lactis B94) on stool frequency, gastrointestinal (GI) transit time, and gastrointestinal symptoms in adults with Prader-Willi syndrome. Participants (18-75 years old, n=36) will be recruited and enrolled in a 20-week randomized, placebo-controlled, crossover study. Study participants will be randomized to B. lactis B94 or placebo each for a 4-week period, preceded by a 4-week baseline and followed by 4-week washouts. Participants will complete daily questionnaires about stool frequency and stool form (transit). Dietary intake will also be assessed. A total of 5 stools (one in each period) will be collected for exploratory microbiota analysis. It is hypothesized that the B. lactis B94 will increase stool frequency, decrease the percentage of slow transit stools, and improve GI symptoms

Probiotics have been shown to improve symptoms of constipation by decreasing gastrointestinal transit time, increasing stool frequency, and normalizing stool form. A 20-week randomized, double-blinded, placebo-controlled crossover study will be carried out. Participants will complete a 4-week baseline period during which stool frequency, stool consistency (transit time), and gastrointestinal symptoms will be collected by paper questionnaire, and participants will collect a single stool. Dietary intake data (3-day) will be obtained during the baseline period. Participants will be randomized on or about day 29 and will consume Bifidobacterium animalis ssp lactis B94 (B. lactis B94) or placebo for 4 weeks, followed by a 4-week washout, 4 weeks on the alternative, and second 4-week washout. Participants will be randomized by sealed envelope method, prepared by a UF faculty member not affiliated with the study.

During the intervention and washout periods, participants will complete daily questionnaires to assess stool frequency and form (Bristol Stool Form Scale). Furthermore, they will complete Gastrointestinal Symptom Rating Scale (GSRS) at weeks 4, 8, 12, 16 and 20(± 3 days). During these same weeks (± 3 days) dietary intake data (28-day record) and stools will be collected (1 per period). Participants' height and weight will be taken at baseline, and weight during weeks 4, 8, 12, 16 and 20.

Inclusion Criteria:

• Are willing and able to provide informed consent.
• Have confirmed diagnosis of PWS
• Are willing to have height and weight measured and provide demographic information (e.g. age, race, sex)
• Are 18-75 years of age
• Are willing to consume B. lactis B94 and placebo each for 4-week periods
• Are willing to complete a daily questionnaire throughout the 20-week period.
• Are willing to complete the Gastrointestinal Symptom Rating Scale (GSRS) monthly throughout the 20-week study
• Are willing to provide information about their dietary intake for 3-days every 4 weeks
• Are willing to provide a valid social security for study payment purposes

Exclusion Criteria:

• Have a milk protein allergy
• Are currently taking medications for diarrhea
• Are currently taking probiotics supplements and do not want to discontinue prior to the start of the baseline period (i.e. those that discontinue will be included)
• Have previously or are currently being treated for any gastrointestinal diseases such as (gastric ulcers, Crohn's, Celiac, ulcerative colitis, etc.).

The B. lactis and placebo will be provided in identical capsules.