The goal of this clinical trial is to learn the effect of Bifidobacterium animalis subsp. lactis Bi66 supplementation on gut function in healthy adults. The main questions it aims to answer are:

1. Does Bi66 affect the gut function(bowel movement frequency and stool form)?
2. Does Bi66 affect the gut microbiota and short-chain fatty acids ? Researchers will compare probiotic Bi66 to a placebo (a look-alike substance that contains no Bi66) to see if Bi66 works to affect gut function.

Participants will

1. Take Bi66 or a placebo every day for 8 months
2. Return the used sachets and stool diaries every 7 days and record bowel movement frequency per week and Bristol Stool Scale
3. Have checkups, including questionnaire survey, physical measurement, dietary survey, collection and detection of biological samples at baseline, week 4, week 8

Inclusion Criteria:

• 18-65 years old;
• Self-reported bowel movement frequency less than 5 times per week;
• Change of weight no more than 3 kg within the last 2 months；
• No consumption of probiotics, prebiotics, synbiotics within the last 2 months

Exclusion Criteria:

• Pregnant or breastfeeding women；
• Smoking；
• BMI < 18 kg/m2 or ≥28 kg/m2；
• Suffering from gastrointestinal disorders such as gastroenteritis, gastroduodenal ulcer, ulcerative colitis, Crohn's disease, intestinal stress syndrome, functional diarrhea, gastroparesis, etc；
• History of gastrointestinal surgery (except appendectomy or hernia repair)；
• Change in dietary habits within the last 1 month (e.g. ketogenic diet, etc.)；
• History of major surgery within the last 3 months or major surgery planned in the next 1 month;
• Use of antibiotics, acid-suppressing drugs, pro-digestive drugs (gastrointestinal, morpholine, cisapride, etc.), anti-constipation drugs, anti- diarrhea drugs, etc., in the last 1 month;
• Participate in a similar intervention study within the last 1 month