## 4 Discussion  
In this study, we investigated common functional gastrointestinal symptoms, namely, abdominal pain, bloating, and bowel habits, in relation to intestinal permeability and fecal microbiota in patients treated with a specific probiotic containing _B. longum_ BB536 and _L. rhamn_ HN001 plus vitamin B6. In this cross-over, double blind, placebo-controlled, short-term study, we found consistent amelioration of some gastrointestinal symptoms, intestinal microbiota, and related metabolism with ZR, compared with placebo. Also, as LI affects approximately 75% of the world population [34], we enrolled the cohort with an established diagnosis of lactose intolerance and persisting symptoms after lactose-free diet for at least six months. We cannot exclude that persisting symptoms could originate from other dairy compounds, namely fats and proteins [35]. Indeed, fats in some dairy products were more likely to cause symptoms  
Figure 6: Significant correlations found between fecal microbiome and metabolome of lactose intolerant patients. in at least two studies [36; 37]. In addition, we cannot exclude that beta-casein proteins also activate gastrointestinal \(\upmu\)-receptors, which, in turn, stimulate motility and abdominal symptoms [38; 39]. Strain-specific bacterial metabolism might operate in the selection of probiotics and symbiotics to decrease the symptoms of gastrointestinal disorders, including LI [34]. Indeed, previous studies demonstrated that _B. longum_, as well as _L. rhamns_, improves general gastrointestinal symptoms, promotes lactose tolerance, and encourages a positive shift in gut microbial composition [40; 41]. Bloating was most effectively corrected by ZR, according to the work of [PERSON] et al. [42]. Even with the lower intake of fibers than the recommended guidelines, constipation showed a trend towards improvement. Similar results have been previously described by [PERSON] et al. [43]. Our data showed that the administration of _B. longum_ BB536 and _L. rhamns_ HN001 plus vitamin B6 for one month did not improve the intestinal permeability and abdominal pain. However, [PERSON] et al. [44] demonstrated that abdominal pain covers an important slice, but not the totality of LI patients with FGIDs. Recent research [45] suggests that probiotic bacteria formulation containing _L. acidophilus_, _L. rhamns_, _L. casei_, _Bifidobacterium breve_, _S. thermophilus_, _B. longum_, and _B. infants_ ameliorate flatulence and bloating in lactose malaborber patients. Our study suggests that similar results also stand for our patients. Thirty days of treatment with _B. longum_ BB536 and _L. rhamns_ HN001 led to a positive shift in intestinal microbial composition. This modulation could be also the result of the intake of vitamin B6, according to the role of this vitamin in the metabolic pathways of microbes [46]. ZR drove an enrichment of genus _Bifidobacterium_. It is associated with lactose tolerance [47] and has been demonstrated to show relief in LI subjects with persistent FGIDs [41]. Recently, it was shown that short-chain galactooligosaccharide (GOS) induced a shift in the microbiome, increasing the relative amount of healthy promoting bacteria (_Lactobacillus_, _Bifidobacterium_, _Faecalibacterium_, and _Roseburia_), and a lowering of potentially detrimental species (_Enterobacteriaceae_) in LI patients [48]. In the present study, we found that the ratios of _Lactobacillus-Bifidobacterium_ and _Bacteroides-Enterobacteriaceae_ were respectively higher in ZR compared with those in PL. A lower ratio was demonstrated to be associated with food intolerance and allergy [49]. According to previous findings [48], _Bifidobacterium_ showed a negative correlation with bloating and abdominal pain. Interestingly, the relative amount of some _Bacteroides_ species was higher in PL compared with ZR. Some species of _Bacteroides_ showed a pro-inflammatory effect [50]. In addition, a different balance of _Lactobacillus_ species was found in ZR compared with PL.