**3. Efficacy of probiotics on gastrointestinal diseases**

Probiotics have a positive effect on intestinal function. They enhance the structure of mucus barrier and make intestinal connections closer by boosting the amount of mucus produced. They decrease inflammation and restore normal bowel movements as well [19, 20]. So, these mechanisms of action lead to probiotics having a major role in the management of gastrointestinal disorders.

### **Figure 2.**

*Distribution of probiotics-disease-related meta-analysis articles based on the category of diseases (2000–2020). The effectiveness of probiotics was defined as the percentage of completed curing or just improvement of some parameters in patients based on the result of each meta-analysis study.*

### **3.1 Diarrhea**

Among the probiotic studies conducted on digestive system diseases, diarrhea is approximately the most studied case. Evidence demonstrates that epidemiological relevance of acute diarrhea, whether caused by virus or associated by using antibiotic, is very high in the world, particularly, in developing countries [21]. Evidence show *Limosilactobacillus reuteri* ATCC 55730 [22, 23] and *Saccharomyces boulardii* [24, 25] play an important role in treating acute diarrhea in children. Research shows that two probiotic strains including *Lactobacillus GG* and *Saccharomyces boulardii* play important role in preventing antibiotic-associated diarrhea in both adult patients and children as well [21, 26]. Research reported that treatment by probiotics could reduce acute diarrheal illness by approximately 1 day [27–32].

Probiotics' preventive or therapeutic effect in almost all types of diarrheas is related to some parameters including strain type, the antimicrobial and anti-inflammatory properties of the probiotic strain, and utilized dosage [11–13]. The effectiveness of probiotics on diarrhea has been imputed to their immunostimulatory effect and also restoring gut microflora to the balance situation [33, 34].

### **3.2 Irritable bowel syndrome (IBS)**

In rodents with intestinal inflammation, it has been proven that probiotics can reduce intestinal cytokine secretion and improve epithelial barrier function [35]. And a reduction of IBS symptoms happened in IBS patients due to enhanced cytokine profile [36] after probiotic supplementation was used [19, 37–39]. A meta-analysis of 15 human studies including 1793 IBS patients also indicated that probiotic therapy reduced pain and symptom severity scores in these people [40]. Furthermore, studies both in humans and animals show that different probiotic strains can be effective in alleviating abdominal pain and decreasing visceral hypersensitivity by changing the expression of neurotransmitters and receptors which are associated with the pain pathways such as the opioid or the cannabinoid receptors [41, 42]. However, taking into account the effectiveness of probiotics based on the type of strain used, duration, dose of administration, and single or combined strains used, can be different in a specific disease. Although many research showed the efficacy of probiotics on IBS patients, however, Connell et al. reported that VSL#3 probiotic supplement which is the combination of eight bacterial strains, not had any positive effect on abdominal pain, stool consistency, abdominal bloating, or quality of life in patients with IBS [43].

### **3.3 Inflammatory bowel disease (IBD)**

IBD is a collective term used to describe Crohn's disease (CD), ulcerative colitis (UC), and nonspecific colitis [44]. The inflammation of the gastrointestinal tract is in common characteristic of these diseases although each of them has distinct features, the inflammation may lead to pain, diarrhea, and bleeding [44]. The precise cause of IBS is unknown; however, it is thought that a multifactor is involved, the complex interplay of genetics and epigenome, environmental factors, and microbiome [45].

It is shown that probiotic administration alleviated the severity of the colitis by decreasing the NF-κB DNA binding activity, and also reducing the accumulation of leukocytes, and downregulating IL-6 and TNF-α production. Probiotics also might be useful in preserving remission and preventing relapse of UC [46, 47]. Other

*Probiotic Effects on Disease Prevention and Treatment DOI: http://dx.doi.org/10.5772/intechopen.109717*

researchers demonstrated the efficacy of probiotics on this disease as well [48, 49]. It should be noted that based on investigating all meta-analysis studies of probiotics on UC published from 2000 to 2020, probiotics supplementations was 100% effective for remission induction and preventing relapse of UC. Therefore, probiotic therapy seems to be a safe and effective method for patient with UC, however, evidence has not proven efficacy of this supplement on CD patients so far [49–52]. A clinical trial confirmed this statement. In a randomized, double-blind, placebo-controlled trial of adult patients with UC and CD, treatment with a multi-strain probiotic supplement or placebo for 4 weeks. The result showed that utilizing probiotics could reduce intestinal inflammation in patients with UC, but not in CD [53].
