Table 2. Characteristics and findings of selected studies.

NM: not mentioned; T2DM: type 2 diabetes mellitus; FBG: fasting blood glucose; HOMA-IR: homeostatic model assessment of insulin resistance; HbA1c: hemoglobin A1c; SCFA: short-chain fatty acid

Author (year)	Patients	Probiotics	Duration (weeks)	Outcomes
Kumar et al. (2022) [28]	150	NM	12	In the context of treating T2DM, the addition of probiotics to metformin as a supplementary therapeutic approach was found to result in reductions in FBG, postprandial blood glucose, and levels of HbA1c when compared to the use of metformin alone. The efficacy of probiotics in combination treatment has not been substantiated by significant findings. Nonetheless, the probiotics trial group exhibited a reduced incidence of gastrointestinal side effects associated with metformin therapy.
Şahin et al. (2022) [29]	156	Bifidobacterium animalis subsp. lactis (BB-12)	12	Patients receiving probiotic supplements showed improved glycemic control and HbA1c decrease, as well as improved treatment compliance and potential effects on the intestinal-pancreatic axis.
Ismail et al. (2021) [30]	150	Bifidobacterium animalis dn-173 010	16	After 16 weeks, probiotic use in T2DM patients improved glycemic control, lipid profile, and inflammatory markers.
Toejing et al. (2021) [31]	50	Lactobacillus paracasei HII01	12	L. paracasei HII01 reduced inflammatory indicators and hyperglycemia by properly regulating the stomach microbiota and so treating endotoxemia and damaged stomach, proposing a possible role as an adjuvant therapy for T2DM.
Sanborn et al. (2020) [32]	200	Lactobacillus rhamnosus GG	12	HbA1c was steady in individuals taking L. rhamnosus GG, whereas it rose in those receiving placebo at the follow-up. L. rhamnosus GG may offer a defense against alterations in blood sugar regulation.
Chen et al. (2023) [33]	58	Bifidobacterium animalis subsp. lactis M8, B. animalis subsp. lactis V9, Lactobacillus casei Zhang, L. plantarum P-8, and L. rhamnosus Probio-M9	12	The findings of this study demonstrated that the co-administration of probiotics and metformin in individuals with T2DM resulted in an augmented hypoglycemic response. The observed impact is likely to have been facilitated through the modulation of the gastrointestinal microbiota, subsequently influencing the metabolism of bile acids and SCFAs. This study provides evidence supporting the benefits of combining metformin and probiotics as a treatment approach for individuals diagnosed with T2DM.
Hasanpour et al. (2023) [34]	100	Bifidobacterium longum, B. breve, Lactobacillus bulgaricus, L. rhamnosus, L. casei, L. acidophilus, and Streptococcus thermophilu	6	Consuming soymilk and probiotics may reduce several cardiovascular risk factors in T2DM patients. FBG and HOMA-IR were not considerably impacted, though.
Velayati et al. (2021) [35]	50	Lactobacillus rhamnosus, Bacillus coagulans, fructooligosaccharide and L. acidophilus	12	This study showed that symbiotic B. coagulans supplementation might reduce metabolic variables and inflammation in T2MD individuals.
Kanazawa et al. (2021) [36]	88	Bifidobacterium breve strain Yakult, Lacticaseibacillus paracasei strain Shirota, and galactooligosaccharides	24	Regarding incendiary markers, there was no method to distinguish between the treatment groups. The stomach environment was undoubtedly slightly altered by synbiotic treatment in obese individuals with T2DM.
Jiang et al. (2021) [37]	101	Bifidobacterium bifidum, Lactobacillus acidophilus, Streptococcus thermophilus	12	This clinical investigation found that administering probiotics in individuals with diabetic nephropathy improved their glycemic control, amplifying their therapeutic potential in clinical settings.
Ming et al. (2021) [38]	300	Bifidobacterium and berberine	16	According to this study, Bifidobacterium may improve the hypoglycemic effects of berberine.
Perraudeau et al. (2020) [39]	76	Clostridium butyricum, Akkermansia muciniphila, C. beijerinckii, Anaerobutyricum hallii and Bifidobacterium infantis	12	In participants with T2DM who were predominantly receiving metformin monotherapy, a unique five-strain probiotic formulation decreased total glucose in comparison to placebo. No changes in the body weight, HOMA-IR, or fasting glucose levels were seen, indicating that the majority of the impact was a decrease in FBG levels during the postprandial period.
Khalili et al. (2019) [40]	40	Lactobacillus casei	8	L. casei supplementation altered serum sirtuin 1 (SIRT1) and fetuin-A levels in people with T2DM in a manner that enhanced glycemic response. A novel recognised method of probiotic action in the treatment of diabetes was introduced by altering their amounts.
Razmpoosh et al. (2019) [41]	60	Lactobacillus, Bifidobacterium, and Streptococcus	6	This study found that using multi-strain probiotic supplements significantly reduced fasting plasma glucose levels when compared among groups, although more research is required to validate the findings.
Madempudi et al. (2019) [42]	79	Lactobacillus casei UBLC42, L. acidophilus UBLA34, L. plantarum UBLP40, Bacillus coagulans Unique IS2, Bifidobacterium breve UBBr01 and fructo-oligosaccharides, L. salivarius UBLS22	12	The reduction in HbA1c values showed that UB0316 (probiotic) considerably improved glycemic management. Additionally, the probiotic-treated patients' weight significantly decreased as compared to the control group.
Sabico et al. (2019) [43]	150	Bifidobacterium bifidum W23, Lactobacillus brevis W63, L. acidophilus W37, L. salivarius W24, L. casei W56, Lactococcus lactis W58, Lactococcus lactis W19, and B. lactis W52	24	In T2DM patients, six months of multi-strain probiotic treatment as a monotherapy dramatically decreased HOMA-IR. Consequently, multi-strain probiotics are a successful adjunctive treatment for diabetes.
Mafi et al. (2018) [44]	60	Lactobacillus with Bifidobacterium	12	Supplementing with probiotics improved indicators of cardiometabolic risk and glycemic control.
Mobini et al. (2017) [45]	46	Lactobacillus reuteri DSM 17938	12	The administration of L. reuteri DSM 17938 for a duration of 12 weeks did not result in any significant changes in HbA1c levels among individuals with T2DM who were undergoing insulin treatment. Nevertheless, the impact of L. reuteri on the insulin response was observed to be significant only in a limited subset of individuals, leading us to hypothesize that this discrepancy could potentially be attributed to the diverse compositions of gut microbiota.
Sabico et al. (2017) [46]	78	Bifidobacterium lactis W52, B. bifidum W23, Lactobacillus brevis W63, L. acidophilus W37, L. salivarius W24, L. casei W56, Lactococcus lactis W58, and Lactococcus lactis W19	12	A multi-strain probiotic supplement was taken by T2DM patients who had not taken any medication for 12 or 13 weeks. This greatly improved HOMA-IR and very slightly decreased abdominal obesity.
Firouzi et al. (2017) [47]	136	Lactobacillus and Bifidobacterium	12	In persons with type 2 diabetes, probiotics only slightly decreased HbA1c and fasting insulin levels.
Feizollahzadeh et al. (2017) [48]	40	Lactobacillus plantarum A7	8	Utilising probiotic soy milk did not decrease inflammation or serum adiponectin; however, it could alter a patient's lipid profile if they had T2DM.
Tonucci et al. (2017) [49]	50	Bifidobacterium animalis subsp. lactis BB-12 and Lactobacillus acidophilus La-5	6	Probiotics reduced T2DM patients' glucose levels across the board, but consumption of mature milk appears to be linked to additional metabolic changes, including a decline in pro-inflammatory cytokines and an increase in acidic corrosive enzymes.