An Update and Overview of the Various Health-related Benefits of Probiotics: A Focus on Clinical Trials Demonstrating Efficacy, Tolerability and Use in Patients with Impaired Glucose Tolerance and Type 2 Diabetes

Recently, probiotics have been investigated as potential therapeutic agents for various diseases. Clinical studies using probiotics have been conducted in humans with impaired glucose tolerance and type 2 diabetes mellitus. Chronic inflammation plays a pivotal role in initiating insulin resistance in the pathogenesis of type 2 diabetes, leading to cardiovascular diseases. Intestinal dysfunction and inflammation have been postulated to trigger systemic chronic inflammation, and it is assumed that the suppression of inflammation in the intestine is the point of activity of probiotics. Therefore, in this review, among the randomised controlled trials that evaluated the effects of probiotics in patients with impaired glucose tolerance and type 2 diabetes, we selected trials that evaluated the indices of glycaemic control and inflammation-related markers. Some trials have shown that the probiotics administration improved glycaemic indices, such as HbA1c levels, and reduced C-reactive protein levels and proinflammatory cytokines, such as IL-6, in the blood, suggesting the suppression of inflammation. Two trials showed improvements in glycaemic indices, implying that they were mediated by IL-10, an anti-inflammatory cytokine. Although a correlation between the suppression of inflammation by probiotics and improvement in glycaemic control has not been documented, one trial revealed that glycaemic control worsened, accompanied by a decrease in anti-inflammatory cytokine levels, after probiotics were discontinued. Other studies have shown that probiotics can reduce blood endotoxin levels and increase intestinal mucin production. These findings suggest that probiotic administration has enormous potential to suppress chronic inflammation in metabolic disorders, leading to improved glycaemic control. Suppression of chronic inflammation has been speculated to prevent vascular diseases in type 2 diabetes.