Study design

This randomized parallel clinical trial was conducted at the Diabetes Association and Endocrine Clinic, Imam Reza Hospital, Tabriz University of Medical Sciences, from February 8, 2019 to May 20, 2019. According to the sample size formula for randomized clinical trials [\(n = \frac{{2 Sp^{2} \left( {Z_{{1 - \frac{\alpha }{2}}} + Z_{1 - \beta } } \right)^{2} }}{{(\mu 1 + \mu 2)^{2} }}\)], considering an α of 0.05, a β of 0.20 (for a power of 80%), and blood concentration of insulin as a key outcome, the sample size was calculated at 19 per group [23]. A total of 185 potentially suitable volunteers referred to the clinic of Imam Reza Hospital were screened for eligibility by a dietitian. Finally, 50 eligible subjects were enrolled in the study, 26 in the taurine group and 24 in the placebo group.

Prior to beginning the study, written informed consent was obtained from all participants. Individuals diagnosed with T2DM with age between 30 and 60 years and a body mass index (BMI) between 25 and 35 kg/m2 were eligible for participation. Exclusion criteria were the use of insulin, being treated with non-steroidal anti-inflammatory drugs or lipid-lowering drugs, intake of supplements within the past 3 months, and changes in physical activity and dietary intake during the study. Patients diagnosed with polycystic ovary syndrome, chronic renal and liver disease, cardiovascular diseases, hypothyroidism, hyperthyroidism, malignancies, gastrointestinal dysfunction and infectious diseases, or patients with physiological conditions such as pregnancy or lactation were also excluded. After stratification for sex, age and BMI values, participants were randomly assigned to consume either taurine or placebo for 8 weeks. Randomization was carried using a block randomization process with a computer-generated random-number table by a statistician. Subjects in the taurine group received capsules containing 1000 mg taurine (Karen Pharma and Food Supplement Co., Iran, > 99% pure) three times a day for 8 weeks following each meal and those in the placebo group received capsules containing microcrystalline cellulose three times daily for the same duration. The appearance of the placebo capsules, such as shape, color and size, was identical to the taurine capsules. The participants were requested to maintain their usual physical activity or dietary intake throughout the experimental period. In order to evaluate the acceptance of the intervention by the participants, they were asked to return the bottle. Participants with good compliance (defined as consumption of over 90% of the capsules) were included in the analysis. Participants and researchers were blinded to the treatment allocation. The study was performed according to the principles provided in the Declaration of Helsinki. The research protocol received the approval of the Ethical Committee of the Tabriz University of Medical Sciences, and was registered with the Iranian Registry of Clinical Trials (http://www.IRCT.IR, identifier: IRCT20121028011288N16). The reporting of this manuscript adheres to CONSORT guidelines [24].

Measurements of anthropometric indices, food intake and physical activity

After completing a general questionnaire on demographic information, medical history, and medication use [25], height and body weight were measured using a wall mounted stadiometer and a Seca scale (Seca, Germany), without shoes and wearing light clothing, respectively. Body mass index was calculated as weight (kg) divided by height2 (m). Waist circumference was measured midway between the lowest rib and the iliac crest and hip circumference was measured around the widest portion of the hip using a non-elastic tape. Data on dietary intake was collected using a 24-h recall procedure across 3 days: two non-consecutive normal days and one holiday. Three-day mean values of energy, macronutrient and micronutrient intakes of all participants were calculated using Nutritionist 4 software (First Databank Inc., Hearst Corp., San Bruno, CA, USA). To evaluate physical activity, all participants completed the International Physical Activity Questionnaire (IPAQ) at baseline and following the eight weeks of the intervention [26].

Blood sampling and biochemical assays

At baseline and after 8 weeks of intervention, 8 mL fasting venous blood samples were collected from each participant. The blood sample was centrifuged at 2500 rpm for 10 min (Orum Tadjhiz Centrifuge, Iran) to separate serum samples, which were kept frozen at − 70 °C until use. Serum total antioxidant capacity (TAC) and malondialdehyde (MDA) were determined using standard spectrophotometry methods and a commercially available total antioxidant capacity assay kit (Navand Salamat, Iran). Reliable spectroscopy methods with commercial kits (RANDOX Lab, Crumlin, UK) were used to assess the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) in red blood cells. Serum glucose was determined by the standard enzymatic method with a Pars Test Kit (Karaj, Iran). Serum insulin concentration was obtained by an ELISA assay with a Monobind kit (USA). Serum tumor necrosis factor alpha (TNF‐α) and Interleukin 6 (IL-6) were determined using ELISA (Human ELISA kit, Crystal Day Bio-Tec). Finally, the serum level of hs-CRP was determined using the immunoturbidimetric method (Bio System Kit, Spain).

Statistical analysis

All data were analyzed using SPSS 23 software (IBM/SPSS Inc., Chicago, IL, USA). Data analysis were carried out based on intention‐to‐treat (ITT) method. The ITT population included all enrolled and randomized participants. The multiple imputation method was used to impute missing values. The Kolmogorov–Smirnov test was used to determine the distribution of data. Quantitative variables with normal distribution were expressed as mean ± standard deviation. Median and interquartile range (IQR) were used to present variables with skewed distribution. A Chi square test was conducted for comparison of qualitative variables. Independent sample Student’s t test and Mann–Whitey test, respectively, were used to compare the percent change of the quantitative parametric data and variables with non-parametric distribution between the intervention and placebo groups. Log transformation was performed for variables with skewed distribution before they were included in the analysis. To detect within-group changes, paired-sample t-tests were used for quantitative parametric data, while data with skewed distribution were analyzed using Wilcoxon’s rank-sum test. Analysis of covariance (ANCOVA) was used to compare mean values of study variables between groups adjusted for differences in baseline values, weight, and energy intake. Statistical significance was considered to be p < 0.05 at a 95% confidence interval (95% CI).