Study population
A total of 9,961 participants from Shanghai Chongming District were included in the baseline survey of the REACTION study from 2011 to 2012, and 7,587 completed the follow-up survey in 2014 [16]. Participants with prediabetes at baseline who developed T2DM during the 3.5-year follow-up period were classified as progressors. After 3.5 years of follow-up, the patients who maintained prediabetes were classified as non-progressors. Progressors were defined as cases and non-progressors as controls. Controls were selected for cases in a 1:1 ratio and matched according to sex, age, fasting blood glucose (FBG), 2h-postprandial blood glucose (PBG), glycated hemoglobin (HbA1c), and body mass index (BMI). Figure 1 shows the patient selection process. This study was approved by the Ethics Committee of Ruijin Hospital Affiliated to Shanghai Jiaotong University School of Medicine. All the participants provided written informed consent.
Characteristics collection and laboratory measurements
Information on lifestyle, disease history, and medication use was obtained using standard questionnaires. A food frequency questionnaire was used to estimate the participants' usual diet at baseline. The frequency ranged from times per day to times per week, month, or year. The sugar-sweetened beverages consumed were recorded in standard bottles (250 mL).
Height, weight, waist circumference, hip circumference, and blood pressure were measured using standardized methods. BMI was derived from weight in kilograms divided by the square of height in meters. The waist-to-hip ratio is the waist circumference divided by the hip circumference. Ultrasonography was used to assess fatty liver disease.
After fasting for 10 h, a 75-g oral glucose tolerance test (OGTT) was performed in the morning. Blood samples were collected during fasting and 2 h post-load. For blood glucose, HbA1c, triglycerides (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT) and serum creatinine measurements, the samples were sent to the central laboratory of Shanghai Institute of Endocrine and Metabolic Diseases. Serum fasting insulin levels (catalog #8K41-74, RRID:AB_3075437) were measured using a chemiluminescent immunoassay. The homeostasis model assessment for insulin resistance (HOMA-IR) was calculated as follows: FPG (mmol/l) × FINS (mU/L)/22.5. Homeostatic model assessment of β-cell function (HOMA-β) was calculated as FINS × 20/(FPG-3.5).
Definition and classification of diabetes subtypes
Prediabetes was defined as 1) impaired fasting glucose (IFG): FBG 6.1–6.9 mmol/L and 2h-PBG < 7.8 mmol/L, or 2) impaired glucose tolerance (IGT): FBG < 6.1 mmol/L and 2h-PBG 7.8–11.0 mmol/L, or 3) IFG + IGT: FBG 6.1–6.9 mmol/L and 2h-PBG 7.8–11.0 mmol/L.
Diabetes was defined as 1) FBG ≥ 7.0 mmol/L, and/or 2) 2h-PBG ≥ 11.1 mmol/L, and/or 3) HbA1c value ≥ 6.5%.
Participants with T2DM were classified according to increased fasting blood glucose levels (F-DM) or 2 h postload glucose level (2h-DM), or both (F-2h-DM) [17].
Serum polyol level measurement
Peripheral venous blood samples were collected after at least a 10 h overnight fast and stored at − 80°C until assay. Samples were thawed on ice at 4°C before preparation.
Quality control samples were prepared by aspirating and mixing equal amounts of each serum sample. 25 µL serum was pipetted into a 1.5 mL centrifuge tube, and then 170 µL pre-cooled methanol was added. The mixture was shaken at 1,450 rpm for 15 min at 10°C (MSC-100; Allsheng Instruments Co., Ltd., Hangzhou, China). After − 20°C for 20 min, the samples were centrifuged at 18,000×g for 20 min at 4°C (Microfuge 20R, Beckman Coulter, Inc., Indianapolis, IN, USA). With a lyophilizer (Labconco, Kansas City, USA) 160 µL supernatant was lyophilized. Lyophilized powder reacted with 50 µL methoxamine solution at 37°C for 2 h. Next, 50 µL MSTFA (Thermo-Fisher Scientific, Fairlawn, NJ, USA) was added, and the reaction continued for 1 h at 37°C. All the standards (Sigma-Aldrich, St. Louis, MO, USA) were accurately weighed and prepared in water to obtain the individual stock solution at a concentration of 5.0 mg·mL− 1. The appropriate amount of each stock solution was mixed to create stock calibration solutions.
Serum polyols were quantified using gas chromatography coupled with time-of-flight mass spectrometry (GC-TOF/MS) system (Pegasus HT, Leco Corp., St. Joseph, MO, USA) as previously described [18]. For separation, a Rxi-5Sil MS capillary column (30 m × 0.25 mm × 0.25 µm i.d., 0.25-µm film thickness; Restek Corporation, Bellefonte, PA, USA) was used. Helium was used as the carrier gas at a constant 1.0 mL/min flow rate. Derivatized samples (1 µL) were injected into the GC/MS instrument. The programmed column temperature was optimized for successful separation. The injection and transfer interface temperature were set to 270 ºC. The source temperature was 220ºC. Electron impact ionization (70 eV) was used for measurements in full scan mode (m/z 50–500). The acquisition rate was set to 15 spectra/s. The instrument was optimized every 48 h.
The original GC-TOF/MS data were processed using Chroma TOF software (v5.51, Leco Corp., USA) for peak integration, calibration, and quantification of each metabolite. The self-developed iMAP platform (v1.0, Metabo-Profile, Shanghai, China) was used for statistical analyses.
Statistical analysis
Continuous variables are shown as means ± standard deviation (SD) or medians (IQR) for skewed variables. Categorical variables are presented as numbers (proportions). Student’s t-test and Wilcoxon rank-sum test were used to compare continuous variables. For comparison of categorical data, the chi-square test or Fisher’s exact test was used.
As continuous variables, multivariate conditional logistic regression was used to assess the association between polyol levels and incident diabetes. Odds ratios (OR) with 95% confidence intervals [CI] were calculated. The first model was adjusted for matching variables, including sex, age, FBG, PBG, HbA1c, and BMI. The second model was additionally adjusted for fasting insulin levels, systolic blood pressure, diastolic blood pressure, hypertension history, and serum creatinine and AST levels. The third model included the variables in Model 2 plus adjustments for current smoking status, alcohol consumption, and beverage intake. Baseline fasting xylitol levels were further grouped into tertiles. The unadjusted difference with a 95% CI was calculated between progressors and non-progressors. The odds of incident diabetes were calculated by comparing patients in the lowest tertile of xylitol levels. Subgroup analysis was conducted based on diabetes subtypes.
Spearman’s correlations were used to assess the relationship between fasting serum xylitol levels and changes in FBG, PBG, and HbA1c levels (follow-up levels minus baseline levels).
A two-sided P-value<0.05 was considered statistically significant. Statistical analyses were performed using SAS 9.3 software (SAS Institute, Cary, NC, USA).