Baseline characteristics of study participants based on fasting blood glucose

The design, sample, and exclusion criteria for this study are shown in Fig. 1. A total of 6,093 participants aged 20 years and older were included in our analyses, and Table 1 shows the baseline characteristics of participants based on fasting blood glucose quartiles. As we can see, there were significant differences in baseline characteristics between subgroups of fasting blood glucose quartiles, with the exception of the income-poverty ratio.

Across all participants, fasting glucose quartiles were positively associated with age, BMI, waist circumference, Apo B, triglycerides, insulin, AIP, and TyG (all P-value less than 0.001). The Q4 group had the highest prevalence of angina, hypertension and diabetes compared to other groups. In addition, males were more prevalent in the Q4 group. The Q1 group had higher education levels and HDL-C levels compared to the Q2, Q3 and Q4 groups.

Association between blood lipids and fasting blood glucose

Tables 2, 3, 4, 5 and 6 reflect the results of the multiple regression analyses. In the uncorrected model, TG and ApoB were positively correlated with fasting glucose (TG: β = 0.562,95% CI: 0.508,0.616) (ApoB: β = 0.980,95% CI: 0.812,1.148) while HDL-C was negatively correlated with FBG (β=-0.692,95% CI: -0.791, − 0.593). These positive correlations remained significant in models 2 and 3 after controlling for confounders. It is difficult to visualise the correlation between LDL-C or TC and FBG from the table. When we analyzed the relationship between TC and fasting blood glucose as a continuous variable, we found that there was no significant positive or negative correlation between the two(P = 0.3635), so we stratified TC and found that after adjusting for the covariates, there was a significant negative correlation between TC and fasting blood glucose when the TC level was 4.84–5.52(β= -0.247, 95% CI: -0.364,-0.129), However, through the test of trend, we found that, when we divided TC into three equal parts, the fasting blood glucose of the strata trends were not consistent(P for trend = 0.064). This suggests that there may not be a linear relationship between TC and fasting blood glucose, or it is not reasonable to divide TC into three equal parts, so we need to further study the relationship between TC and fasting blood glucose by other statistical methods. Therefore, we plotted correlation-fitted smoothed curves for the associations between lipids and FBG. Figures 2, 3 and 4 show generalised additive models with smoothed functions illustrating the linear associations between FBG and TG, HDL-C, and ApoB. As can be seen in Figs. 5 and 6, FBG first decreases with increasing TC and LDL-C and then gradually increases. The threshold effect of TC and FBG was analysed using a two-segmented linear regression model, and in Table 7, the inflection point was found to be 5.17 mmol/L (TC < 5.17 mmol/L:β= -0.19,95% CI: -0.27, -0.10; TC > 5.71 mmol/L: β = 0.21,95% CI: 0.11,0.30). In Table 8, the inflection point of the U-shaped curve for the association between LDL-C and FBG was 2.3 mmol/L (LDL-C < 2.3 mmol/L: β = -0.64,95% CI: -0.84,-0.43; LDL-C > 2.3 mmol/L: β = 0.10,95% CI: 0.03,0.17). In contrast, a multivariate logistic regression model found that patients with LDL-C > 2.3 mmol/L had a 1.40-fold higher risk of diabetes than those with LDL-C ≤ 2.3 mmol/L (odds ratio = 1.40, 95% CI: 1.07,1.83).

Subgroup analyses based on gender and race and history of diabetes mellitus

In subgroup analyses based on gender and race, the positive correlation between TG and FBG remained for men (β = 0.187,95% CI: 0.109,0.266) and women (β = 0.480,95% CI: 0.395, 0.565), as well as for blacks (β = 0.563,95% CI: 0.347,0.778), non-Hispanic whites (β = 0.284,95% CI: 0.205,0.363) and Other/Multiracial (β = 0.274,95% CI: 0.158,0.391), but not among Mexican-Americans and Other Hispanics. As shown in Fig. 7, for Mexican Americans and other Hispanics, the association between TG and FBG was an inverted U-shaped curve, whereas in whites, the association was an S-shaped curve. In men and in all races, HDL-C was negatively associated with FBG almost universally, and in women the association between HDL-C and FBG was an L-shaped curve. In subgroup analyses stratified by sex and race, the correlation between ApoB and FBG was absent. After distinguishing whether diabetes was present or not, we found that the effects of TG and HDL-C on blood glucose were consistent, but TC and LDL-C were negatively correlated in non-diabetic patients, whereas they were positively correlated in diabetic patients.