Of the 501 participants, 169 (34%) were diagnosed with type 2 diabetes with median follow-up time of 9.6 years (IQR: 5.6–13.5 years). Demographic characteristics are reported in Table 1. Adults who developed type 2 diabetes were more likely to be female, had higher body mass indices (37.0 ± 0.5 kg/m2 versus 31.8 ± 0.4 kg/m2) and body fat (36.2 ± 0.6% versus 31.2 ± 0.5%). Glucose and insulin responses during the MMTT are shown (Fig. 1A, B). Glucose and insulin AUCs were significantly lower for the MMTT compared with the OGTT (p-values < 0.001). At baseline, participants who later developed diabetes had increased glucose and insulin AUCs/iAUCs in response to the MMTT than non-progressors (Table 1).

Means and SDs for glucose (A) and insulin (B) responses by those who developed diabetes and those who did not. MMTT responses are displayed by and closed black circles with solid lines and open circles with dashed lines for participants who developed type 2 diabetes (+T2D) n = 169(34%) and those who did not (−T2D) n = 332(66%). OGTT responses are displayed similarly with closed and open squares.

The HR and 95% confidence intervals from the Cox proportional hazards models assessing the prospective relationship between MMTT glucose AUC/iAUCs on development of diabetes are reported in Fig. 2A and Supplemental Table S1. In unadjusted analyses, glucose AUC180-min (HR: 1.98, 95% CI: 1.67, 2.34, p < 0.0001), AUC240-min (HR: 1.93, 95% CI: 1.62, 2.31, p < 0.0001), and iAUC180-min (HR: 1.43, 95% CI: 1.20, 1.71, p < 0.0001) were associated with an increased risk of diabetes while iAUC240-min (HR: 1.16, 95% CI: 0.98, 1.38, p = 0.09) was not. After further adjustment for covariates (age, sex, body fat [%], M, AIR, SWIA heritage) in three subsequent models AUC180-min (model 4 HR: 1.44, 95% CI: 1.10, 1.88, p = 0.007) and AUC240-min (model 4 HR: 1.41, 95% CI: 1.09, 1.84, p = 0.01) remained associated with increased risk of diabetes. In the final model, after further adjustment for AIR and SWIA, iAUC180-min was no longer associated with increased risk of diabetes (HR: 1.13, 95% CI: 0.91, 1.40, p = 0.27). We also examined glucose and insulin AUC/iAUCs from the OGTT for comparison. These are shown in Supplement Table 3, and as expected, glucose AUC/iAUCs predicted the development of diabetes.

Model 1 (unadjusted); Model 2 adjusted for age, sex, body fat (%); Model 3 further adjusted for M; Model 4 further adjusted for AIR and full vs. non-full Southwestern Indigenous American heritage (SWIA).

The HR and 95% confidence intervals from the Cox proportional hazards models assessing the prospective relationship between MMTT insulin AUC/iAUCs on development of diabetes are reported in Fig. 2B and Supplemental Table S2. In the unadjusted analyses and adjustment for age, sex, and body fat (%), insulin AUC180-min, AUC240-min, iAUC180-min, and iAUC240-min were all independently associated with increased risk of diabetes (p-values < 0.01). However, after adjustment for M (model 3) and further adjustment for AIR and SWIA heritage (model 4) the association between insulin AUC/iAUCs with risk of diabetes was attenuated (p-values > 0.05). As expected, insulin AUC/iAUCs from the OGTT predicted the development of diabetes (Supplemental Table S3).

The HR and 95% confidence intervals from the Cox proportional hazards models assessing the prospective relationship between MMTT glucose/insulin peaks, rise from fasting, and decline from peak on development of diabetes are reported in Table 2. Peak glucose was consistently associated with increased risk of diabetes across all four models (model 4 HR: 1.28, 95% CI: 1.03, 1.61, p = 0.03). The association between glucose rise from fasting on risk of diabetes was attenuated after adjustment for AIR and SWIA heritage. Glucose decline from peak was protective against diabetes in models 1 (unadjusted), 2 and 3 but not the final model 4. Insulin peak, rise from fasting and decline from peak were not associated with the risk of diabetes after adjusting for age, sex, and body fat (%) in model 2. Results were similar using the rate of change for rise from fasting and decline from peak to adjust for time (data not shown).

The calculated DI using surrogate measures from the MMTT and OGTT were included in model 2. For MMTT, this measure of DI did not predict diabetes (HR: 0.91; 95% CI: 0.81, 1.03, p = 0.12) but did for OGTT (HR: 0.89; 95% CI: 0.81, 0.99, p = 0.03).

Correlation coefficients between MMTT and OGTT glucose variables are reported in Supplemental Table S4. Due to the moderate to strong correlations between MMTT and OGTT variables C-statistics were computed. C-statistics provide a global measure of model discrimination and were calculated to compare MMTT variables that remained significant after adjustments for age, sex, body fat (%), M, AIR, and SWIA heritage (AUC180-min, AUC240-min, and peak MMTT glucose) to the corresponding OGTT variable, fasting, 60-minute and 120-minute glucose (Supplemental Table S5). The C-statistics for MMTT AUC180-min, AUC240-min, and peak glucoses were similar to OGTT 60-min glucose (AUC180-min 0.72 vs. 0.74, p = 0.31; AUC240-min 0.71 vs. 0.74, p = 0.23; peak 0.71 vs. 0.74, p = 0.07) and OGTT 120-min glucose (AUC180-min 0.72 vs 0.71, p = 0.86; AUC240-min 0.71 vs. 0.72, p = 0.90; peak 0.71 vs. 0.69, p = 0.53). Similarly, MMTT AUC180-min and AUC240-min were similar to the OGTT AUC180-min (AUC180-min 0.72 vs. 0.74, p = 0.15; AUC240-min 0.71 vs. 0.74, p = 0.90) while MMTT peak glucose were also similar to OGTT peak glucose (0.71 vs. 0.73, p = 0.18). The only difference occurred between MMTT glucose AUC180-min and fasting glucose from the OGTT (0.72 vs. 0.69, p = 0.03) indicating the MMTT glucose AUC180-min is slightly more predictive of diabetes compared to fasting glucose from the OGTT.