Characterization of Beverage Viscosity Based on the International Dysphagia Diet Standardisation Initiative and Its Correspondence to the Japanese Dysphagia Diet 2021

<b>Background/Objective:</b> The International Dysphagia Diet Standardisation Initiative (IDDSI) and the Japanese Dysphagia Diet 2021 (JDD2021) are prominent systems that classify thickened beverages for dysphagia management. We herein aim to establish a correspondence between these systems through systematic viscosity measurements. <b>Methods:</b> We analyzed 49 thickened beverage samples using an E-type viscometer, IDDSI flow test, and JDD syringe test. <b>Results:</b> Receiver operating characteristic analysis revealed the following viscosity cutoffs for IDDSI levels: 0-1 at 72.0 mPa·s (area under the curve [AUC] 0.94), 1-2 at 112.0 mPa·s (AUC 0.95), and 2-3 at 303.0 mPa·s (AUC 0.97). Multiple regression analysis revealed that beverage characteristics, including fat, sodium content, and settling time, significantly influenced viscosity (R² = 0.803). The findings established that IDDSI Level 0 corresponds to a thinner viscosity than JDD Stage 1 and Stage 1 (0-72 mPa·s), Level 1 to Stage 1 (72-112 mPa·s), Level 2 to Stages 1-2 (112-303 mPa·s), and Level 3 to Stage 3 (>303 mPa·s). Moreover, the measurement methods had high correlations (r = 0.83-0.93, <i>p</i> < 0.001). <b>Conclusions:</b> The comprehensive map between IDDSI and JDD2021 classifications developed from the results of this study enables healthcare providers to translate between the classification systems, improving dysphagia management internationally while supporting evidence-based care and global research.