Validation of Two Portion Size Estimation Methods for Use with the Global Diet Quality Score App

<b>Background/Objectives</b>: The Global Diet Quality Score (GDQS) was developed to provide crucial information on diet quality. The GDQS app standardizes the collection of GDQS data using portion size estimation at the food group level with 3D cubes of pre-defined size. Playdough was proposed as a possible alternative method. This validation study assessed whether the GDQS obtained using cubes or playdough with the GDQS app was equivalent to the GDQS estimated by weighed food records (WFRs) for the same 24 h reference period. <b>Methods</b>: We used a repeated measures design where 170 participants aged 18 years or older estimated portion sizes using both the WFR and the GDQS app with cubes and playdough. To assess the equivalence between the GDQS-WFR and GDQS-cubes or GDQS-playdough, we utilized the paired two one-sided t-test (TOST), with 2.5 points pre-specified as the equivalence margin. We used the Kappa coefficient to quantify agreement between WFR, risk of poor diet quality outcomes, and food group consumption using the cubes or playdough. <b>Results</b>: GDQS-WFR, GDQS-cubes, and GDQS-playdough were all equivalent within the pre-specified 2.5-point margin (<i>p</i> = 0.006 for cubes and <i>p</i> < 0.001 for playdough). The cubes (κ = 0.5685, <i>p</i> < 0.0001) and playdough (κ = 0.5843, <i>p</i> < 0.0001) showed moderate agreement with WFR when classifying individuals at risk of poor diet quality outcomes. There was substantial to almost perfect agreement between the cubes and playdough methods and WFR for 22 out of the 25 GDQS food groups. Liquid oils exhibited the lowest agreement (κ = 0.059, 27.7% agreement, <i>p</i> = 0.009). <b>Conclusions</b>: Using the GDQS app with cubes or playdough was equivalent to the WFR in assessing diet quality as measured by GDQS. These findings contribute to the growing body of research, validating simplified tools for dietary assessment and paving the way for more frequent evaluations of diet quality.