Day-to-Day Variability in Meal Timing and Its Association with Body Mass Index: A Study Using Data from a Japanese Food-Logging Mobile Application

<b>Background/Objectives</b>: Discrepancies in mealtimes between weekdays and weekends-often referred to as "eating jetlag"-have been linked to a higher body mass index (BMI). However, in modern societies characterized by diverse work patterns, misalignment between mealtimes and the internal circadian rhythm may result not only from weekday-weekend differences but also from day-to-day variability. The aim of this study was to quantitatively assess and visualize daily mealtime variability over a 1-month period using food log data and to investigate the association between breakfast time irregularity and BMI. <b>Methods</b>: We conducted a retrospective, cross-sectional analysis using food log data (<i>n</i> = 1072; 742 women and 360 men) extracted from a popular Japanese food-logging mobile application. Mealtime irregularity was quantified using composite phase deviation (CPD). Data were stratified by sex and age tertile. <b>Results</b>: Approximately 18% of participants (women and men) exhibited irregular breakfast timing (CPD > 1 h). Multivariate regression analysis revealed that an evening-type chronotype was primarily associated with BMI among younger women, whereas breakfast time irregularity was associated with BMI among older women. <b>Conclusions</b>: Our findings suggest that daily mealtime variability is an additional chrono-nutritional factor associated with BMI. Furthermore, the chrono-nutritional factors most strongly associated with BMI may differ by age.