From Self-Monitoring to Better Health: Changes in Diet Quality, Body Composition and Cardiometabolic Markers Following Use of the Dietary Diary App in a Single-Arm Study

Background/Objectives: Despite the rapid development of personalised nutrition, new tools are still needed to support and improve diet quality and prevent diet-related diseases in individuals who do not seek or require personalised dietary advice. The study analysed changes in diet quality, body composition and cardiometabolic markers in two groups of the Dietary Diary app (DDApp) users: (i) after 10 weeks in invited app users, (ii) after one year in voluntary app users who, after 10 weeks, were not encouraged to continue using the app. Methods: The app underwent testing with 62 adult participants (aged 18–69 years). App users’ body composition, systolic (SBP) and diastolic (DBP) blood pressure, total cholesterol (TC) and fasting blood glucose in capillary blood (FBG) were measured before (62 users), after 10 weeks (62) and after a year (30). Dietary data were collected, and pro-Healthy Diet Index (pHDI) and non-Healthy Diet Index (nHDI) were calculated. Results: After 10 weeks, invited app users recorded decreased nHDI (mean ± standard deviation: from 15.5 ± 6.2 to 12.1 ± 4.9% points; p < 0.001), body weight (81.5 ± 18.3 to 79.7 ± 18.0 kg; p < 0.001), body mass index (28.6 ± 6.1 to 28.0 ± 6.1 kg/m2; p < 0.001), waist circumference (91.2 ± 14.8 to 89.7 ± 14.4 cm; p = 0.019), fat content (36.6 ± 7.9 to 35.1 ± 8.5% bw.; p < 0.001), FBG (108.3 ± 30.1 to 93.5 ± 17.3 mg/dL; p < 0.001), TC (199.1 ± 36.6 to 169.3 ± 25.5 mg/dL; p < 0.001), SBP (131.3 ± 16.5 to 123.5 ± 15.6 mmHg; p < 0.001), DBP (84.8 ± 9.8 to 80.1 ± 9.3 mmHg; p < 0.001), along with increased fat-free mass/fat mass ratio (1.9 ± 0.9 to 2.1 ± 0.9; p < 0.001) and muscle mass/fat mass ratio (0.9 ± 0.5 to 1.0 ± 0.5; p < 0.001). After a year, voluntary app users had decreased nHDI (from 16.7 ± 6.7 to 13.5 ± 6.1% points; p = 0.017), visceral fat tissue (1.9 ± 1.8 to 1.8 ± 1.7 kg; p = 0.045) and SBP (129.9 ± 15.9 to 125.0 ± 13.5 mmHg; p = 0.015). Conclusions: DDApp may support dietary behaviour change through self-monitoring, with favourable short-term reductions in nHDI and improvements in body composition and selected cardiometabolic markers. Some benefits persisted after one year.