The use of a food logging app in the naturalistic setting fails to provide accurate measurements of nutrients and poses usability challenges.

Objective: MyFitnessPal is the most popular commercial nutrition weight loss app. The aim of this study was to assess how individuals in naturalistic settings performed when recording their dietary intake in MyFitnessPal, and their usability experiences with the app.

Methods: Adults not regularly using MyFitnessPal (N = 43) logged their dietary intake in the app for 4 consecutive days and completed two researcher-administered 24-h recalls collected based on the Automated Multiple Pass Method. Food items from 24-h recalls were coded into food categories and foods omitted from corresponding MyFitnessPal records were calculated. Comparative validity of energy and macronutrient outputs from MyFitnessPal were compared against 24-h recalls using paired t tests. Inductive thematic analysis was applied to app usability responses.

Results: Individuals omitted a mean of 18% (SD, 15) of food items, particularly energy-dense and nutrient-poor foods from MyFitnessPal records. Relative to 2-day 24-h recalls, 4-day MyFitnessPal records significantly underestimated mean energy intake by 1863 kJ (SD, 2952 kJ, P = 0.0002) and intake of all macronutrients. Although 80% of participants rated MyFitnessPal as easy to use, only 20% said they would continue use, citing challenges in matching foods, estimating portion size, and logging being time-consuming, as affecting motivation for long-term use.

Conclusions: Large discrepancies in nutrient measurements from MyFitnessPal indicate suboptimal performance with using the app to record intake, particularly given food omissions in records and difficulties encountered with app usability relating to the food database and input of portion sizes. Stand-alone use of MyFitnessPal is therefore cautioned and guidance from dietitians is necessary to support use of nutrition apps in collecting accurate dietary data.