Objective versus Self-Reported Energy Intake Changes During Low-Carbohydrate and Low-Fat Diets

OBJECTIVE: To compare self-reported with objective measurements of energy intake changes (∆EI) during a 1-year weight loss intervention with subjects randomized to low-carbohydrate versus low-fat diets. METHODS: We used repeated body weight measurements as inputs to an objective mathematical model to calculate ∆EI(Model) to compare with self-reported energy intake changes assessed by repeated 24-hr recalls (∆EI(Recall)). RESULTS: ∆EI(Recall) indicated a relatively persistent state of calorie restriction of ~500–600 kcal/d at 3, 6, and 12 months with no significant differences between the diets. ∆EI(Model) demonstrated large early decreases in calorie intake >800 kcal/d followed by an exponential return to ~100 kcal/d below baseline at the end of the year. Accounting for self-reported physical activities did not materially affect the results. Discrepancies between ∆EI(Model) and ∆EI(Recall) became progressively greater over time. The low-carbohydrate diet resulted in ∆EI(Model) that was 162±53 kcal/d lower than the low-fat diet over the first 3 months (p=0.002), but no significant diet differences were found thereafter. CONCLUSIONS: Self-reported ∆EI measurements were inaccurate. Model-based calculations of ∆EI found that instructions to follow the low-carbohydrate diet resulted in greater calorie restriction than the low-fat diet in the early phases of the intervention, but these diet differences were not sustained.