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

Objective This study aimed 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 Repeated body weight measurements were used as inputs to an objective mathematical model to calculate ∆EI Model and to compare with self‐reported energy intake changes assessed by repeated 24‐hour recalls (∆EI Recall ). Results ∆EI Recall indicated a relatively persistent state of calorie restriction of ~500 to 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.