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

Abstract

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.

Trial registration: ClinicalTrials.gov NCT01826591.

Figure 1.

A) Mean body weight changes measured during the DIETFITS trial clinic visits (●) or self-reported by subjects at group counseling sessions (○) for all 414 subjects with complete clinic weight data. B) Mean self-reported energy intake changes (■) indicated a relatively persistent reduction in energy intake whereas the model-based measurements (○ from self-reported weights and ● from clinic weights) followed an exponential time course (solid curve). Error bars indicate 95% CI.

Figure 2.

A) Mean body weight changes for the 209 subjects in the low-carbohydrate diet group (♦ clinic and ◊ self-reported) and B) the 205 subjects in the low-fat (▲clinic and Δ self-reported) diet group. C) Mean model-based measurements of energy intake changes in the low-carbohydrate diet group (▲ from clinic weights and Δ from self-reported weights ) and the low-fat diet group (◊ from self-reported weights and ♦ from clinic weights) both followed an exponential time courses (solid curve and dashed curve for low-carbohydrate and low-fat diets, respectively). * indicates p<0.05 between diet groups and the error bars indicate 95% CI.

Figure 3.

Individual weight changes at 12 months for subjects assigned to the low-fat diet (left column) and low-carbohydrate diet (right column) were significantly correlated with model-calculated changes in energy intake averaged over A) 6–12 months; B) 3–6 months; and C) 0–3 months.