Calibration of the brief food frequency questionnaire among patients on dialysis

Estimating dietary intake is challenging in patients with chronic diseases. The aim of this study was to calibrate the Block Brief 2000 food frequency questionnaire (BFFQ) using 3-day food diary records among patients on dialysis.

Data from 3-day food diary records from 146 patients new to dialysis were reviewed and entered into National Cancer Institute self-administered 24-hour dietary recall (ASA24), a web-based dietary interview system. The information was then re-entered omitting foods reported in the diaries that were not in the BFFQ to generate a "BFFQ-restricted" set of intakes. We modeled each major dietary component (i.e., energy [total calories], protein, carbohydrate, fat) separately using linear regression. The main independent variables were BFFQ-restricted food diary estimates computed as the average of the 3 days of diaries, restricted to items included in the BFFQ, with the unrestricted 3-day food diary averages as dependent variables.

The BFFQ-restricted diary energy estimate of 1,325 ± 545 kcal was 87% of the energy intake in the full food diary (1,510.3 ± 510.4, P < .0001). The BFFQ-restricted diary carbohydrate intake was 83% of the full food diary (156.7 ± 78.7 g vs. 190.4 ± 72.7, P < .0001). The BFFQ-restricted fat intake was 90% of the full-diary-reported fat intake (50.1 ± 24.1 g vs. 56.4 ± 21.6 g, P < .0001). Daily protein intake assessments were not statistically different by BFFQ-restricted diary and full diary assessment (63.1 ± 28.5 vs. 64.1 ± 21.4 g, P = .60). The associations between BFFQ-restricted diary intake and unrestricted intake were linear. Three-day diary-reported intake could be estimated from BFFQ-restricted intake with r2 ranging from 0.36 to 0.56 (P < .0001 for energy [total calories], protein, carbohydrate, and fat). Final equations did not include adjustments for age, sex, or race because the patterns of associations were not significantly different.

Energy and macronutrient estimates by BFFQ are lower than estimates from 3-day food diaries, but simple calibration equations can be used to approximate total intake from BFFQ responses.