Self-reported energy intake by FFQ compared with actual energy intake to maintain body weight in 516 adults.

It is generally assumed that a FFQ is not suitable to estimate the absolute levels of individual energy intake. However, in epidemiological studies, reported nutrients by FFQ are often corrected for this intake. The objective of the present study was to assess how accurately participants report their energy intakes by FFQ. We compared reported energy intake with actual energy intake needed to maintain stable body weights during eleven controlled dietary trials. FFQ were developed to capture at least 90 % of energy intake. Participants, 342 women and 174 men, with a mean BMI of 22.8 (SD 3.1) kg/m2 filled out the FFQ just before the trials. Energy intakes during the trials were calculated from provided foods and reported free-food items, representing 90 and 10 % of energy intake, respectively. Mean reported energy intake was 97.5 (SD 12.7) % of actual energy intake during the trials; it was 98.9 (SD 15.2) % for women and 94.7 (SD 16.3) % for men (P = 0.004 for difference between sexes). Correlation coefficients between reported and actual energy intakes were 0.82 for all participants, 0.74 for women and 0.80 for men. Individual reported energy intake as a percentage of actual intake ranged from 56.3 to 159.6 % in women and from 43.8 to 151.0 % in men. In conclusion, the FFQ appeared to be accurate for estimating the mean level of energy intakes of these participants and for ranking them according to their intake. However, the large differences found on the individual level may affect the results of epidemiological studies in an unknown direction if nutrients are corrected for energy intakes reported by FFQ.