Background: The reproducibility of the measurement of ad libitum energy intake (EI) is not well known. Furthermore, it is not known whether standardized conditions before the test day influence this measure.
Objective: The objective was to examine the reproducibility and power of the measurement of ad libitum EI with and without prior diet standardization.
Design: Fifty-five healthy, normal-weight men were tested in 2 groups, one with (D, n = 32) and one without (ND, n = 23) prior diet standardization, on 2 different identical occasions. They were given a standardized energy-fixed breakfast and then an ad libitum lunch 4.5 h later. Reproducibility was assessed by the coefficient of repeatability.
Results: No effect of prior diet standardization was seen on the reproducibility of ad libitum EI (P = 0.56), but diet standardization increased ad libitum EI significantly (P < 0.001). The correlation between ad libitum EI on the 2 test days was r = 0.861 (R(2) = 0.742, P < 0.0001) and r = 0.654 (R(2) = 0.428, P < 0.001) in the D and ND groups, respectively. The coefficient of repeatability and CV were 1478 kJ and 8.9% compared with 1831 kJ and 14.5% in the D and ND groups, respectively. A paired design with a study power of 0.8 requires 17 and 26 subjects, with and without prior diet standardization, respectively, to detect a difference of 500 kJ in EI.
Conclusions: The ad libitum test meal used to measure spontaneous EI is reproducible, and the reproducibility does not seem to be influenced by prior standardization. However, prior diet standardization exerts a significant effect on ad libitum EI.