Effect of daytime on resting energy expenditure and thermic effect of food in obese adolescents.

The aim of this study was to investigate the effect of daytime on resting energy expenditure (REE) and thermic effect of food (TEF), REE and TEF were measured in 11 obese boys (mean age +/- SD 11.9 +/- 1.6 years; body mass index 30.1 +/- 4.0 kg/m2) by indirect calorimetry (SensorMedics 2900) on two consecutive days: early in the morning (7:30 a.m.) on one day and at 12 noon on the other day or vice versa. REE was measured for 30 minutes and TEF for 180 minutes after a 600 kcal liquid meal containing 13% protein, 39% fat and 48% carbohydrates. REE measured in the morning was not statistically different from that measured at noon (2191 +/- 358 vs 2223 +/- 319 kcal/24 hours) and morning values were highly correlated with afternoon values (r2 = 0.805). Therefore we conclude that the effect of daytime is negligible for measurements of REE if the patients are in a postabsorptive state and avoid physical activity and stress 10-12 hours prior to measurements. The thermic effect of food calculated in the morning also was not statistically significantly from values found in the afternoon (8.2 +/- 8.8% in the morning and 6.6 +/- 7.5% in the afternoon). However, because of very high within-patient variability the correlation between morning and afternoon values was weaker than for REE (r2 = 0.289).