The variability in resting metabolic rate (RMR), diet-induced thermogenesis (DIT), and fuel utilization rates as well as the impact of several factors on RMR and DIT were assessed in several studies with a total of 103 males and females. The intraindividual CV of RMR and respiratory quotients was 5-6%. The intraindividual variability in DIT and fuel utilization rates was substantially higher. RMR did not change from morning to afternoon. The menstrual cycle phase did not affect RMR and DIT. DIT after mixed meals of 1.3-2.6 MJ could be assessed with good accuracy in 3 h. It is concluded that the low reproducibility of DIT implies that sample sizes of < 10 individuals with one measurement per subject and per treatment have power levels < 80% of assessing true, relatively large (50%) treatment effects or between-group differences in DIT.