The thermic effect of food (TEF), defined as the increase in metabolic rate after ingestion of a meal, has been studied extensively, but its role in body weight regulation is controversial. We analyzed 131 TEF tests from a wide range of subjects ingesting meals of varying sizes and compositions. Each test lasted 6 h. Of the total 6-h TEF, 60% of the total had been measured after 3 h. 78% after 4 h, and 91% after 5 h. We developed a three-parameter curve to fit the data, which reduced noise and gave additional information about the TEF. The area under this parametric curve was positively correlated with fat-free mass (FFM) and meal size (MS) and negatively correlated with meal size squared (MS2) with an R2 of 0.35. The usual area under a curve created by connecting the data points of a line was correlated with the same factors but with an R2 of 0.28. The peak of the parametric curve was positively correlated with FFM and MS and negatively correlated with MS2, percent body fat, and meal composition. The time at which the peak occurred correlated positively with MS and percent fat in the meal. Our analysis suggests that an inadequate measurement duration of the TEF could lead to errors. In general, we recommend that the TEF be measured for > or = 5 h.