Background and aims: The aim of this study was to evaluate changes in fat oxidation rate during 40 min of continuous exercise and identify the intensity at the highest fat oxidation rate (Fatmax). Methods: A total of 14 sedentary males with age, body height, weight, and BMI averages of 29.3 +/- 0.7 years, 178.3 +/- 1.7 cm, 81.1 +/- 3.9 kg, and 25.4 +/- 0.9 kg/m(2), respectively, were included in the study. Fatmax was determined using an indirect calorimeter with an incremental treadmill walking test at least after 12 h of fasting. On a separate day, at least after 12 h of fasting, the participants walked for 40 min within their predetermined individual Fatmax heart rate and speed ranges. Results: The initial fat oxidation rate was not sustained within the first 16 min of exercise and was reduced; however, carbohydrate oxidation reached a stable level after nearly 10 min. Conclusions: In sedentary individuals, during low-intensity physical activity, fat oxidation rates may not be sustainable as expected from Fatmax testing. Therefore, when exercise is prescribed, one should consider that the fat oxidation rate might decrease in sedentary overweight individuals.