Background/Objectives <p>Individuals with constitutional thinness are characterized by an atypical resting energy metabolism and body composition. The aim of the present study was to compare patterns of energetic changes from a fasted resting state to i) postprandial resting, ii) fasted exercise, and (iii) postprandial exercise between females with constitutional thinness and normal-weight, considering body weight and composition characteristics.</p> Subjects/Methods <p>Body composition (X-ray absorptiometry) and maximal aerobic capacities were assessed in 17 female adults with constitutional thinness and 17 normal-weight controls. Their energy expenditure and substrate oxidation were assessed by indirect calorimetry on four occasions: i) in a fasted resting state, ii) two hours after a calibrated meal in a resting state, iii) after a 30-min cycling exercise performed postprandially, and iv) after a 20-min walking exercise in a fasted state.</p> Results <p>Females with constitutional thinness had significantly lower body weight (<i>p</i> &lt; 0.001), absolute fat-free mass (<i>p</i> &lt; 0.001) and percentage of fat mass (<i>p</i> = 0.04). Energy expenditure was lower for all the energetic stimuli considered (from <i>p</i> = 0.02 to <i>p</i> &lt; 0.001) in participants with constitutional thinness compared to normal-weight controls, and differences disappeared after adjustment for body weight or composition.</p> Conclusions <p>The present results do not indicate any impaired energetic responses to any energetic stimuli in females with constitutional thinness. Results challenge the hypothesis of a spender energy profile in individuals with constitutional thinness, and highlight the necessity to gain insight into the role of body composition in energetic regulation in this population.</p>