Energy expenditure adjusted for body composition differentiates constitutional thinness from both normal subjects and anorexia nervosa

Constitutional thinness ( CT) is characterized by a low and stable body mass index ( BMI) without any hormonal abnormality. To understand the weight steadiness, energetic metabolism was evaluated. Seven CT, seven controls, and six anorexia nervosa ( AN) young women were compared. CT and AN had a BMI < 16.5 kg/m(2). Four criteria were evaluated: 1) energy balance including diet record, resting metabolic rate ( RMR) ( indirect calorimetry), total energy expenditure ( TEE) ( doubly labeled water), physical activity; 2) body composition ( dual-energy X-ray absorptiometry); 3) biological markers ( leptin, IGF-I, free T3); 4) psychological profile of eating behavior. The normality of free T3 ( 3.7 +/- 0.5 pmol/l), IGF-I ( 225 +/- 93 ng/ml), and leptin ( 8.3 +/- 3.4 ng/ml)confirmed the absence of undernutrition in CT. Their psychological profiles revealed a weight gain desire. TEE ( kJ/day) in CT ( 8,382 +/- 988) was not found significantly different from that of controls ( 8,793 +/- 845) and AN ( 8,001 +/- 2,152). CT food intake ( 7,565 +/- 908 kJ/day) was found similar to that of controls ( 7,961 +/- 1,452 kJ/day) and higher than in AN ( 4,894 +/- 703 kJ/day), thus explaining the energy metabolism balance. Fat-free mass ( FFM) ( kg) was similar in CT and AN ( 32.5 +/- 2.9 vs. 34.1 +/- 1.9) and higher in controls ( 37.8 +/- 1.6). While RMR absolute values ( kJ/day) were lower in CT ( 4,839 +/- 473) than in controls ( 5,576 +/- 209), RMR values adjusted for FFM were the highest in CT. TEE-to-FFM ratio was also higher in CT than in controls. Energetic metabolism balance maintains a stable low weight in CT. An increased energy expenditure-to-FFM ratio differentiates CT from controls and could account for the resistance to weight gain observed in CT.