Physical Capacities are Differentially Altered in Anorexia Nervosa and Constitutional Thinness.

INTRODUCTION: Physical capacities partly depend on body mass and composition. Anorexia nervosa (AN) and constitutional thinness (CT) are two distinct low-weight conditions with different muscle phenotypes. While AN is associated with impaired physical performance, this remains unclear in CT. This study aims to directly compare physical capacities in women with AN, CT, and normal-weight controls (CON). METHODS: 84 women (29 AN, 24 CT, 31 CON) were recruited. Measurements included baroreflex sensitivity, handgrip and leg extension strength, cardiopulmonary exercise testing, and subjective fatigue evaluation FACIT-F questionnaire. A subset (7 AN, 6 CT, 15 CON) also underwent quadriceps-intermittent-fatigability (QIF) test of knee extensor muscles. RESULTS: DXA-derived fat-free mass (FFM) was higher in AN than in CT. Lower body maximal strength, normalized to bodyweight, was lower in AN and CT compared to CON, and similar in AN and CT. VO2peak (% predicted) was lower (p<0.0001) in AN (86.5±24.1%) and CT (80.4±14.6%) than in CON (104.2±24.0%). Maximal aerobic power (MAP) was lower in AN (112.4±37.4W) and CT (92.2±19.7W) than in CON (151.5±37.3W, p=0.0009 and p<0.0001, respectively) and lower in CT than AN (p=0.023). FFM-normalized VO₂peak and MAP did not differ significantly between AN and CT. Peak lactate was lower in AN and CT vs. CON, and lower in CT than AN. Subjective fatigue was greater in AN and CT than in CON. CT completed fewer QIF stages than CON. DISCUSSION: This study demonstrates lower aerobic and muscular capacities in both AN and CT. AN-related deficits may result from undernutrition. CT-related alteration may reflect lower mass and specific phenotype of skeletal muscle, reduced exercise conditioning remaining a possible but unproven contributor. Tailored physical interventions may improve outcomes in both conditions.