Orexigenic and anorexigenic hormones in CT contrast with an adaptative profile characterizing AN. The hormones appear to be valuable biomarkers for distinguishing these 2 categories of severely underweight subjects.

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.

Context: Low fat mass and hormonal or nutritional deficiencies are often incriminated in bone loss related to thinness. Constitutional thinness has been described in young women with low body mass index (BMI) but close-to-normal body composition, physiological menstruation, no hormonal abnormalities, and no anorexia nervosa (AN) psychological profile.Objective: Our objective was to determine whether constitutional thinness is associated with impaired bone quality.Design, Setting, and Participants: This was an observational, cross-sectional study on 25 constitutionally thin and 44 AN young women with similar low BMI (Ͻ16.5 kg/m 2 ) and 28 age-matched controls. Main Outcome Measures:Femoral and lumbar spine bone mineral density by dual-energy x-ray absorptiometry, distal tibia and radius bone architecture and breaking strength by three-dimensional peripheral quantitative computed tomography, and bone turnover markers were determined. Results:Constitutionally thin subjects displayed a higher percentage of fat mass than AN subjects but had similar lumbar and femoral bone mineral density, which were significantly lower than in controls (P Ͻ 0.001). Constitutionally thin subjects displayed more markedly impaired trabecular and cortical bone parameters in the distal tibia than in the radius. AN bone structure was impaired only in subjects with a long history of disease. Calculated breaking strength was decreased in constitutional thinness and long-standing AN in both the radius and the tibia. Bone markers in constitutionally thin subjects were similar to those of controls. Osteoprotegerin to receptor activator of nuclear factor B ligand ratio was higher in constitutionally thin subjects than in controls or AN women. Conclusions:Young women with constitutional thinness present an unexpectedly high prevalence of low bone mass (44%) associated with small bone size, overall diminished breaking strength, but normal bone turnover. Mechanisms related to insufficient skeletal load and/or genetics are proposed to explain this new phenotype of impaired bone quality. In middle-aged and elderly patients, bone loss has been related to thinness, mainly in a context of low fat mass (FM) (2). Consumptive (3), infectious (4), digestive (5), or chronic Abbreviations: AN, Anorexia nervosa; bALP, bone alkaline phosphatase; BMD, bone mineral density; BMI, body mass index; BV/TV, relative bone volume as a percentage of total volume, derived from trabecular density; CT, constitutional thinness; CTh, absolute thickness of cortical bone; CV, coefficient of variation; 3D-pQCT, three-dimensional peripheral quantitative computed tomography; D100, mean whole bone (cortical and trabecular) density; Dcomp, bone density of cortical bone; DHEAS, sulfate salt of dehydroepiandrosterone; Dinn, density of the central part of trabecular bone;Dmeta,densityofthesubcorticalareaoftrabecularbone;Dtrab,densityofthetrabeculararea of bone; DXA, dual-energy x-ray absorptiometry; FFM, fat-free mass; FM, fat mass; HA, hydroxyapatite; MOI, moment of inertia; OPG, oste...

No abstract