To compare the resting energy expenditure (REE) and the REE/Fat-free-mass (FFM) quotient in children with constitutional leanness (CL) and children with normal body weight, and to describe the within-family clustering of CL.
We have studied 18 children and teenagers with CL, 10 girls and 8 boys, and 18 gender and age matched normal controls, with the same pubertal stage. All were recruited from the outpatient pediatric clinic nutrition unit. None of the children with CL showed symptoms of chronic illness, they had normal laboratory results, they had a normal caloric food intake, and they did not agree with the DSM-IV-TR criteria for anorexia nervosa. We describe the body mass index (BMI) of children and their parents. The children were classified according to Cole's recently published BMI cut-offs for thinness: under 18.5 points in CL group, stable at least in the last year, and between 18.5 and 25 cutt-offs in the control group. The body composition was calculated by anthropometric methods (skinfold thickness measurements). In addition REE was measured using fasting indirect calorimetry.
The CL group had a higher mean percentage of FFM, and a mean FM significantly less, relative to controls (p < 0.001). The average absolute REE was significantly lower in the CL group (1,106.55 ± 240.72 kcal) than the control group (1,353.33 ± 270.01 kcal/dia) (p < 0.01). However, the REE adjusted for FFM showed a mean significantly greater in the CL group (41.39 ± 2.26 kcal/kg FFM) (Mean confidence interval (CI) 95 %: 40.33-42.45) than the controls (37.37 ± 3.06 kcal/kg FFM) (Mean CI 95 %: 35.93-38.81) (p < 0.001). Finally, in the family study, the mean BMI of fathers of CL group was significantly lower (p < 0.01), but there were not any differences in the mean BMI of mothers. Among parents with BMI known, 8 of 35 parents of CL group had an BMI lower 18.5, and only 2 of 36 parents in the control group (p < 0.05).
This increased energy expenditure-to-FFM ratio differentiates between CL and controls. These metabolic differences are probably genetically determined.