The Quételet index revisited in children and adults.

BACKGROUND The body mass index (BMI) is based on the original concept that body weight increases as a function of height squared. As an indicator of obesity the modern BMI assumption postulates that adiposity also increases as a function of height in states of positive energy balance. OBJECTIVE To evaluate the BMI concept across different adiposity magnitudes, in both children and adults. METHODS We studied 975 individuals who underwent anthropometric evaluation: 474 children and 501 adults. Tetrapolar bioimpedance analysis was used to assess body fat and lean mass. RESULTS BMI significantly correlated with percentage of body fat (%BF; children: r=0.893; adults: r=0.878) and with total fat mass (children: r=0.967; adults: r=0.953). In children, body weight, fat mass, %BF and waist circumference progressively increased as a function of height squared. In adults body weight increased as a function of height squared, but %BF actually decreased with increasing height both in men (r=-0.406; p<0.001) and women (r=-0.413; p<0.001). Most of the BMI variance in adults was explained by a positive correlation of total lean mass with height squared (r(2)=0.709), and by a negative correlation of BMI with total fat mass (r=-0.193). CONCLUSIONS Body weight increases as a function of height squared. However, adiposity progressively increases as a function of height only in children. BMI is not an ideal indicator of obesity in adults since it is significantly influenced by the lean mass, even in obese individuals.