Is BMI a relevant marker of fat mass in 4 year old children? Results from the MINISTOP trial.

Abstract

Due to the increase in childhood obesity, identifying children with excess body fat as early as possible is essential. Body mass index (BMI) is commonly used as a marker of body fat in children, adolescents, and adults, yet whether BMI is a valid marker of body fat in pre-school aged children remains to be confirmed. Therefore, we analyzed the associations of BMI with fat and fat-free mass in healthy 4-year-old Swedish children. The study comprised of 303 children (135 girls) participating in the MINISTOP obesity prevention trial. Fat and fat-free mass were measured using air displacement plethysmography and we computed fat mass index (FMI) and fat free mass index (FFMI) as fat and fat free mass (kg)/height2 (m). BMI was positively yet weakly associated with percent fat mass (boys: r2 = 0.120, P < 0.001 and girls: r2 = 0.224, P < 0.001). There was a strong association between BMI and the FMI (boys: r2 = 0.468, P < 0.001 and girls r2 = 0.598, P < 0.001) as well as between BMI and the FFMI (boys: r2 = 0.621, P < 0.001 and girls: r2 = 0.499, P < 0.001). Children classified as normal weight had a wide range of percent fat mass (12.3 to 35.3%) and FMI (1.75 to 5.78 kg/m2). BMI was strongly associated to both FMI and FFMI. Therefore, caution is needed when interpreting body fat status based on BMI values in pre-school children.