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

Background/objectives 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. Subjects/methods 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)/height(2) (m). Results BMI was positively yet weakly associated with percent fat mass (boys: r(2) = 0.120, P< 0.001 and girls: r(2) = 0.224, P < 0.001). There was a strong association between BMI and the FMI (boys: r(2) = 0.468, P < 0.001 and girls r(2) = 0.598, P < 0.001) as well as between BMI and the FFMI (boys: r(2) = 0.621, P < 0.001 and girls: r(2) = 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/m(2)). Conclusions 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.