The body mass index (BMI) characterizes the relative proportion between a child’s weight and height. It is a valid predictor of adiposity (fat tissue), and is therefore the best clinical standard for defining obesity in adults and in children older than two years of age.

The BMI is calculated from the weight and square of the height as follows:

BMI = body weight (kg) ÷ height (meters) squared

BMI varies with age, gender, and pubertal stage, so growth standards are based on BMI percentiles. The calculated BMI is plotted on a BMI reference chart to determine the BMI percentile. There is a separate chart for boys and girls. These measures are very accurate, but clinical judgment must be used in applying these criteria to a specific patient.

A child with a BMI greater than the 85th percentile is overweight and a child who’s BMI is greater than the 95th percentile is obese.
Childhood obesity is a national problem that needs our attention. We know that currently 15% of 6 – 19 year olds are at or above the 95% for BMI on a standard growth chart. Preventing obesity during childhood is critical, because habits formed during youth frequently carry into adulthood; an obese 4-year-old has a 20% chance of becoming obese as an adult, and an obese teenager has up to an 80% chance of becoming an obese adult.

Obesity in childhood can come with an array of health problems such as psychosocial issues, fatty liver, orthopedic problems, and sleep apnea. Obese children and teens have risk factors for cardiovascular disease, high blood pressure, high cholesterol, and abnormal glucose intolerance.

The underlying cause for children is normally an “energy imbalance.” They are taking in more calories than they burn. Focusing on health habits is the best way to approach a high BMI. Modeling healthy food choices and routine physical activity can go a long way. Encourage healthy foods such as fruits, vegetables, whole grains, and protein. Limit screen time and promote exercise.

The AAP recommends measuring BMI yearly beginning at age 2. This recommendation is not meant to cause shame or label anyone “fat.” The goal is to promote health from a young age. Remember, healthy children turn into healthy adults!