When does my child have a weight issue?

Adults are considered to have overweight when their BMI reaches 25 kg/m2. Obesity, however, starts at 30 kg/m2. Unlike adults, children undergo rapid physical development, which changes the normal BMI range with age. Consequently, the static limits that apply to adults cannot be used to assess children’s state of nutrition

So how is the physical development of children and adolescents assessed?

First, BMI percentile curves can be used to assess physical development. They are based on the measurements of a reference population and are age- and gender-specific. Different percentile curves for height, weight, BMI and head circumference can be used to assess a child’s development. For example, a 10-year-old boy with a BMI of 16.5 kg/m2 would be on the 50th percentile according to the WHO. The BMI of half of the boys his age would be higher, and the other half lower.

Children and adolescents are considered to have overweight from the 90th percentile upwards in Europe, and above the 85th percentile in the USA. Accordingly, obesity begins at the 97th and 95th percentile respectively. In addition to percentiles, height, weight and BMI can also be expressed by numerical values in the form of so-called z scores or standard deviation scores (SDS). These indicate the extent to which a measured value deviates from the mean value of the reference population. A value of 0 corresponds to the mean value of the comparison group. Negative scores correspond to values below, and positive scores to values above, the mean value. They are also directly related to percentiles and can be converted into them. A score of +1 corresponds to the 84th percentile and a score of +2 to the 97.5th percentile. The WHO defines a child over five years of age as having overweight if the score exceeds 1, and having obesity above a score of 2.

One of the advantages of the z scores is that they map clinically relevant changes better than percentiles, especially in the extreme range, and are also linearly scaled, which makes it easier to compare results between different age groups.

In view of the serious health consequences of excess weight and obesity, children and adolescents should have regular routine weight checks. The values measured can be easily interpreted using freely accessible percentile curves and BMI and z-score calculators. However, it should be noted that body weight and BMI measurements alone have a major weakness – they are unable to distinguish between fat and muscle mass. The results can be particularly affected by relative changes in muscle and fat content. In principle, however, regular weighing is a quick and easy screening method to detect weight problems in children and adolescents and to treat them at an early stage.