The arithmetic dilemma when defining thinness, overweight and obesity in stunted populations

Background: Critical cut-off values of BMI-for-age z-scores (BAZ) are used to define “thinness”, “overweight” and “obesity”, but the validity of these cut-off values needs to be questioned in populations that are shorter or taller than the reference. We hypothesized that the prevalence of thinness, overweight, and obesity depends on population height and performed a random simulation. Methods: We created virtual child populations aged 2-10 years with normally distributed height expressed as height-for-age z-scores (HAZ) and weight expressed as weight-for-age z-score (WAZ), based on WHO growth standards and references, with a correlation r=0.7 between height and weight. We adjusted weight-for-height and calculated BAZ. Results: BAZ depends on height and age. In short children (mean HAZ=-2 to HAZ=-3), the prevalence of thinness falls to less than 1% in the youngest and rises up to 10% (mean HAZ=-2) and up to 13% (mean HAZ=-3) at age 10 years. The prevalence of obesity rises to up to 7% in the shortest and youngest and falls close to zero at age 10. Short young children and tall older children are more prone to be misclassified as overweight. Conclusions: The prevalence of thinness, overweight and obesity depends on height and age. The coexistence of being short and being overweight – currently referred to as “double burden of malnutrition” – needs consideration as to what extent this condition is a health issue or reflects calculation artefacts. The arithmetic dilemma particularly affects young children in short populations. We suggest abstaining from defining “thinness”, “overweight”, or “obesity” by BMI z-scores. Different states of under- and malnutrition should rather be classified by direct or indirect measures of body fat.