via Obesity Science to Practice, Editors Gareth Williams and Gema Fruhbeck (pub 2009). I lent this book via the British Library, SRP £70. "Cut off levels for overweight and obesity reflect the increasing risk of metabolic, CVD as BMI increases above an optimal range of 21-23 kg/m2 - which remains the recommended median goal for Caucasion populations" pg 21 (see WHO (2000) Obesity: Preventing and Managing Global EpidemicWHO/NUT/NCD, 2000 and the 5 part pdf links 1,2,3,4,5)
What is the median BMI of a population? Lowering the median to 21 (first curve) significantly reduces the proportion obese.