Overweight is defined as having more body fat than is optimally healthy. Being overweight is common where food supplies are plentiful and lifestyles are sedentary. Excess weight has reached epidemic proportions with more than one billion adults either overweight or obese, and over 40 million children under five years old overweight worldwide.1 In 2011 in England, about 30 per cent of boys and girls aged between two and 15 were either overweight or obese. 2
BMI in children and young people
The degree to which a person is overweight is usually measured by body mass index (BMI), which is the weight in kilograms divided by the height in metres squared (kg/m2). Overweight is defined as a BMI of 25 or more, pre-obesity as a BMI between 25 and 30, and obesity as a BMI of 30 or more. Assessing the BMI of children is more complicated than for adults because it changes as they grow and mature, and growth patterns differ between boys and girls. Thresholds that take into account a child's age and sex are used to assess whether their BMI is too high or too low. These are usually derived from a reference population, known as a child growth reference, with the data shown in BMI centile charts. In a clinical assessment, a child or young person on or above the 98th centile is classified as obese. A child or young person on or above the 91st centile, but below the 98th centile, is classified as overweight.