Definition of Obesity

Obesity and being overweight are well-known descriptions and everyone has a rough idea of their meaning, but they are also technical terms with clear definitions defined by the World Health Organisation (WHO) based on the Body Mass Index (BMI – see box for an explanation).

BMI is an effective measure of weight status at a population level but can be less accurate for assessing healthy weight in individuals, especially for certain groups (e.g. athletes, the elderly) where a slightly higher BMI is not necessarily unhealthy. BMI is therefore often supplemented by measuring waist circumference and by considering individual circumstances. However, these complexities mean that, while BMI is well understood by clinicians and professions related to nutrition, it does not always provide a clear guide for the majority of people.

For children the situation is more complicated. There is no fixed BMI to define being obese or overweight since this varies with gender and with growth and development. Parents can get an indication of their child’s weight status by checking their position on a standard growth chart but should seek expert guidance before acting on such information.

Although not perfect, leading experts have concluded that BMI is the best measure we have. However, we will keep this under review as part of our commitment to invest in our knowledge.

Body Mass Index (BMI)

BMI is measured by dividing a person’s weight (in kilograms) by the square of their height (in metres). The calculation produces a figure that can be compared to various thresholds that define whether a person is overweight or obese. For adults these thresholds are: