Measuring overweight (including obesity)

How did we measure overweight (including obesity)?

Body mass index (BMI) is an internationally recognised standard for classifying overweight and obesity in adults. It is calculated by dividing a person’s weight in kilograms by the square of their height in metres. A BMI of 25 kg/m2 or higher is considered overweight, while obesity is a higher-risk subcategory of being overweight (30 kg/m2 or higher). A BMI of 25.0–29.9 kg/m2 is classified as overweight (but not obese). Throughout the report, we refer to living with 'overweight (including obesity)' as the risk factor of being in either BMI category above 25 kg/m2.

Overweight (including obesity) is a complex condition and is influenced by social, environmental and economic factors which can affect a person’s ability to maintain a healthy weight. For further information, see the box below.

While BMI can be a useful measure for monitoring trends in the population, it does not necessarily reflect differing body composition, fat or muscle mass distribution between individuals. Particularly among older persons, other indicators of overweight (including obesity) may be more appropriate in determining health risk (Woo 2015). Further considerations for muscle mass and bone density may be needed before recommending intentional weight loss for older age groups.

Understanding BMI: Richard and Adrian’s body mass index

Given that body mass index (BMI) incorporates measures of both height and weight, it is important to understand the relationship between the two that determines one’s BMI.

Richard and Adrian are brothers. Richard weighs 90 kg and is 178 cm tall, giving him a BMI of 28 kg/m2. Adrian weighs the same, though stands taller at 186 cm, giving him a BMI of 26 kg/m2. While the two share the same weight, their BMI differs because of their different heights. To achieve the target scenario (described further below) of a 1-unit reduction in BMI (that is, 1 kg/m2), Richard would have to reduce his weight by approximately 3.2 kg and Adrian by 3.5 kg.

While categories of BMI are described in this report, health risk develops gradually along a continuous BMI scale where individual risk differs even within the same BMI category. For example, while Adrian and Richard would be considered in the same category (that is, overweight but not obese), the additional risk of developing overweight-related diseases is greater for Richard due to his higher BMI.

Additionally, while being underweight (that is, having a very low BMI) also presents additional risk of disease, the risk factor presented in this report assesses only burden attributable to high BMI.