Obesity in New Zealand

How obesity is measured

The most common method for measuring obesity rates, for population groups, is classifying by an index called the body mass index (BMI).

BMI is an index of weight adjusted for height, and is calculated by dividing weight in kilograms by height in metres squared (kg/m2). BMI is often used as an indirect measure of body fatness because it is relatively simple to measure and is correlated with total body fat.

How can I work out whether I'm obese?

The normal BMI range is 18.5–24.9 kg m²

Adults with a BMI of 25.0–29.9 are considered overweight

Adults with a BMI of 30 or greater are considered obese

Changes to the way obesity is measured

In the 1997 National Nutrition Survey and the 2002/03 New Zealand Health Survey, the World Health Organization (WHO) BMI cut-off points were used to classify overweight and obesity (25 and 30 respectively) in European/Other and Asian adults (Table 3). Higher BMI cut-off points were used to classify Māori and Pacific adults as overweight and obese (26 and 32 respectively), based on evidence that at an equivalent BMI, Māori and Pacific adults had a lower percentage of body fat than European adults (Swinburn 1998).

For the 2006/07 New Zealand Health Survey ethnic-specific BMI cut-off points were not used. This follows international recommendations from the World Health Organization (WHO).

The issue of whether different BMI cut-off points are needed for different ethnic groups has been debated for many years, with much of the international debate focused on BMI cut-off points for Asian populations. The basis of the argument for ethnic-specific BMI cut-off points is that the association between BMI, percent body fat and body fat distribution differs across ethnic groups.

However, the WHO BMI cut-off points are intended to identify people or populations at increased risk of health conditions associated with increasing BMI, not to estimate percent body fat per se. Furthermore, the associations between BMI, percent body fat and body fat distribution differ by factors other than ethnicity, such as gender and age. The differences are primarily due to the fact that BMI does not distinguish between weight associated with muscle and weight associated with fat; nor does BMI provide information on the distribution of body fat (World Health Organization 2000).

Although the WHO BMI cut-off points were developed primarily using data from populations of European origin, the health risks associated with increasing BMI are continuous and graded and begin at a BMI below 25 in all population groups. Therefore, the most recent WHO Expert Consultation recommended that principal BMI cut-off points should be retained as the international classification for all adults (World Health Organization 2004).

The most recent WHO Expert Consultation (World Health Organization 2004) also recommended that additional BMI cut-off points be added as points for public health action. This is in recognition of the fact that the risk of disease increases as BMI increases, even within the ‘normal’ range (Table 4). It is recommended that all countries use these additional BMI cut-off points for reporting purposes with a view to facilitating international comparisons.

International classification of adult underweight, normal range, overweight and obesity according to BMI

Classification	BMI (kg/m2)
	Principal cut-off points	Additional cut-off points
Underweight	< 18.50	< 18.50
Severe thinness	< 16.00	< 16.00
Moderate thinness	16.00−16.99	16.00−16.99
Mild thinness	17.00−18.49	17.00−18.49

Normal range	18.50−24.99	18.50−22.99

		23.00−24.99
Overweight	≥ 25.00	≥ 25.00
Pre-obese	25.00−25.99	25.00−27.49

		27.50−29.99
Obese	≥ 30.00	≥ 30.00
Obese class I	30.00−34.99	30.00−32.49
		32.50−34.99
Obese class II	35.00−39.99	35.00−37.49
		37.50−39.99
Obese class III	≥ 40.00	≥ 40.00

Source: World Health Organization 2006

In addition to complying with international standards, there are two other important reasons for ceasing to use ethnic-specific BMI cut-off points in adults. Firstly, it is to ensure consistency between adults and children. There are no ethnic-specific BMI cut-off points for children, and they would be extremely difficult to develop because cut-off points are gender- and age-specific.

Secondly, an increasing proportion of the population identify with multiple ethnic groups, and it is unclear which BMI cut-off points should be used for people identifying with two ethnic groups with different BMI cut-off points (eg, Māori and European). Without ethnic-specific BMI cut-off points for multi-ethnic groups it would be necessary to prioritise ethnicity before classifying BMI categories.

Adopting international BMI cut-off points for all adults affects Māori and Pacific adults, but not European/Other or Asian adults. This change results in a decrease in the proportion of Māori and Pacific adults in the normal and overweight categories, and an approximately 11 percentage point increase in the proportion of Māori and Pacific adults classified as obese.