EAT FOR HEALTH Australian Dietary Guidelines

APPENDICES

137

Z-scores and percentiles: Converting between measures

While percentiles are most commonly used in Australia, in some situations, particularly for research, it is more
appropriate to use the Z-scores (standard deviations above or below the mean). The conversion of percentiles to
Z-scores requires a table of normal distributions. The 50th percentile is a Z score of 0, the 90th percentile is +1.28,
and 10th percentile is –1.28. Growth charts are available in both formats and a calculator is available on the uS CDC
website.^1080 BMI can be converted into a BMI Z-score using a BMI-for-age growth chart and the formula: Z-score =
((BMI/M)l-1)/(lS), where M, l, and S are values selected from reference tables corresponding to the age of the
child in months.^35

Differences between IOTF criteria and the new WHO growth standard

The IOTF uses cut-off points based on WHO adult BMI but extrapolated for use in children based on six international
data sets. In comparison, the WHO growth standards are a standard rather than a reference. The standard is how
children should grow as opposed to a reference which describes how they grew at a particular time and place.
The WHO growth standards reflect the growth of children who were predominantly breastfed for the first
4–6 months of life and were breastfed until 12 months of age. In addition, the data for babies was taken from
those who were single, term births and those whose mothers did not smoke. There are major differences in
the percentage of children classified as obese or overweight according to the reference used, ranging from
5% to 25% in a study of 3-year old children.^1078 The study concluded that ‘the IOTF reference and cut-offs could
be preferable for the identification of overweight and obesity both at individual and population levels because
they are at least based on a crude association with ill health later in life, namely the definition of overweight and
obesity at age 18 years’.^1078

H2 How is healthy weight measured in adults?

For adults, the standard WHO BMI cut-off points are most commonly used for assessment of obesity. BMI is
used to define weight status in adults (except older adults) as follows.

Table H2: WHO body mass index classification

Classification (WHO) BMI (kg/m^2 )

Underweight < 18.5

Healthy weight 18.5 – 24.9

Overweight 25.0 – 29.9

Obese class I 30.0 – 34.9

Obese class II 35.0 – 39.9

Obese class III > 40.0

However, this classification may not be suitable for all population groups:118 , 119

• some groups may have equivalent levels of risk at a lower BMI (e.g. people of Asian origin) or higher BMI
(e.g. people of Polynesian origin including Torres Strait Islanders and Mãori)^120

• while specific BMI ranges have not been developed, Aboriginal people have a relatively high limb to trunk ratio
and may have equivalent levels of risk at a lower BMI

• a higher BMI range may be desirable for people aged over 70 years.

It is important to measure weight and height accurately to assess overweight and obesity, as self-reported data is
usually inaccurate.

Waist circumference is increasingly being used assessing risk of chronic disease in adults, and provides a better
estimate of risk than BMI.110 0,1101 Risk is increased at ≥80 cm and high at ≥88 cm for women and increased at
≥94 cm and high at ≥102 cm for men.121,169 As with BMI, thresholds for other ethnic groups may differ from those
for people of European descent.