Guidelines Dietary - Eat For Health

Z-scores and percentiles: Converting between measuresWhile percentiles are most commonly used in Australia, in some situations, particularly for research, it is moreappropriate to use the Z-scores (standard deviations above or below the mean). The conversion of percentiles toZ-scores requires a table of normal distributions. The 50 th percentile is a Z score of 0, the 90 th percentile is +1.28,and 10 th percentile is –1.28. Growth charts are available in both formats and a calculator is available on the US CDCwebsite. 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 thechild in months. 35Differences between IOTF criteria and the new WHO growth standardThe IOTF uses cut-off points based on WHO adult BMI but extrapolated for use in children based on six internationaldata sets. In comparison, the WHO growth standards are a standard rather than a reference. The standard is howchildren 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 first4–6 months of life and were breastfed until 12 months of age. In addition, the data for babies was taken fromthose who were single, term births and those whose mothers did not smoke. There are major differences inthe percentage of children classified as obese or overweight according to the reference used, ranging from5% to 25% in a study of 3-year old children. 1078 The study concluded that ‘the IOTF reference and cut-offs couldbe preferable for the identification of overweight and obesity both at individual and population levels becausethey are at least based on a crude association with ill health later in life, namely the definition of overweight andobesity at age 18 years’. 1078H2How is healthy weight measured in adults?For adults, the standard WHO BMI cut-off points are most commonly used for assessment of obesity. BMI isused to define weight status in adults (except older adults) as follows.Table H2: WHO body mass index classificationClassification (WHO) BMI (kg/m 2 )Underweight < 18.5Healthy weight 18.5 – 24.9Overweight 25.0 – 29.9Obese class I 30.0 – 34.9Obese class II 35.0 – 39.9Obese class III > 40.0However, 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 ratioand 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 isusually inaccurate.Waist circumference is increasingly being used assessing risk of chronic disease in adults, and provides a betterestimate of risk than BMI. 1100,1101 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 thosefor people of European descent.AppendicesNational Health and Medical Research Council137