Draft Australian Dietary Guidelines (PDF, 3MB) - Eat For Health

A3.1.6 Z-scores and percentiles: Converting betweenmeasuresWhile percentiles are most commonly used in Australia, in some situations, particularly forresearch, it is more appropriate to use the Z-scores (standard deviations above or below themean). The conversion of percentiles to Z-scores requires a table of normal distributions. The50th percentile is a Z score of 0, the 90th percentile is +1.28, and 10th percentile is –1.28.Growth reference charts are available in both formats and a calculator is available on the internet(http://www.cdc.gov/growthcharts/ [952]). BMI can be converted into a BMI Z-score using a BMIfor-agegrowth chart and the formula: Z-score = ((BMI/M)L-1)/(LS), where M, L, and S are valuesselected from reference tables corresponding to the age of the child in months [36].A3.1.7 Differences between IOTF criteria and the newWHO growth referenceThere are major differences in the percentage of children classified as obese or overwightaccording to the reference used, ranging from 5% to 25% in a study of 3 year old children [951].These authors concluded that ‘the IOTF reference and cut-offs could be preferable for theidentification of overweight and obesity both at individual and population levels because they are atleast based on a crude association with ill health later in life, namely the definition of overweightand obesity at age 18 years’ [951].A3.2 How is healthy weight measured inadults?For adults the standard WHO BMI cutoff points are most commonly used for assessment ofobesity, and separate cutoffs are available for Asian populations.Body mass index (BMI) is a measure of body size that is widely used as an index of relative risk ofmortality and morbidity at the population level [687, 691]. The association of mortality with BMI isa J-shaped curve with the lowest risk – normal healthy weight – within the range 18.5 to 25 [687,691]. The cutoff point is 28 for older Australians.BMI = weight (in kilograms)/height (in metres) 2 can be used to define weight status in adults(except older adults) as follows:Underweight