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New international standard definitions

Anthea M Magarey, Lynne A Daniels and T John C Boulton
Med J Aust 2001; 174 (11): 561-564.

Research

Prevalence of overweight and obesity in Australian children and adolescents: reassessment of 1985 and 1995 data against new standard international definitions

Anthea M Magarey, Lynne A Daniels and T John C Boulton

MJA 2001; 174: 561-564
For editorial comment, see Baur; see also Eckersley

Abstract - Methods - Results - Discussion - Acknowledgements - References - Authors' details
- - More articles on Paediatrics


Abstract

Objective: To review the prevalence of overweight and obesity in Australian children and adolescents in two national samples, 10 years apart, using the new standard international definitions of the International Obesity Task Force Childhood Obesity Working Group.
Design: Body mass index (BMI) cut-off points defining overweight and obesity were applied to the individual BMI values in the two cross-sectional samples.
Setting: Australian community.
Participants: 8492 schoolchildren aged 7-15 years (Australian Health and Fitness Survey, 1985) and 2962 children aged 2-18 years (National Nutrition Survey, 1995).
Main outcome measure: Prevalence of overweight and obesity.
Results: In the 1985 sample, 9.3% of boys and 10.6% of girls were overweight and a further 1.4% of boys and 1.2% of girls were obese. In the 1995 sample, overall 15.0% of boys (varied with age from 10.4% to 20.0%) and 15.8% of girls (varied with age from 14.5% to 17.2%) were overweight, and a further 4.5% of boys (2.4%-6.8%) and 5.3% of girls (4.2%-6.3%) were obese. The prevalence of overweight and obesity in the 1995 sample peaked at 12-15 years in boys and 7-11 years in girls. In schoolchildren aged 7-15 years, the rates represent a relative risk of overweight in 1995 compared with 1985 of 1.79 (95% CI, 1.59-2.00) and of obesity of 3.28 (95% CI, 2.51-4.29). Compared with previous estimates from these samples, the revised prevalence data are slightly higher for the 1985 data and considerably higher for the 1995 data.
Conclusion: The secular trend of increasing overweight and obesity in the decade from 1985 and the high prevalence rates in Australian children and adolescents are a major public health concern.

The rapidly increasing prevalence of obesity in young people is a major global public health concern,1 not only because of the health consequences in childhood and adolescence,1-4 but also because of the greater risk of obesity in adulthood.3 To develop and evaluate appropriate public health policies, the trends in childhood obesity need to be monitored.

While, in adults, the body mass index (BMI) cut-offs internationally accepted as definitions of overweight and obesity (25 kg/m2 and 30 kg/m2, respectively) are based on increased risks of morbidity and mortality,5 no such outcome-based definition exists for children. Definitions for children and adolescents have generally been statistical, with percentile cut-offs based on variably representative and sized national anthropometric data sets, making it difficult to adequately monitor secular changes in body size and prevalence of obesity. Furthermore, there has been no agreement on either the most appropriate adiposity index or the cut-off points. Thus, it has not been possible to interpret and compare prevalence and intervention studies, nationally or internationally.

In 1997, the International Obesity Task Force (IOTF) Childhood Obesity Working Group accepted BMI as a consistent, pragmatic (ie, reasonable although imperfect) index of adiposity in children and adolescents.6 The IOTF has recently published cut-off points to define overweight and obesity in children and adolescents based on adult overweight and obesity BMI ranges.7 While this remains a statistical definition and can not be quantified in terms of current or future morbidity, it allows analysis of national data to determine point prevalence and identify secular trends, and enables meaningful international comparisons. For example, a secular trend in body fatness, estimated by BMI, from 1985 to 1997 was found in children aged 7-12 years in New South Wales and Victoria, but the lack of accepted standards meant that no estimates were made of the prevalence of overweight and obesity.8,9

Using these new international definitions, we determined the prevalence of overweight and obesity in Australian children and adolescents aged 2-18 years in two nationally representative samples 10 years apart — the Australian Health and Fitness Survey of 1985 (AHFS85)10 and the National Nutrition Survey of 1995 (NNS95).11 In previous estimates of the prevalence of overweight using these samples, any secular trend could not be identified because different definitions were used.12,13


Methods

National surveys
The data collection procedures for the two surveys are described in Box 1. The original data published from the AHFS85 survey included percentiles for height and weight only. Subsequently published were BMI percentiles by age and sex, and the prevalence of overweight in the age group 12-14 years (but not in those 7-11 years of age).12 Summary statistics of prevalence of overweight and obesity in NNS95 participants have also been published.13Box 2 shows the prevalence of overweight in these two surveys.

Data for analysis of the AHFS85 sample were supplied on disc by the Australian Council on Health, Physical Education and Recreation and those for the NNS95 were provided by the Australian Bureau of Statistics as a confidentialised unit record file.

New international standard definitions
The new international standard definitions for overweight and obesity published by the International Obesity Task Force were based on six large, nationally representative cross-sectional growth studies from birth to 25 years (Brazil 1989, Great Britain 1979-1993, Hong Kong 1993, the Netherlands 1980, Singapore 1993, and the United States 1963-1980).7 For each of these, centile curves of BMI were drawn that at age 18 years passed through the BMI cut-off points of 25 kg/m2 and 30 kg/m2 for adult overweight and obesity, respectively. The resulting curves were averaged to provide age- and sex-specific BMI cut-off points to define overweight (cut-off 1) and obesity (cut-off 2) in half-year intervals from 2 to 18 years.7

Re-estimate of overweight and obesity prevalence
We applied the new international cut-off points to each child in the two data sets to re-estimate the prevalence of overweight and obesity. For each age, the cut-off point at the mid-year value was applied (eg, for those aged 7 years, we used the cut-off at 7.5 years). For data with one-year age groups, as in these two samples, this will give an essentially unbiased estimate.7 We grouped children in the AHFS85 into two age bands and those in the NNS95 into five age bands to facilitate comparison between the two studies.

Statistical analysis
Analyses were performed using the Stata statistical package.14Relative risk (RR) of overweight (≥ cut-off 1) or obesity (≥ cut-off 2) within years (controlling for sex) and between years (controlling for age and sex) was determined using "Epitab - tables for epidemiologists"14 and reported as RR (95% CI).


Results

For each age group, the proportion of boys and girls classified, according to the new definitions, as overweight (BMI ≥ cut-off 1 and < cut-off 2) and obese (BMI ≥ cut-off 2) in the AHFS85 and NNS95 samples are listed in Box 3.

In the AHFS85 sample:

  • In the age group 7-15 years, 9.3% of boys and 10.6% of girls were overweight and a further 1.4% of boys and 1.2% of girls were obese.

  • In the two age groups 7-11 years and 12-15 years, the relative risks of overweight or obesity did not differ between boys and girls and between age groups.

    In the NNS95 sample:

  • In the age group 2-18 years, 15.0% of boys and 15.8% of girls were overweight and a further 4.5% of boys and 5.3% of girls were obese.

  • In the age group 7-15 years, 15.3% of boys and 16.0% of girls were overweight and a further 4.7% of boys and 5.5% of girls were obese.

  • Overall, there was no difference between boys and girls in relative risk of overweight (RR, 1.08; 95% CI, 0.94-1.24).

  • In the age group 7-11 years, the relative risk of overweight was significantly greater for girls than boys (RR, 1.53; 95% CI, 1.16-2.02; P = 0.002).

  • Overall or within any age group, there was no difference in relative risk of obesity between boys and girls.

    Data from NNS95 v data from AHFS85:

  • The relative risk of overweight was 1.79 (95% CI, 1.59-2.00; P < 0.001), independent of sex and after controlling for age.

  • The relative risk of obesity was 3.28 (95% CI, 2.51-4.29; P < 0.001) and independent of sex and age.


  • Discussion

    We used the new international cut-off points to estimate the prevalence of overweight and obesity in two nationally representative samples of Australian children from 1985 and 1995, respectively.7 Depending on age, 13%-26% and 19%-23% of Australian boys and girls, respectively, aged 2-18 years are overweight or obese, with prevalence peaking at 12-15 years in boys and 7-11 years in girls. The prevalence of obesity is 2%-7% in boys and 4%-6% in girls.

    Previous estimates of the prevalence of overweight and obesity in the AHFS85 used the 85th percentile of BMI for the age group 12-14 years in the United States National Health and Nutrition Examination Survey II (NHANES II; 1976-1980) to define overweight (Box 1).12 In the NNS95 sample, children aged 2-8 years were classified as high weight for height if their standard deviation (Z) score of weight for height was greater than + 2, based on World Health Organization reference values.11 For older subjects, the 85th and 95th BMI percentiles derived from NHANES I data of 1971-1974,15 which are lower than those from the later NHANES II data, were used to define "at risk of overweight" and overweight, respectively. The reason for the use of the older NHANES data for the later 1995 survey is not clear. These differences in reference populations and approaches to classifying participants as overweight or obese clearly limit comparison of prevalence rates and assessment of trends.

    Our revised estimates of prevalence of overweight in the 1985 and 1995 samples, respectively, are slightly higher and considerably higher than those previously reported.12,13 The risks of both overweight and obesity in 1995 are significantly higher than in 1985 and the increase in prevalence is higher than previously reported.

    Methodological differences between the two surveys must be taken into consideration when making these comparisons. Although the effect of sampling on the trend is unknown, the different method of determining age in the 1985 sample may result in a very few subjects being misclassified as acceptable weight and slightly fewer being misclassified as overweight. The possible net underestimate of prevalence of overweight would be extremely small and could not account for the large increase in prevalence of overweight from 1985 to 1995.

    Furthermore, our prevalence rates are high by international standards. The revised prevalence figures for 1985 for both overweight and obesity fall at about the mid-point of the range of the international reference populations at age 18 years.7 However, prevalence figures from the 1995 sample indicate rates for both overweight and obesity that are higher than all the international reference populations and considerably higher than those of 1985. Comparison with recently published trends from 1974 to 1994 in British children aged 4-11 years16 indicates that more Australian boys aged 7-11 years were overweight in 1985 and 1995 than British boys, although the decade increase was similar. For girls, prevalence of overweight in Australia in 1985 was slightly higher than in Britain, but considerably higher in 1995 (ie, the decade increase was greater in Australian girls). Similarly, prevalence of overweight in 1994 in Australian children aged 4-6 years was considerably higher than in British children (boys, 13.4% v 6.4%; girls, 19.5% v 9.2%).

    The new standard definitions for childhood overweight and obesity are not directly related to measurable outcomes in morbidity and mortality in adulthood, although dyslipidaemia, elevated blood pressure and insulin resistance are associated with these levels of adiposity in children.2-4 Overweight and obesity in childhood and adolescence increase the risk of overweight in adulthood, but these new BMI centile curves give no information on the degree of tracking within the different levels of adiposity (ie, the extent to which individuals maintain their level of adiposity). Longitudinal studies are urgently needed to assess the change in BMI percentiles during growth, and the long term health outcomes associated with these defined levels of adiposity, so that overweight and obesity in childhood can be defined by biological endpoints.17

    The secular trend towards increasing overweight and obesity in the decade from the mid-1980s and the high prevalence figures described here are of major public health concern and should stimulate urgent action: treatment of those already overweight and obese, prevention of progression of overweight to obesity, and prevention of overweight.18 The International Obesity Task Force proposes three levels at which prevention should occur:

    • Universal/public health prevention directed at the whole population;

    • Selective prevention directed at subgroups at increased risk of developing obesity; and

    • Targeted prevention directed at high risk individuals who are overweight but not yet obese.1

    The National Health and Medical Research Council (NHMRC) report Acting on Australia's weight places particular emphasis on strategies, at the population level, to change the macroenvironment of food supply and opportunities for physical activity.18 The global epidemic of increasing overweight and obesity in both adults and children is attributed to changing lifestyle.1 Partly because of methodological difficulties in assessment,19 no data exist in Australia on either the activity or inactivity levels of children and adolescents in the past 15 years, nor how these may have changed.

    Several factors, however, suggest that the increased prevalence of overweight and obesity in Australian children may be attributed to decreasing activity, increasing inactivity and increasing food energy intake:

    • a study of fitness levels in Australian children from 1985 to 1997 reported that these have declined, suggesting a decrease in physical activity;20

    • it is generally recognised that the time spent in sedentary activities, such as television watching and playing computer games, has increased in the past decade;

    • a comparison of the dietary intakes of children aged 10-15 years from the national surveys of 198521 and 199513 showed that mean daily energy intake has increased by 0.5-2.0 MJ in the 10-year interval; and

    • a comparison of the energy intake of children aged 4-8 years in 1995 with that of Adelaide children in the early 1980s suggested that energy intake in this age group has also increased by 0.5-1.5 MJ.22

    Health professionals have a major role to play in promoting, to all family members, the benefits of a healthy diet, an increase in habitual physical activity and a decrease in inactivity.23-25 Such strategies have major implications for maintenance of a healthy weight, and the prevention of overweight, obesity, and future cardiovascular disease risk.



    Acknowledgements

    We wish to thank Adrian Esterman, biostatistician, Paul Hakendorf, clinical epidemiologist, and Sonia Anderton, statistical consultant, Flinders University, for their advice on analysis.


    References

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    2. Dietz WH. Health consequences of obesity in youth: childhood predictors of adult disease. Pediatrics 1998; 101 (3 Suppl March): 518-525.
    3. Power C, Lake JK, Cole TJ. Measurements of long-term health risks of child and adolescent fatness [review]. Int J Obes Relat Metab Disord 1997; 21: 507-526.
    4. Guo SS, Roche AF, Chumlea WC, et al. The predicitve value of childhood body mass index values for overweight at age 35 y. Am J Clin Nutr 1994; 59: 810-819.
    5. Manson JE, Willett WC, Stampfer MJ. Body weight and mortality among women. N Engl J Med 1995; 333: 677-685.
    6. Bellizzi MC, Dietz WH. Workshop on childhood obesity: summary of the discussion [review]. Am J Clin Nutr 1999; 70: 173S-175S.
    7. Cole TJ, Bellizzi MC, Flegal KM, Dietz WH. Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ 2000; 320: 1240-1243.
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    9. Lazarus R, Wake M, Hesketh K, Waters E. Change in body mass index in Australian primary school children, 1985-1997. Int J Obes Relat Metab Disord 2000; 24: 679-684.
    10. Pyke JE. Austalian Health and Fitness Survey 1985. Parkside, SA: The Australian Council for Health, Physical Education and Recreation Inc, 1987.
    11. McLennan W, Podger A. National nutrition survey user's guide 1995. Canberra: AGPS, 1998.
    12. Harvey PWJ, Althaus M-M. The distribution of body mass index in Australian children aged 7-15 years. Aust J Nutr Diet 1993; 50: 151-153.
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    15. Must A, Dallal GE. Reference data for obesity: 85th and 95th percentiles of body mass index (wt/ht2) and triceps skinfold thickness. Am J Clin Nutr 1991; 53: 839-846.
    16. Chinn S, Rona RJ. Prevalence and trends in overweight and obesity in three cross sectional studies of British children, 1974-94. BMJ 2001; 322: 24-26.
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    18. National Health and Medical Research Council. Acting on Australia's weight: a strategic plan for the prevention of overweight and obesity. Canberra: NHMRC, 1997.
    19. Goran MI. Measurement issues related to studies of childhood obesity: assessment of body composition, body fat distribution, physical activity and food intake. Pediatrics 1998; 101 (3 Suppl March): 505-518.
    20. Dollman J, Olds T, Norton K, Stuart D. The evolution of fitness and fatness in 10-11 year old Australian children: changes in distributional characteristics between 1985 and 1997. Pediatr Exer Sci 1999; 11: 108-121.
    21. Department of Community Services and Health. National dietary survey of schoolchildren (aged 10-15 years), 1985: no 2 — nutrient intakes. Canberra: AGPS, 1989.
    22. Magarey A, Boulton TJC. Food intake during childhood: percentiles of food energy, macronutrient and selected micronutrients from infancy to eight years of age. Med J Aust 1987; 147: 124-127.
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    (Received 22 Sep 2000, accepted 15 Feb 2001)



    Authors' details

    Department of Public Health, Flinders University of South Australia, Adelaide, SA.
    Anthea M Magarey, PhD, DipNutDiet, National Health and Medical Research Council (NHMRC) Research Fellow;
    Lynne A Daniels, PhD, DipNutDiet, Associate Professor.

    Women's and Children's Health, The Nepean Hospital, Penrith; and Western Clinical School, University of Sydney, NSW.
    T John C Boulton, MD, FRACP, Clinical Professor of Paediatrics.

    Reprints will not be available from the authors.
    Correspondence: Dr A M Magarey, Nutrition Unit, Department of Public Health, Flinders University of South Australia, Bedford Park, SA 5034.
    anthea.magareyATflinders.edu.au

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    1: Data collection procedures for the 1985 Australian Health and Fitness Survey (AHFS85) and the 1995 National Nutrition Survey (NNS95)

    Australian Health and Fitness Survey 1985 (n = 8492)
    The AHFS85 was conducted by the Australian Council on Health, Physical Education and Recreation from May to October 1985 inclusive.10 A representative sample of Australian schoolchildren aged 7-15 years was selected using a two-stage probability sampling design. Firstly, 109 schools were selected randomly from a list of all primary and secondary schools with 10 or more students enrolled in each age and sex category. The probability of selecting each school was proportional to the number of children enrolled at that school. All States and Territories were included and a good geographical distribution was achieved. Secondly, within selected schools, 15 students in each age and sex category were sampled from enrolment lists using a systematic selection process. The age of participants was determined in whole years as at 30 June 1985. Measurements were made by trained observers using standardised procedures.

    National Nutrition Survey 1995 (n = 3007)
    The NNS95 was a joint project of the Australian Bureau of Statistics and the Commonwealth Department of Health and data were collected from February 1995 to March 1996.11 The NNS sample was a systematically selected subsample of private dwellings of the National Health Survey (NHS), conducted on a multistage area sample. Up to two eligible people per household (in urban areas) and three per household (in rural areas) were randomly selected from the subsample. Weighting factors were applied to adjust for non-respondents. All States and Territories and urban, rural and remote areas were represented. Age was determined as a whole year at the most recent birthday. Measurements were made by trained observers using standardised protocols.

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    2: Previously published prevalences of overweight in Australian children from the 1985 and 1995 surveys, using the older definitions of overweight
    Age (years)
    Sex 2-3 4-8 9-11 12-15 16-18

    Australian Health and Fitness
    Survey 198512 (12-14 years)
    Overweight* Boys 8.9%
    Girls 9.1%
    National Nutrition Survey 199513
    High weight for height † Boys 3.9% 4.9%
    Girls 6.8% 5.9%
    At risk of overweight ‡ Boys 13.3% 20.3% 14.7%
    Girls 16.3% 11.9% 13.7%
    Overweight &Para; Boys 7.0% 7.2% 7.0%
    Girls 10.1% 6.5% 6.0%

    * Body mass index (BMI) ≥ 85th centile (National Health and Nutrition Examination Survey [NHANES II])
    † Z score (standard deviation score of weight for height) > +2
    ‡ BMI ≥ 85th and < 95th centile (NHANES I)
    &Para; BMI ≥ 95th centile (NHANES I)
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    3: Number of Australian children in each age group in the AHFS85 and the NNS95 samples and number (%) of children classified as overweight* and obese,† according to the new standard definitions7
    Age (years)
    Boys 2-3 4-6 7-11 12-15 16-18

    Australian Health and Fitness
    Survey 198510 (n = 2425) (n = 1877)
    Overweight 235 (9.7%) 165 (8.8%)
    Obese 36 (1.5%) 36 (1.9%)
    National Nutrition Survey 199513 (n = 164) (n = 270) (n = 457) (n = 360) (n = 264)
    Overweight 24 (14.6%) 28 (10.4%) 53 (11.6%) 72 (20.0%) 50 (18.9%)
    Obese 4 (2.4%) 8 (3.0%) 17 (3.7%) 22 (6.1%) 18 (6.8%)
    Girls

    Australian Health and Fitness
    Survey 198510 (n = 2443) (n = 1747)
    Overweight 269 (11.0%) 176 (10.1%)
    Obese 46 (1.9%) 23 (1.3%)
    National Nutrition Survey 199513 (n = 164) (n = 262) (n = 430) (n = 339) (n = 252)
    Overweight 27 (16.5%) 40 (15.3%) 74 (17.2%) 49 (14.5%) 37 (14.7%)
    Obese 10 (6.1%) 11 (4.2%) 27 (6.3%) 15 (4.4%) 15 (6.0%)

    * Overweight: body mass index (BMI)≥ cut-off 1 and † Obese: BMI † cut-off 2.
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    Anthea M Magarey
    Lynne A Daniels
    T John C Boulton