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Despite this, the NHMRC expert report on prevention of obesity and overweight¹ found few population-based data on Australians' weight-control behaviours. As the report noted, to understand weight-control behaviours, it is first necessary to answer the question: "Who perceives themselves as overweight?".¹

To date, no national Australian studies have addressed this question. Most Australian studies of weight-control attitudes and behaviours have used small and unrepresentative samples and have focused on women, particularly younger women.³ Although two studies used larger, representative samples of adults in Adelaide and Melbourne to investigate the relationship between body mass index (BMI) and perceptions of overweight,^4,5 both calculated BMI from self-reported height and weight. Self-reported measures are known to lead to underestimates of BMI.^6-8

The 1995 National Health Survey⁹ and National Nutrition Survey¹⁰ provided a unique opportunity to investigate the relationship between Australian adults' subjective perceptions of their weight and their measured BMIs. I used data from these surveys to investigate:

• to what extent Australian adults with different BMIs perceive themselves to be overweight; and

• the BMI at which adults perceive themselves to be overweight.

The 1995 National Nutrition Survey was conducted on a random subsample of the households interviewed for the National Health Survey. Participants in the National Nutrition Survey answered detailed questions about their current food consumption and had their heights and weights measured by specially trained interviewers two to three weeks after the National Health Survey interview.¹⁰

Response rates were 97% for the National Health Survey⁹ and 61% for the National Nutrition Survey.¹⁰ Non-responders were more likely to be characterised by one or more of the following: high income, older than 59, unmarried, or unemployed.

The current study was based on a sample of 5076 men and 5576 women aged 18 and over for whom height and weight measurements were obtained by the National Nutrition Survey. The sample represents 98.4% of respondents (excluding pregnant women).

Population percentages were estimated from sample percentages using the weighting factors and methods of the Australian Bureau of Statistics (ABS),¹⁵ which adjust for the sampling method (multistage, cluster sampling). This ensured that the percentages represented as far as possible the adult Australian population. For percentages close to or equal to zero, exact confidence intervals were calculated using the binomial distribution.¹⁶ All other confidence intervals were estimated using the relative standard errors provided by the ABS.²

Among people with a BMI 25, an estimated 49.3% of men and 72.0% of women considered themselves overweight (95% CIs, 48.1%-50.5% and 70.8%-73.1%, respectively). Estimated percentages of those who considered themselves overweight are shown in Box 2 by age group. At all ages, women were more likely to consider themselves overweight than men; the difference was significant for all except those aged 70 years and over. Women in this age group were less likely to consider themselves overweight than younger women, while men in their 20s or those 80 years and over were less likely to consider themselves overweight than men of other ages.

Among people with a BMI < 25, an estimated 3.4% of men and 12.4% of women considered themselves overweight (95% CIs, 2.8%-4.1% and 11.4%-13.3%, respectively). For men, there was little variation with age (Box 2), and, although there were statistically significant differences between some age groups for women, these differences were small.

The estimated percentage of people who considered themselves overweight at each measured BMI is shown in Box 3. Data for men and women in different age groups were analysed separately. For men, there were no significant differences between age groups (not shown). For women, the only significant difference was between women aged under 60 and older women. The percentages of both sexes who considered themselves overweight increased with BMI; the lowest BMI at which at least half the respondents considered themselves overweight was 26 to < 27 for women aged 18-59 years, and 28 to < 29 for older women and men.

A potential limitation of the study was the response rate to the National Nutrition Survey, which was low by ABS standards for household surveys. However, adjustment of results to represent the total population would be expected to minimise the effect of response bias.

The results of this study contrast with those of two other Australian studies.^4,5 These found that around 25% of men and 45% of women of acceptable weight considered themselves overweight, while only about 15% of overweight men and 5% of overweight women considered their weight to be acceptable. Both earlier studies estimated BMI from self-reported height and weight, which are known to underestimate BMI.^6-8 For example, based on self-reported height and weight, 36% of women and 52% of men aged 18 and over were overweight or obese in Australia in 1995,¹⁴ considerably fewer than the 49% of women and 64% of men found to be overweight or obese by measurement in the National Nutrition Survey.² Therefore, in the earlier studies, some people classified as having acceptable weight would have been overweight, and those classified as overweight would mostly have had BMIs well in excess of 25.

In addition, when respondents in the earlier studies were asked to assess their weight, they were given the choice of "slightly overweight",^4,5 "very overweight"^4,5 or "extremely overweight"⁴ (rather than the single option of "overweight" used in the current study). The option "slightly overweight" was chosen by nearly all who considered themselves overweight but were actually of acceptable weight, and by well over half those who considered themselves overweight and were actually overweight or obese. Thus, another possible explanation for the difference in my results is that, when given the choice of classifying oneself as "acceptable weight" or "overweight", those who consider themselves slightly overweight generally opt for "acceptable weight".

The results of this study suggest that, for men and for women aged 60 and over, the meaning of "overweight" differs considerably from the medical definition. For men, "overweight" appears to mean a BMI greater than 28. Only a minority of men consider themselves overweight at lower BMIs, while 30% of men with a BMI of 30-31 consider their weight acceptable. This is consistent with the findings of a recent study of weight-control practices of adults in an Australian rural community; when asked about their weight goals in the next year, only one in five overweight men wished to attain a BMI below 25.¹⁷ Some highly muscular men may have a BMI above 25 and not be overweight, but, given the increase in BMI with age, this group is likely to be small. Men's perceptions may be influenced by the high proportion who are overweight; it is so common that being moderately overweight may be seen as normal.

For women aged 60 and over, the meaning of "overweight" appears similar to that for men, but for younger women, the meaning is closer to the medical definition. However, for many of these younger women, a BMI in the upper part of the acceptable range constitutes being "overweight". A factor influencing these women's attitudes may be the mass media focus on images of very thin (young) women. The difference in attitudes between women of different ages may reflect a change in attitude with age; as a substantial proportion of older women are overweight, older women may consider it normal. There may also be a generational difference in women's attitudes, possibly related to changes in media images of the ideal female form.

For most people, achieving and maintaining weight loss involves considerable lifestyle change, which is unlikely to be achieved unless people consider the reasons are compelling. My study suggests that, for clinical and public health weight reduction programs to be successful, they must first convince many overweight people that their weight is an issue which needs addressing.

1. National Health and Medical Research Council. Acting on Australia's weight: A strategic plan for prevention of overweight and obesity. Canberra: AGPS, 1997.
2. Australian Bureau of Statistics. National Nutrition Survey: selected highlights, Australia, 1995. Canberra: AGPS, 1995. (Catalogue No. 4802.0.)
3. Crawford D, Owen N. The behavioural epidemiology of weight control. Aust J Public Health 1994; 18: 143-148.
4. Crawford D, Worsley A. Present and desired body weights of Australian adults: a cause for concern? Community Health Stud 1987; 11: 62-67.
5. Paxton S, Sculthorpe A. Weight-loss strategies and beliefs in high and low socioeconomic areas of Melbourne. Aust J Public Health 1994; 18: 412-417.
6. Hill A, Roberts J. Body mass index: a comparison between self-reported and measured height and weight. J Public Health Med 1998; 20: 206-210.
7. Kuskowska Wolk A, Bergstrom R, Bostrom G. Relationship between questionnaire data and medical records of height, weight and body mass index. Int J Obes Relat Metab Disord 1992; 16: 1-9.
8. Nieto Garcia FJ, Bush TL, Keyl PM. Body mass definitions of obesity: sensitivity and specificity using self-reported weight and height. Epidemiology 1990; 1: 146-152.
9. Australian Bureau of Statistics. National Health Survey: users' guide. Canberra: AGPS, 1995. (Catalogue No. 4363.0.)
10. Australian Bureau of Statistics. National Nutrition Survey: users' guide. Canberra: AGPS, 1995. (Catalogue No. 4801.0.)
11. SPSS Inc. SPSS. Release 9.0.1. Chicago, Ill: SPSS Inc, 1999.
12. Microsoft. Excel 97 SR-1. Microsoft, 1997.
13. National Health and Medical Research Council. Reports of the 98th and 100th sessions. Canberra: AGPS, 1984 and 1985.
14. Australian Bureau of Statistics. How Australians measure up. Canberra: AGPS, 1998. (Catalogue No. 4359.0.)
15. Australian Bureau of Statistics. National Nutrition Survey: technical paper for confidentialised unit record file. Canberra: AGPS, 1995. (Catalogue No. 4807.0.)
16. Armitage P, Berry G. Statistical methods in medical research. Oxford: Blackwell, 1994.
17. Crawford D, Owen N, Broom D, et al. Weight control practices of adults in a rural community. Aust N Z J Public Health 1998; 22: 73-79.

Reprints will not be available from the author.
Correspondence: Ms S M Donath, Key Centre for Women's Health, Faculty of Medicine, Dentistry and Health Sciences, The University of Melbourne, Parkville, VIC 3052.
s.donathATkcwh.unimelb.edu.au

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