Monitoring Body Weight
Monitoring of body weight is usually undertaken to assess whether body fat has accumulated to such an extent that it may have an adverse effect on health. Excess weight is a major risk factor for chronic disease such as cardiovascular disease, type 2 diabetes, high blood pressure, asthma back pain and some cancers, as well as increased death rates (AIHW 2017). Body Mass Index (BMI) is an internationally recognised measure for monitoring body weight in populations (WHO 2000).
Body Mass Index
Body mass index is measured using a person's weight and height. The Body Mass Index (BMI) = weight in kilograms divided by the square of the height in metres.
In adults, the BMI is classified into four categories: underweight (BMI <18.5 kg/m2), healthy weight (BMI 18.5-<25 kg/m2), overweight (BMI 25-<30 kg/m2), and obese (BMI ≥30 kg/m2). This classification, established by the WHO, commonly includes a further subclassification of obesity into three classes: class I (BMI 30.0-34.9 kg/m2), class II (BMI 35.0-39.9 kg/m2) and class III (BMI ≥ 40.0 kg/m2).
BMI does not necessarily reflect body fat distribution or describe the same degree of fatness in different individuals. The BMI standard classification categories may not be suitable for all ethnic groups and are not suitable for children. For children as their BMI changes substantially with age and growth patterns that varies between boys and girls, a separate classification of overweight and obesity based on age and sex is used (Cole et al 2000).
BMI measured or self-reported
When height and weight data are collected in surveys, they may be measured (for example, Australian Health Survey 2011-12) or rely on self-reported data (as in the NSW Population Health Survey). It has been found that on average people tend to overestimate their height and underestimate their weight, leading to an underestimate of BMI, both in Australia (ABS 2012) and internationally (Krul et al 2010; Maukonen et al 2018). Further, the level of mis-reporting for height and weight varies by gender, age, geographic region as well as across BMI categories. A validation study of 1997 NSW Population Health Survey data reported that the prevalence of overweight and obesity was underestimated by 23% for men and 15% for women (Flood et al. 2000). Consequently, rates of overweight and obesity based on self-reported data are likely to be underestimates of the true rates and are not directly comparable with rates based on measured data. Nevertheless, while not as accurate as measured values, the BMI calculated from self-reported height and weight still provides useful data for ongoing surveillance of population health.
However, the BMI classification is not suitable for everyone. Compared with the rest of the population, some ethnic groups may have equivalent levels of chronic disease risk at lower BMI (for example people born in Asia) or higher BMI (for example people born in Polynesia). The differences originate from different body proportions in various ethnic groups, while the BMI classification is based on the international standards developed for adult people of European descent.
Waist circumference
Another way of monitoring body weight is measuring waist circumference. It measures abdominal obesity, that is, the fat in the abdominal and around internal organs providing a tool to assess risk of metabolic complications like type 2 diabetes and body fat distribution. Increasing waist measurement is associated with an increasing risk of chronic disease. Waist circumference is used less frequently in self-reported surveys than the BMI as people are more likely to know their height and weight than their waist circumference.
Assessment of adverse effects of body weight on health
BMI tends to increase as body fat increases. This relationship makes BMI a useful measure of overweight and obesity in populations, however, as discussed above, its usefulness is not universal and for the individual other measures may offer better insight.
Because individuals with greater muscle mass have higher BMIs as muscle tissue is heavier than fat (adipose) tissue, a thorough assessment of possible adverse effects of body weight on the health of an individual should include not only the measurement of BMI, but also the distribution of body fat (via measurement of waist circumference and skin fold thickness), evaluation of regular physical activity and nutrition, other cardiovascular risk factors (such as blood cholesterol and high blood pressure) and existing health conditions (such as diabetes).
References
Australian Bureau of Statistics 2012 Research Paper: Investigating the Discrepancy between Measured and Self-Reported BMI in the National Health Survey. 1351.0.55.039 https://www.abs.gov.au/ausstats/abs@.nsf/mf/1351.0.55.039 (accessed on 05/05/2021)
Australian Institute of Health and Welfare. 2017 Impact of overweight and obesity as a risk factor for chronic conditions: Australian Burden of Disease Study. Australian Burden of Disease Study series no.11. Cat. no. BOD 12. Canberra: AIHW.
Australian Institute of Health and Welfare. 2020a Overweight and obesity : an interactive insight Web report Cat. no: PHE 251 Canberra AIHW https://www.aihw.gov.au/reports/overweight-obesity/overweight-and-obesity-an-interactive-insight/contents/what-is-overweight-and-obesity (accessed on 05/05/2021)
Australian Institute of Health and Welfare. 2020b A framework for monitoring overweight and obesity in Australia. Cat. no. PHE 272. Canberra: AIHW.
Cole TJ, Flegal KM, Nichols D, Jackson AA. Body mass index cut offs to determine thinness in children and adolescents: international survey. British Medical Journal 2007; 335 (7612): 194-202. Available at https://www.bmj.com/content/335/7612/194 (Accessed 13 May 2021).
Cole TJ, Bellizzi MC, Flegal KM, Dietz WH. Establishing a standard definition of child overweight and obesity worldwide: international survey. British Medical Journal 2000; 320: 1240-3. Available at https://www.bmj.com/content/320/7244/1240 (Accessed 13 May 2021).
Flood V, Webb K, Lazarus R, Pang G. Use of self-report to monitor overweight and obesity in populations: Some issues for consideration. Australian and New Zealand Journal of Public Health 2000; 24: 96-9.
Krul AJ, Daanen H, Choi H, Self-reported and measured weight, height and body mass index (BMI) in Italy, the Netherlands and North America European Journal of Public Health, Volume 21, Issue 4, August 2011, Pages 414–419, https://doi.org/10.1093/eurpub/ckp228 (Accessed 13 May 2021).
Maukonen M, Männistö S, Tolonen H. A comparison of measured versus self-reported anthropometrics for assessing obesity in adults: a literature review. Scand J Public Health. 2018 Jul;46(5):565-579. doi: 10.1177/1403494818761971. Epub 2018 Mar 10. PMID: 29528773.
National Health and Medical Research Council. Clinical practice guidelines for the management of overweight and obesity in adults, adolescents and children in Australia. Melbourne: National Health and Medical Research Council, 2013. Available at https://www.nhmrc.gov.au/guidelines-publications/n57 (Accessed 13 May 2021).
WHO (World Health Organization) 2000. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. WHO technical report series 894. Geneva: WHO.