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Body mass and physical measurements
Body Mass Index (BMI) or Quetelet's index is a useful tool, at a population level, for measuring trends in body weight and helping to define population groups who are at higher risk of developing long-term medical conditions associated with a high BMI, for example Type 2 diabetes and cardiovascular disease.
Waist circumference reflects mainly subcutaneous abdominal fat storage, and according to a World Health Organisation (WHO) joint report has been shown to positively correlate to disease risk. The scale used for determining risky waist circumference is as recommended by the World Health Organisation, (See Obesity: preventing and managing the global epidemic. Report of a WHO Consultation, 2000).
Physical measurements were obtained for all persons, excluding pregnant women, aged 2 years and over who agreed for the measurements to be taken.
Self-perceived body mass was asked of all persons aged 15 years and over in the 2017-18 NHS. It was not asked where a proxy was reporting on behalf of the respondent.
Physical measurements were taken towards the end of the survey. All physical measurements were voluntary, and women who volunteered at this point in the interview that they were pregnant were not measured. Interviewers used digital scales to measure weight (maximum 200kg), a stadiometer to measure height (maximum 210cm), and a metal tape measure (which avoided the risk of the tape stretching) to measure waist circumference (maximum 200cm). Thorough interviewer training identified the points at which waists were to be measured as recommended by a World Health Organisation report as well as how to take the measurements with the least amount of respondent discomfort. For waist measurements, interviewers held the end of the tape at the appropriate point and asked the respondent to turn around until the tape met, or asked the respondent to hold the end of the tape and walked around them until the tape met.
Interviewers encouraged respondents to remove their shoes and any heavy clothing, e.g. jumpers, before they took measurements, however, this was voluntary, and may not have occurred in some cases. Interviewers were not required to record if they thought clothing may have impacted significantly on measurements. Weight was recorded in kilos to one decimal point, and height and waist measurements were recorded in centimetres to one decimal point. Waist measurements were taken by placing the tape measure across the top of the belly button. If a respondent's waist measurement was more than two meters (the maximum measurement of the tape measure), interviewers were instructed to record this as 200.0cm. If a respondent's weight was self-reported to be more than 200 kilograms (the maximum measurement of the scales used) the interviewer was instructed to record 999.9 kg and include a comment in the instrument, however some interviewers recorded weights above 200kg and these records were included in output.
In order to validate the height and waist measurements, a random 10% of respondents were selected to be measured an additional time. If this second measurement of height or waist varied by more than one centimetre then a third reading was taken. Weight measurements were only taken once. For output purposes only the first measurement is used.
Body mass index
(a) Child cut-offs identified in this table are in terms of Adult cut-offs. While the formula to calculate BMI scores is the same for adults and children, the classification of children's BMI is different to that of persons aged 18 years and over, and takes into account individual age and sex. BMI cut-off ranges for children 2 to 17 years of age are included in Appendix 4: Classification of BMI for Children. Half-year cut-off points are used to calculate children's BMI scores for persons aged 2 to 17. Two versions of the data item are available, one using the mid-year cut-off (used in the National Health Survey: First Results, 2017-18) and one using the whole-year cut-off.
Waist circumference reflects mainly subcutaneous abdominal fat storage, and according to a World Health Organisation (WHO) joint report has been shown to positively correlate to disease risk. The scale used for determining risky waist circumference is as recommended by the WHO, (See Obesity: preventing and managing the global epidemic. Report of a WHO Consultation, 2000). As with BMI, the cut-off points in this scale are designed for people of European origin. However, as ethnicity cannot be determined, the same cut-off points are used for all respondents.
Waist circumference guidelines, Adults
Respondents were asked whether they considered themselves to be:
This question was not asked where:
The questionnaire, data items and related output categories for this topic are available in pdf / Excel spreadsheet format from the Downloads page of this product.
Points to be considered when interpreting data for this topic include the following.
Comparability with 2014-15
Body mass and physical measures data are considered directly comparable between the 2017-18 and 2014-15 NHS.
A new imputation method was utilised in 2017-18, however this had minimal impact on results. For more information please see the chapter on Imputation.
New scales allowing measurements of up to 200kg were used in NHS 2017-18. In addition, a new stadiometer was used to measure height for greater accuracy. For this reason, an additional height measure was taken to analyse variation for quality assurance. Despite these changes, the estimates are considered comparable with 2014-15.
More information regarding comparisons between 2014-15 NHS and previous cycles is available in the National Health Survey: Users' Guide, 2014-15 (cat. no. 4363.0).
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