Body mass and physical measurements

Definition

This topic refers to respondents' physical measurements and other self-perception questions including:

• The height (cm), weight (kg) and waist circumference (cm) of respondents as measured during the interview
• Body mass index (BMI) derived from the height and weight physical measurements
• Self-perceived body mass.

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).

Population

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.

Methodology

Physical measurements

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

Body mass index (BMI) scores were derived using Quetelet's metric body mass index which is calculated as weight (kg) divided by height (m)². BMI scores are commonly grouped for output. Whist the cut off points for different ethnic groups can vary, the World Health Organisation recommends that the same cut off points be used for international classification. The detailed output classifications available are shown in the table below. 2017-18 NHS publications use aggregated categories for estimates in published tables.

Category		BMI score
Underweight
	Class 3	Less than 16
	Class 2	16.0 to less than 17.0
	Class 1	17.0 to less than 18.5
Normal range
	Normal	Adult: 18.5 to less than 20.0; Child: 18.5 to less than 25.0(a)
	Normal (Adult only)	20.0 to less than 25.0
Overweight
		25.0 to less than 30.0
Obese
	Class 1	Adult: 30.0 to less than 35.0; Child: 30.0 or more (a)
	Class 2 (Adult only)	35.0 to less than 40.0
	Class 3 (Adult only)	40 or more

(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

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.

	Not at risk	Increased risk	Substantially increased risk
Males	Less than 94 cm	94 cm or more	102 cm or more
Females	Less than 80 cm	80 cm or more	88 cm or more

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Respondents were asked whether they considered themselves to be:

• Acceptable weight
• Underweight
• Overweight.

This question was not asked where:

• A female had previously identified themselves as currently pregnant (or raised it at this point).
• A proxy was being used for a respondent aged 15-17 years
• A proxy was being used for an adult interview and the respondent was not present.

Data items

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.

Interpretation

Points to be considered when interpreting data for this topic include the following.

• BMI was calculated for persons for whom height and weight were measured or imputed (for further information see Imputation). For 2017-18, 64.1% of respondents aged 2 years and over had their height and weight measured (i.e. This comprises 56.1% of children aged 2-17 who had their height and weight measured, and 66.2% of respondents 18 years and over who had their height and weight measured. However, to allow for the calculation of BMI for all fully responding persons, imputation was used to obtain values for respondents for whom physical measurements were not taken. The results presented as part of the 2017-18 NHS include both measured and imputed data.
• Non-response rates for physical measurements were higher in 2017-18 than in 2014-15; for example, the non-response for BMI for adults in 2017-18 was 33.8% compared with 26.8% in 2014-15.
• While BMI is a useful tool to assess and monitor changes in body mass at the population level, it may be an inappropriate measure of the body fatness of certain populations and certain individuals; for example, those whose high body mass is due to muscle rather than fat, those with osteoporosis who have lower than usual BMI, or those of non-European background, whose risk levels are not accurately reflected in the BMI cut-off points used. BMI can, however, be used in conjunction with waist circumference which provides a second indicator for people at risk.
• Respondents were not specifically asked whether they were pregnant, however they were able to volunteer this during the interview. If the respondent volunteered that they were currently pregnant, the interviewer was instructed not to take their measurements and to explain to the respondent that the measurements during the course of a pregnancy will not accurately reflect their general (i.e. non-pregnant) measures.
• If physical measurements were taken prior to being informed that the respondent was pregnant, interviewers were instructed to include a comment in the instrument confirming that the measurements taken were of a pregnant respondent, their results were excluded.

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).