The mass (SI unit = kilogram, kg) of a body is a composite measure of total body size. It is probably the most common anthropometric measure in population health sciences, often used in the assessment of obesity.
In common usage, human body mass is often referred to as weight. In fact, weight is the force exerted on an object due to gravity (SI unit = newton, N). A measure of weight in N allows estimation of human body mass in kg using the formula W = m*g and standard gravity. In addition to the term weight often being used instead of human body mass, assessment of human body mass is also often referred to as weighing.
If conducted at multiple time points, assessment of weight provides information on an individual’s physical status, showing the amount of mass that an individual has gained or lost over time. It can be combined with a standardising variable such as age or height to produce indices that allow a simple, but crude assessment of health status, such as weight-for-height Z-scores in young children or body mass index (kg/m²) in adults. Weight assessment is also used in equations for predicting energy expenditure and in indices of body composition.
A variety of scales are available for measuring weight such as beam balances and spring and digital scales. Digital scales are preferable as easier to read than scales with moving dials (see Figure A.3.4). Many digital scales, also known as body composition monitors can estimate body composition (e.g. body fat) using Bioelectrical Impedance Analysis (BIA) technology.
The scales need to be calibrated for accurate assessments of weight using standardised cast iron bar weights (see Figure A.3.5). It is common practice to carry out calibration procedures on a monthly basis. Ideally, if scales are moved on a regular basis, they should be calibrated daily.
Figure A.3.4 Spring scale with a moving dial.
Source: MRC Epidemiology Unit.
Figure A.3.5 Cast iron bar trade stamped weights for calibration use.
Source: MRC Epidemiology Unit.
In children and adolescents:
Adults and older individuals:
Assessment of weight is a fundamental anthropometric measurement and is very commonly used in a variety of health related studies. Estimation of weight is important for assessing growth and development, the construction of anthropometric indices such as weight for height, and for estimating physiological outcomes such as basal energy expenditure.
In addition to being used in isolation, weight is used in combination with other anthropometric dimensions to derive the following anthropometric indices:
Measurements of weight require very little data processing to derive final estimates of weight. Estimates should be quality checked during data processing in the same manner as other health related variables, for example by checking for outliers and data entry errors. There are no specific steps which are unique to weight. In some circumstances it may be necessary to convert units taken in imperial units to metric units.
An overview of weight assessment methods is outlined in Table A.3.6.
Table A.3.6 Characteristics of weight assessment methods.
|Number of participants||Large|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||Low|
|Risk of reactivity bias||No|
|Risk of recall bias||No|
|Risk of social desirability bias||No|
|Risk of observer bias||Yes|
|Suitability for field use||High|
|Participant literacy required||No|
Considerations relating to the use of weight methods in specific populations are described in Table A.3.7.
Table A.3.7 Use of weight assessment methods in different populations.
|Infancy and lactation||Suitable.|
|Toddlers and young children||Suitable.|
|Other||Suitable. The capacity of most of the scales commercially available is between 150-200 kg. If participants’ weight is above this, a bariatric platform with hand rails for stability can be used. It can weigh up to 500 kg.|
Factors contributing to minor weight fluctuations:
Bathrooms scales are not accurate and inappropriate to use in a professional setting. In the UK, medical equipment must be approved to either the Class III or Class IV standard, depending on the intended usage. Class III scales are the default standard for medical establishments, also used in medical research.
Refer to section: practical considerations for objective anthropometry
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