Body weight is the sum of individual body components: eg heart, muscle, plasma and skeleton. These components are grouped into compartments: the fat compartment (ie adipose tissue) and the fat-free compartment (also known as lean body mass). Lean body mass may be further divided into muscle and viscera, skeleton and water (both intracellular and extracellular fluids). The lean body mass is the biologically active tissue and changes in this compartment are important.
Weight should be measured using either beam balance or well-calibrated digital scales with the subject in light clothing and no shoes. In addition, the height is usually measured. Care must be taken to ensure the spine is fully extended, the heels are planted firmly on the ground and the head held in the correct plane.
A number of techniques to measure body compartments are available:
To determine what is normal weight, several different approaches can be adopted but most are in the context of a given weight for a particular height:
In Australia it is the standard to assess normality of weight using body mass Menu (BMI) which is weight(kg)/height(m)2 and this has been shown to correlate well with adiposity. For adults a BMI of 20 to 25 is considered to be in the healthy weight range, from 18 to 20 slightly underweight and below 18 underweight while values above 25 indicate overweight and above 30 indicate obesity. For children the healthy BMI range will be at lower values.
When you are deciding if an individual's weight is normal it is important to establish a weight history. If an adult had a BMI of 25 for many years but loses 20% of their body weight in two months this new weight is probably not normal nor healthy for that individual even if their weight remains within the healthy weight range.
It may be necessary to assess individual body compartments to determine normality. Some athletes eg. weight lifters, may have extra large muscle mass and may be misclassified as overweight or a patient with a disease that causes fluid retention, such as liver failure, may be misclassified. An individual with a small skeleton and muscle mass may be considered normal when they actually have above normal percentage of their body as fat.
The most readily accessible way to assess body compartments is arm anthropometry. Large population surveys have measured upper arm anthropometry ie the mid-upper arm circumference with a tape measure and the fat skinfold over the triceps muscle using a calliper.
The triceps indicate adiposity while the calculated muscle circumference indicates lean body. Reference percentiles are available for both children and adults with values less than the fifth percentile evidence of depletion and those below 10 indicating moderate depletion.
Assessing growth and determining healthy weight range.
This provides a good summary of how to measure and interpret growth in a child or adolescent and gives a guide to determining what a likely healthy weight range is for a child or adolescent. The absolute weight is, however, not the only information on which decisions should be made on deciding if there is a problem or not as eating behaviours and psychological status are equally as important. It is part of the Eating disorders toolkit , which has been provided as a comprehensive document on the website for the Centre for Eating and Dieting Disorders
Normal growth in childhood smp.sydney.edu.au/compass/teachingactivity/view/id/768
Shils, M.E., Olson, J.A. & Shike, M, (eds) 1999, Modern nutrition in health and disease, 9th edn, Williams & Wilkins, Baltimore.
South, M. & Isaacs, D. (eds) 2007, Practical paediatrics, 7th edn., Churchill Livingstone, Edinburgh.
Pietrobelli A, Tato L. Body composition measurements: from the past to the future.Acta Paediatrica Supplement. 2005;94 Suppl 448:S8-13.
opac.library.usyd.edu.au/record=b3192844~S4
CDC growth charts