body mass

Last reviewed 05/2021

Body weight refers to the mass of the individual in S.I. units of kilograms (kg). It is extremely variable both within a population and in the individual with time. It is dependent on both genetic and environmental factors.

Weight varies with height and build; attempts have been made to standardise weight independent of the latter variables to indicate nutritional state. One approach is to compare weights of populations with similar builds; however, previously, standard populations were not objectively and metrically assessed as to their build. Physique is a relatively difficult parameter to accurately and quickly quantify. Hence, the use of the body mass index, a measure of weight that is independent of height, has become a more widespread and practical guide.

In practice, the trend of weight change of an individual with time is a useful tool for monitoring pathology or therapeutic intervention. Ideally, it should be carried out by the same observer every day in standardized conditions. Yet, even weight trends must be interpreted with caution:

  • body compartments are not distinguished e.g. acute weight decrease may just reflect fluid loss
  • energy source is not distinguised e.g. starvation in healthy individuals catabolizes fat whereas protein may be degraded in hypercatabolic states

Nevertheless, weight is a key medical index used for:

  • monitoring satisfactory growth in neonates, infants and children
  • assessing general nutritional status
  • assessing response to treatment e.g. heart failure and diuresis with loop diuretics
  • determining suitability for surgery e.g. weight loss of 10-15% is associated with significant postoperative morbidity