Bodyweight is the most widely used measure of nutritional status in clinical practice. However, while weight provides a simple, readily obtainable and usually fairly precise measure, it remains a one‐dimensional metric and as such has limitations. In contrast, an understanding of anatomy and physiology and in particular the changes that can occur in body composition, in addition to frank weight gain or loss, provides valuable clinical insight (Thibault et al. [178]). In the so‐called ‘two‐compartment’ model of body composition, bodyweight is described in terms of, firstly, fat‐free mass, that is bones, muscles and organs, which includes the hepatic carbohydrate energy store glycogen, and secondly, fat mass or adipose tissue (Thibault et al. [178]). In health, water constitutes up to 60% of total bodyweight. It is distributed throughout the fat‐free and fat compartments with approximately two‐thirds present as intracellular and one‐third as extracellular fluid. Thus, a healthy 70 kg male comprises 42 kg of water, which amounts to 60% of total bodyweight. This is made up of 28 L of intracellular fluid and 14 L of extracellular fluid (Santanasto et al. [157]).

Body composition and nutritional status are closely linked and are dependent on a number of factors, including age, sex, metabolic requirements, dietary intake and the presence of disease. Depletion of lean body mass occurs in both critical illness and injury; it negatively affects physical functioning and immune function, and can be associated with increased mortality and length of hospital stay (Carlsson et al. [30], Jensen et al. [71], Shachar et al. [162]). Generalized loss of skeletal muscle mass and strength with a risk of adverse outcomes (such as physical disability, poor quality of life and death) is called sarcopenia. Sarcopenia plays a major role in the increased frailty and functional impairment that come with age (van den Berg et al. [184]). Shifts in body water compartments are readily observed in conditions such as ascites and oedema (Jaffee et al. [70], Schol et al. [159]). Both of these conditions result from abnormally increased extracellular water and, despite a gain in total bodyweight, are indicative of worsening outcome.

The physiological characteristics of the different body compartments can be exploited by various assessment tools and techniques, such as bioimpedance analysis (BIA) or measurement of cross‐sectional areas of muscle from computed tomography (CT) scanning, to determine changes indicative of nutritional risk (Santanasto et al. [157]). Such changes are masked if weight alone is used as the sole measurement.