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This process is dependent on digestive enzymes secreted by the pancreas and lining of the intestinal tract which act on specific nutrients. Bile, from the liver, is required to emulsify fat, thus enabling it to be broken down by digestive enzymes. The absorption of nutrients is dependent on an active, or energy-dependent, transport across the intestinal epithelium lining the digestive tract. Villi, finger-like projections, increase the surface area of the small intestine to aid absorption. Most nutrients are absorbed from the small intestine although some require specific sites within the gastrointestinal tract; for example, vitamin B12 is absorbed in the terminal ileum. Mucosa shed into the lumen of the intestine is broken down and absorbed along with fluid and electrolytes secreted into the lumen during the process of digestion. The volumes of fluid secreted into the gut are large and may amount to 8–9 L a day when combined with an oral intake of 1.5–2 L daily. The majority of fluid is reabsorbed in the small intestine with the remainder being absorbed in the large intestine. Bacteria in the large intestine metabolize non-starch polysaccharides (dietary fibre), increasing faecal bulk and producing short chain fatty acids which are absorbed and metabolized for energy.

Related theory

Bodyweight is the most widely used measure of nutritional status in clinical practice. However, whilst 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 (Battezzati et al. 2003). 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 (van Loan 2003). Water comprises 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. The same ‘typical’ male contains approximately 12 kg of muscle, also referred to as lean body mass, and 12 kg of fat (Geissler and Powers 2005).

Body composition and nutritional status are closely linked and are dependent upon a number of factors including age, sex, metabolic requirements, dietary intake and the presence of disease. Depletion of lean body mass occurs in both acute trauma and chronic inflammatory conditions and negatively affects functional and immune capacity (Roubenoff and Kehayias 1991). Loss of skeletal lean body mass is a natural phenomenon of ageing and similarly compromises functional capacity. Shifts in body water compartments are readily observed in conditions such as ascites and oedema (Bedogni et al. 2003). Both of these conditions result from an abnormally increased extracellular water compartment 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 to determine changes indicative of nutritional risk (Bedogni et al. 2006). Such changes are masked if weight alone is used as the sole measurement.

Evidence-based approaches

Rationale

Nutritional support, to maintain or replete body composition, should be considered for anybody unable to maintain their nutritional status by taking their usual diet (NCCAC 2006). These include the following.

Patients unable to eat their usual diet (e.g. because of anorexia, mucositis, taste changes or dysphagia) should be given advice on modifying their diet.

Patients unable to meet their nutritional requirements, despite