Introduction

It goes without saying that without food there can be no life, that food is a basic human right, and that it behoves every doctor to pay attention to the nutritional needs of their patients. Nevertheless, approximately one-third of all patients admitted to an acute hospital will have evidence of protein-calorie malnutrition and two-thirds will leave hospital either malnourished or having lost weight. Against this background it is important to recognize that in Western Society there is now an epidemic of obesity. Whilst obese individuals generally have a matching increase in lean body mass, there is a subgroup with underlying muscle wasting (sarcopenic obesity) who are at high risk of metabolic syndrome and postoperative complications. Patients with sarcopenic obesity are difficult to recognize clinically due to the fact that their muscle wasting is obscured by overlying fat.

Malnutrition has damaging effects on psychological status, activity levels and appearance. Paradoxically, in the surgical patient a low body fat content may sometimes be viewed as an advantage, making technical aspects of surgery easier. There is, however, clear evidence that patients with severe protein depletion have a significantly greater incidence of postoperative complications, such as pneumonia and wound infection, and a prolonged hospital stay.

Nutritional disorders in surgical practice have two principal components. First, starvation can be initiated by the effects of the disease, by restriction of oral intake, or both. Simple starvation results in progressive loss of the body’s energy and protein reserves (i.e. subcutaneous fat and skeletal muscle). Second, there are the metabolic effects of stress/inflammation; namely, increased catabolism and reduced anabolism. These result in a variety of changes, including a low serum albumin concentration, accelerated muscle wasting and water retention. Although malnutrition may be the result of starvation, in most surgical patients it results from a combination of reduced food intake and metabolic change (Fig. 3.1).

Fig. 3.1 Mechanisms linking the effects of disease/surgery on patient outcomes.

Assessment of nutritional status

The main energy reserves in the body are found in subcutaneous and intra-abdominal fat. Loss of fat reserves does not usually impair function. In contrast, there are no true protein reserves in the body. Thus, in the face of starvation or stress, structural tissues such as skeletal muscle and the gut are autocannibalized to liberate amino acids, resulting in functional impairment that can eventually impede recovery.

The key elements of nutritional assessment include current food intake, levels of energy and protein reserves, and the patient’s likely clinical course (Fig. 3.2). Patients who have not eaten for 5 days or more require nutritional support, and those with symptoms such as anorexia, nausea, vomiting or early satiety are at risk of a reduced food intake and hence undernutrition. Levels of energy reserves are most easily assessed by examining for loss of subcutaneous fat (skinfolds), whereas protein depletion is most commonly manifest as skeletal muscle wasting (Fig. 3.3). A history of weight loss of more than 10–15% is highly significant. Patients can also be assessed according to their body mass index – BMI = weight (kg)/height (m²). The normal BMI is 18.5–24.9. A value less than 18 is suggestive of significant protein-calorie undernutrition. Finally, it is important to recognize that in assessing the nutritional status of patients, knowledge of their likely clinical course is vital (Fig. 3.4). For example, if patients are well nourished, they should be able to withstand the brief period of fasting associated with major surgery. However, if patients are severely malnourished (e.g. weight loss of 15%, BMI 17), then even a short further period of starvation or catabolism may make them so critically undernourished that this may become life-threatening in itself. Taken together, a patient’s food intake, level of reserve and likely clinical course should alert the astute clinician to the need for nutritional support and should be part of the routine daily appraisal of every patient during a surgical ward round.

Fig. 3.2 Nutritional assessment in surgical patients.

Fig. 3.3 Protein-energy malnutrition in a surgical patient, illustrating depleted muscle and subcutaneous fat stores.

Fig. 3.4 Alterations in nutritional status associated with weight loss.

Assessment of nutritional requirements

Energy and protein/nitrogen requirements vary, depending on weight, body composition, clinical status, mobility and dietary intake. For most patients, an approximation based on weight and clinical status is sufficient. Relevant values are given in Table 3.1. Few adult patients require more than 25–30 kcal/kg/day (approximately 1800–2200 kcal in an adult of average body mass). Additional calories are unlikely to be used effectively and may even constitute a metabolic stress. Particular caution must be exercised when refeeding the chronically starved patient because of the dangers of hypokalaemia and hypophosphataemia (notably cardiac dysrhythmias).

Table 3.1 Estimation of energy and protein requirements in adult surgical patients

* Grams of protein can be converted to the equivalent amount of nitrogen by dividing by 6.25.

The most common method for assessing protein/nitrogen requirement is based on body weight (Table 3.1). Although more accurate assessment for patients receiving nutritional support can be derived from measurement of 24-hour urinary urea excretion, which can be converted to an estimate of 24-hour urinary nitrogen loss, this is seldom necessary in routine clinical practice.

Enteral diets will usually provide protein whereas parenteral nutrition provides the nitrogen (N) in the form of amino acids. The nitrogen equivalent of protein can be calculated by multiplying nitrogen requirement by a conversion factor of 6.25. In practice, nitrogen requirements are usually estimated based upon predicted calorie intake and the level of metabolic stress. Most patients will require 1gN per 200 kcal of energy provided daily (typically 10gN) in the absence of sepsis but this may increase to as much as 18–20g N in critically ill, catabolic and septic patients. Even if losses are in excess of this, more than 18 g nitrogen/day (equivalent to 112 g protein) is seldom given because it is unlikely to be used effectively. It is usually impossible to prevent substantial loss of protein reserves and lean body mass in critically ill patients and the aim of meeting requirements in such patients is primarily to limit losses resulting from catabolism.

Causes of inadequate intake

The ideal way for surgical patients to take in enough nutrients is for them to eat or drink palatable food. Unfortunately, the catering budget is often far too low for the provision of appetizing food, and wastage of unwanted food can account for up to 40% of that served. Other reasons for a poor food intake include the patient being too weak and anorexic, or having a mechanical problem such as obstruction of the gastrointestinal tract. Patients with increased metabolic demands may have some difficulty in taking sufficient food to meet such demands. Patients with a normal functional gut may also have a reduced food intake due simply to the cumulative effects of repeated periods of fasting to undergo investigations such as endoscopy or radiology.

Some patients suffer from what is best described as ‘intestinal failure’, i.e. a state in which the amount of functioning gut is reduced below a level where enough food can be digested and absorbed for nourishment. Intestinal failure can be acute (when it is usually reversible) or chronic (when it is frequently permanent). Acute intestinal failure is relatively common, especially after abdominal surgery when it commonly results from the development of surgical complications, whereas chronic intestinal failure is comparatively rare. The principal causes of acute intestinal failure are mechanical intestinal obstruction and paralytic ileus, frequently associated with abdominal sepsis, as well as intestinal fistula formation, in which bowel content is lost externally or short-circuited (internal fistula) before it can be adequately digested and absorbed. Chronic intestinal failure may result from short bowel syndrome, following extensive small bowel resection, extensive small bowel disease, such as Crohn’s disease, and motility disorders, such as chronic intestinal pseudo-obstruction. In some patients with short bowel syndrome, the remaining intestine may adapt over a period of months or years by a process of progressive dilatation and mucosal hyperplasia, allowing the patient to regain nutritional independence. Reconstructive surgery may also improve the function or even be employed to increase the functional length of remaining intestine in selected cases.

Specialized nutritional treatment is required in patients with intestinal failure if the patient is to remain adequately nourished. The provision of nutrition in many patients with acute intestinal failure is further complicated by the metabolic consequences of ongoing inflammation or sepsis. As a general rule, this results in increased energy requirements and impaired ability to utilize administered nutrients, rendering nutritional support less effective. The priority in providing effective nutritional support for such patients is therefore to simultaneously eliminate sepsis.