Nutrition

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4 Nutrition

Chapter Contents

Dietary requirements 119
Protein–energy malnutrition 121
Nutritional support in the hospital patient 124
Nutritional support in the home patient 130
Refeeding syndrome 130

Dietary requirements

Energy

Food is necessary to provide the body with energy. The SI unit of energy is the joule (J), and 1 kJ = 0.239 kcal. The conversion factor of 4.2 kJ, equivalent to 1 kcal, is used in clinical nutrition.

Energy expenditure gives a reasonably accurate estimate of dietary requirements. Daily energy expenditure is the sum of:

• the basal metabolic rate (BMR)
• the thermic effect of food eaten
• occupational activities
• non-occupational activities.
• BMR. This is usually taken from standardized tables that require knowledge of the subject’s age, weight and sex.
• Physical activity.The physical activity ratio (PAR) is expressed as multiples of the BMR for both occupational and non-occupational activities of varying intensities.
• Total daily energy expenditure:

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The estimated ‘average’ daily requirement is:

• for a 55-year-old female: 8100 kJ (1940 kcal)
• for a 55-year-old male: 10 600 kJ (2550 kcal).

This figure is at present made up of 50% carbohydrate, 35% fat and 15% protein plus or minus 5% alcohol. In developing countries, however, carbohydrate may constitute more than 75% of the total energy input, and fat less than 15% of the total energy input.

Energy requirements increase during the growing period, with pregnancy and lactation, and sometimes following infection or trauma. In general, the increased BMR associated with inflammatory or traumatic conditions is counteracted or more than counteracted by a decrease in physical activity, so that total energy requirements are not increased.

In the basal state, energy demands for resting muscle are 20% of the total energy required, abdominal viscera 35–40%, brain 20% and heart 10%. There can be more than a 50-fold increase in muscle energy demands during exercise.

Protein

In the UK the adult daily reference nutrient intake (RNI) for protein is 0.75 g/kg, with protein representing at least 10% of the total energy intake. Most affluent people eat more than this, consuming 80–100 g of protein per day. The total amount of nitrogen excreted in the urine represents the balance between protein breakdown and synthesis. In order to maintain nitrogen balance, at least 40–50 g of protein are needed. The amount of protein oxidized can be calculated from the amount of nitrogen excreted in the urine over 24 hours using the following equation (most proteins contain about 16% of nitrogen):

image

In practice, urinary urea is more easily measured and forms 80–90% of the total urinary nitrogen (N). In healthy individuals urinary N excretion reflects protein intake. However, urinary N excretion does not match intake either in catabolic conditions (negative N balance), or during growth or repletion following an illness (positive N balance).

Protein contains many amino acids.

• Indispensable (essential) amino acids. These are tryptophan, histidine, methionine, threonine, isoleucine, valine, phenylalanine, lysine and leucine. They cannot be synthesized and must be provided in the diet.
• Dispensable (non-essential) amino acids. These can be synthesized in the body, but some may still be needed if there are insufficient precursors in the diet.

Animal proteins, such as those in milk, meat and eggs, are of high nutritional value, as they contain all indispensable amino acids. Conversely, many proteins from vegetables are deficient in at least one indispensable amino acid.

In developing countries, adequate protein intake is achieved mainly from vegetable proteins. By combining foodstuffs with different low concentrations of indispensable amino acids (e.g. maize with legumes), protein intake can be adequate, provided enough vegetables are available.

Amino acids may be utilized to synthesize products other than protein or urea. For example, haem requires glycine.

Fat

Dietary fat is chiefly in the form of triglycerides, which are esters of glycerol and free fatty acids. Fatty acids vary in chain length and in saturation. Unsaturated fatty acids are monounsaturated or polyunsaturated. The hydrogen molecules related to these double bonds can be in the cis or the trans position; in most natural fatty acids in food, they are in the cis position.

The essential fatty acids (EFAs) are linoleic and α-linolenic acids, both of which are precursors of prostaglandins. Eicosapentaenoic and docosahexaenoic acids are also necessary, but can be made to a limited extent in the tissues from linoleic and linolenic; thus a dietary supply is not essential.

Synthesis of triglycerides, sterols and phospholipids is very efficient, and even with low-fat diets, subcutaneous fat stores can be normal.

Dietary fat provides 37 kJ (9 kcal) of energy per gram. A high fat intake has been implicated in the causation of cardiovascular disease, cancer (e.g. breast, colon and prostate), obesity and type 2 diabetes. It is suggested that the consumption of saturated fatty acids should be reduced, accompanied by an increase in monounsaturated fatty acids, e.g. oleic acids (the ‘Mediterranean diet’), or polyunsaturated fatty acids, e.g. linoleic acid. Any increase in polyunsaturated fats should not, however, exceed 10% of the total food energy, particularly as this requires a big dietary change.

Increased consumption of partially hydrogenated vegetable and fish oils in margarines has led to a higher trans fatty acid consumption, and their intake should not be above more than the current estimated average of 5 g per day or 2% of the dietary energy. This is because trans fatty acids (also called trans fats) behave as if they were saturated fatty acids, with an increase in the risk of cardiovascular disease. A total ban of trans fats is in place in many countries.

The current recommendations for fat intake for the UK are as follows:

• Total fat intake should be no more than 35% of the total dietary energy, and restriction to 30% is desirable.
• Saturated fatty acids should provide ~10% of the dietary energy.
• cis-Monounsaturated acids (mainly oleic acid) should continue to provide approximately 12% of the dietary energy.
• cis-Polyunsaturated acids should provide 6% of dietary energy, and are derived from n-6 and n-3 polyunsaturated fatty acids, which should provide approximately 0.5% of total energy intake.

Cholesterol is found in all animal products. Eggs are particularly rich in cholesterol, which is virtually absent from plants. The average daily intake in the UK is 300–500 mg. Cholesterol is also synthesized, and only very high or low dietary intakes will significantly affect blood levels.

Essential fatty acid deficiency may accompany protein–energy malnutrition (PEM), but it has been clearly defined as a clinical entity only in patients on long-term parenteral nutrition who are given glucose, protein and no fat. Alopecia, thrombocytopenia, anaemia and a dermatitis occur within weeks, with an increased ratio of triene (n-9) to tetraene (n-6) in plasma fatty acids.

Carbohydrate

Carbohydrates are readily available in the diet, providing 17 kJ (4 kcal) per gram of energy (15.7 kJ (3.75 kcal) per gram monosaccharide equivalent).

Carbohydrate intake comprises the polysaccharide (starch), the disaccharides (mainly sucrose) and the monosaccharides (glucose and fructose). Carbohydrate is cheap compared with other foodstuffs; a great deal is therefore eaten, usually more than required.

Protein–energy malnutrition

Starvation uncomplicated by disease is relatively uncommon in developed countries, although some degree of undernourishment is seen in very poor areas. Most nutritional problems occurring in the population at large are due to eating wrong combinations of foodstuffs, such as an excess of refined carbohydrate or a diet low in fresh vegetables. Undernourishment associated with disease is common in hospitals and nursing homes. Surgical complications, with sepsis, are a common cause.

The majority of the weight loss, leading to malnutrition, is due to poor intake secondary to the anorexia associated with the underlying condition. Disease may also contribute by causing malabsorption and increased catabolism, which is mediated by complex changes in cytokines, hormones, side-effects of drugs and immobility. The elderly are particularly at risk of malnutrition because they often suffer from diseases and psychosocial problems such as social isolation or bereavement.

• In the fed state, insulin/glucagon ratios are high. Insulin promotes synthesis of glycogen, protein and fat, and inhibits lipolysis and gluconeogenesis.
• In the fasted state, insulin/glucagon ratios are low. Glucagon acts mainly on the liver and has no action on muscle. It increases glycogenolysis and gluconeogenesis, as well as increasing ketone body production from fatty acids. It also stimulates lipolysis in adipose tissue. Catecholamines have a similar action to glucagon but also affect muscle metabolism. These agents both act via cyclic adenosine monophosphate (cAMP) to stimulate lipolysis, producing free fatty acids that can then act as a major source of energy.
• During weight loss, uncomplicated by disease, the proportion of lean to fat tissue loss (or proportion of energy derived from protein metabolism) is greater in thin than in overweight/obese individuals.

Clinical features

Patients are sometimes seen who have loss of weight or malnutrition as the primary symptom (failure to thrive in children). Mostly, however, malnourishment is only seen as an accompaniment of some other disease process, such as malignancy. Severe malnutrition is seen mainly with advanced organic disease or after surgical procedures followed by complications. Three key features that help in the detection of chronic PEM in adults are described below. These features are also used in the Malnutrition Universal Screening Tool (see Fig. 4.1):

image

Fig. 4.1 Malnutrition Universal Screening Tool (MUST).

Reproduced with permission from the British Association for Parenteral and Enteral Nutrition (BAPEN): http://www.bapen.org.uk.

• The body mass index (BMI): calculated as weight/height2 (kg/m2):

• probable risk of chronic PEM: < 18.5 kg/m2
• possible risk of chronic PEM: 18.5–20 kg/m2
• little or no risk of chronic PEM: > 20 kg/m2.

In patients with oedema or dehydration the BMI may be somewhat misleading.

• Weight loss in the previous 3–6 months

• > 10%, high risk
• 5–10%, possible risk
• < 5%, low/no risk of developing PEM.
• Acute disease effect:

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