Nutritional assessment

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Nutritional assessment

Malnutrition is a common problem worldwide, and in developed countries it is associated particularly with poverty and alcoholism. It is also encountered among patients in hospital. Various studies have shown that patients may have evidence, not only of protein-calorie malnutrition, but also of vitamin and mineral deficiencies, especially after major surgery or chronic illness.

Malnutrition to the layman usually means starvation, but the term has a much wider meaning encompassing both the inadequacy of any nutrient in the diet as well as excess food intake. The pathogenesis of malnutrition is shown in Figure 52.1.

Fig 52.1 The development of malnutrition.

Malnutrition related to surgery or following severe injury occurs because of the extensive metabolic changes that accompany these events: the ‘metabolic response to injury’ (pp. 110–111).

The assessment of a patient suspected of suffering from malnutrition is based on:

laboratory investigations including biochemistry.

History

Past medical history may point to changes in weight, poor wound healing or increased susceptibility to infection. The ability to take a good dietary history is one of the most important parts of a full nutritional assessment. Taking a dietary history may involve recording in detail the food and drink intake of the patient over a 7-day period. More usually, however, a few simple questions may yield a lot of useful information about a person’s diet. Depending on the background to the problem, different questions will be appropriate. For example, in the wasted patient, questions about appetite and general food intake may suggest an eating disorder such as anorexia nervosa, but in the patient presenting with a skin rash, details of the specific food groups eaten will be required to help exclude a dietary cause. In the patient at increased risk of coronary heart disease, questions on saturated fat intake may be most revealing.

Examination

Simple anthropometric measurements will include height, weight, arm circumference and skin-fold thickness.

Body mass index (weight in kg divided by the height squared in metres) is a reasonable indicator of nutritional state, except when the patient is oedematous. Arm circumference is an indicator of skeletal muscle mass, while skin-fold thickness is proportional to body fat levels. In addition, general physical examination may reveal signs of malnutrition in the skin, nails, hair, teeth and mucous membranes.

Biochemistry

A number of biochemical tests are used to complement the history and examination in assessing the general nutritional status of a patient. None are completely satisfactory and should never be used in isolation. The most common tests include:

Protein. Serum albumin concentration is a widely used but insensitive indicator of protein nutritional status. It is affected by many factors other than nutrition, e.g. hepatic and renal diseases and the hydration of the patient. Serum albumin concentration rapidly falls as part of the metabolic response to injury, and the decrease may be mistakenly attributed to malnutrition.

Blood glucose concentration. This will be maintained even in the face of prolonged starvation. Ketosis develops during starvation and carbohydrate deficiency. Hyperglycaemia is frequently encountered as part of the metabolic response to injury.

Lipids. Fasting plasma triglyceride levels provide some indication of fat metabolism, but are again affected by a variety of metabolic processes. Essential fatty acid levels may be measured if specific deficiencies are suspected. Faecal fat may be measured both qualitatively and quantitatively in the assessment of malabsorption. However this test is not commonly available, and is certainly not popular with laboratory staff.

Unlike the assessment of overall status, biochemical measurements play a key role in identifying excesses or deficiencies of specific components of the diet. Both blood and urine results may be of value. Such assays include:

Vitamins. These organic compounds are vital for normal metabolism. Usually they are classified by their solubility; they are listed in Table 52.1 and their average adult daily requirements shown in Figure 52.2. Some assays are available to measure the blood levels of vitamins directly, but often functional assays that utilize the fact that many vitamins are enzyme cofactors are used. These latter assays may help identify gross abnormalities. However, to detect subtle deficiencies and the increasing problem of excess intake, quantitative measurements are required.

Table 52.1

Classification of vitamins

Vitamins	Deficiency state	Lab assessment
Water soluble
C (Ascorbate)	Scurvy	Plasma or leucocyte levels
B₁ (Thiamin)	Beri-beri	Red cell levels
B₂ (Riboflavin)	Rarely single deficiency	Red cell levels
B₆ (Pyridoxine)	Dermatitis/Anaemia	Red cell levels
B₁₂ (Cobalamin)	Pernicious anaemia	Serum B₁₂, full blood count
Folate	Megaloblastic anaemia	Serum folate, RBC folate, full blood count
Niacin	Pellagra	Urinary niacin metabolites (not commonly available)
Fat soluble
A (Retinol)	Blindness	Serum vitamin A
D (Cholecalciferol)	Osteomalacia/rickets	Serum 25-hydroxycholecalciferol
E (Tocopherol)	Anaemia/neuropathy	Serum vitamin E
K (Phytomenadione)	Defective clotting	Prothrombin time

Fig 52.2 Average adult daily requirements of vitamins.

Major minerals. These inorganic elements are present in the body in quantities greater than 5 g. The main nutrients in this category are sodium, potassium, chloride, calcium, phosphorus and magnesium. All of these are readily measurable in blood and their levels in part reflect dietary intake.

Trace elements. Inorganic elements present in the body in quantities less than 5 g are often found in complexes with proteins. The essential trace elements are shown in Figure 52.3.

Fig 52.3 Average adult daily requirements of essential trace elements.

Preoperative nutritional assessment

Nutritional assessment is not only necessary following surgical procedures. Patients need to be in good nutritional condition before an operation and the assessment should be done well in advance to allow build-up of reserves before surgery (Fig 52.4).

Fig 52.4 Selection of patients for preoperative nutritional support.

Case history 41

A 68-year-old man with motor neurone disease is admitted because of severe anorexia and weight loss. Suspecting malnutrition, the house officer requests a battery of biochemical tests including serum vitamin E and selenium.

• How useful will these be in the management of this patient?

Comment on page 168.

Clinical note

Accurate measurement of height and weight are the most important features of global nutritional assessment at all stages of life – from the neonatal period to old age. These are also often very poorly recorded in patient notes.

Nutritional assessment

Nutritional assessment is important in every patient.

Malnutrition is common and usually reflects the inadequacy of any nutrient or nutrients in the diet.

History, examination and laboratory investigations are complementary.

A variety of biochemical investigations may assist in the diagnosis of nutritional deficiencies and in the monitoring of patients undergoing nutritional support.

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