| Biochemical markers of nutrition
|
| Urinary nitrogen excretion is important in assessing the body nitrogen balance
|
| The nitrogen balance is used to assess the body protein requirements. Nitrogen balance is a difference between the intake of nitrogen and its excretion. The positive nitrogen balance means that the intake exceeds loss. Negative nitrogen balance signifies that the loss exceeds intake.
|
The quantity of proteins oxidized by the body is estimated by measuring the 24 h urinary nitrogen excretion. Ninety percent of the excreted nitrogen appears in the urine (and 80% of this is present as urea ). The rest is excreted in the stool, hair, and sweat. Nitrogen excretion adjusts to protein intake over 2 to 4 days. The measurement of urinary nitrogen (or urea) excretion is the most reliable way of assessing daily protein requirements. Unfortunately the method is tedious, and the test requires the 24 h urine collection. As a guide, most people require 1-1.2 g protein/kg body weight/day. More detailed requirements are listed in Table 21.1.
|
| Several plasma proteins have been used as markers of nutritional state
|
| The concentration of a protein in plasma may reflect the nutritional state, and the time-period such measurement reflects is dependent on the half-life of the particular protein. The proteins which have been most commonly used for this purpose are albumin, transthyretin (prealbumin), and transferrin.
|
| page 306 |  | | page 307 |
| Many studies confirmed the link between liver albumin synthesis (albumin half-life is approximately 20 days) and the nutritional state. Transthyretin, which has a half-life of two days, has been also extensively used in nutritional assessment. Transthyretin is synthesized in the liver and forms a complex with retinal-binding protein in plasma. Transferrin, an iron-binding protein is also synthesized in the liver has a half-life of approximately 10 days. Unfortunately, the interpretation of plasma concentrations of nutritionally-relevant proteins is often difficult, because their concentration is not determined exclusively by the state of nutrition. For instance, although the albumin concentration in plasma reflects the rate of its synthesis in the liver, it is also heavily dependent on the state of hydration; albumin concentration is low in overhydrated patients. In addition, albumin, transthyretin and transferrin are all affected by the acute phase response (transferrin and transthyretin concentrations increase during the acute-phase reaction and albumin concentration decreases:
albumin is called a negative acute-phase reactant). All this means that, as many other biochemical variables, they cannot be interpreted in isolation; still, they are helpful when seen as a part of the entire clinical/laboratory picture.
|
| Full nutritional assessment also involves measurements of vitamins and trace metals. This is particularly important in patients who remain on long-term parenteral nutrition. With regard to these measurements, the interpretation of laboratory results is complicated by the fact that their concentration in blood may not reflect the status of body stores (see Chapter 10).
|
| General laboratory tests provide useful supplementary information related to the nutritional status
|
| Other laboratory tests provide important supplementary information during a nutritional assessment of hospitalized patients, and particularly those who are candidates for parenteral nutrition (see below). For instance, the measurement of haemoglobin may uncover iron deficiency. Testing the liver (Chapter 28) and kidney function (Chapter 22), the measurements of serum electrolytes such as sodium, potassium chloride, bicarbonate, calcium, phosphate and magnesium, and the assessment of iron metabolism, provide useful information. Finally, assessing daily fluid intake and loss is essential in patients who are being considered for nutritional support.
|
| A WOMAN WITH RENAL FAILURE AND WEIGHT LOSS |
| A 59-year-old lady was admitted to the Renal Unit with recurrent infections and recurrent endocarditis. She had been treated with hemodialysis for 5 years and over a period of 2 years had lost 33% of her body weight. She had a very poor appetite and had been taking two cartons (1.5 kcal/ml) of milkshake sip feeds daily. She was anuric and her fluid intake was restricted to 1000 ml daily. Her height was 1.68 m and she weighted 52.7 kg: the BMI was 18. Her most recent biochemistry results revealed persistent hyperkalaemia ranging between 5.7 and 6.2 mmol. |
| Comment. Because of her poor nutritional status and continuous weight loss she needs to continue taking nutritional supplements to minimize further weight loss. However, due to hyperkalaemia, a low-electrolyte sip feed should be considered. She was initially taking 22 mmol of potassium from the 1.5kcal/ml milkshake. Subsequently this was changed to a low electrolyte sip feed, which provided only 12 mmol in total. The UK Renal Nutrition Group Standards recommend that daily potassium intake in this type of patient should be less than 1 mmol/kg/body weight. Low electrolyte/volume sip feeds should always be considered for renal patients and the serum concentration needs to be closely monitored. (See Chapter 22.) |
|
