Malnutrition protein turnover

Finally, both constant infusion and pulse label techniques using isotopic labels to estimate whole body protein turnover share the common premise that there is a homogeneous metabolic nitrogen pool of which the plasma constitutes an integral part. That this is in fact an oversimplification, has been shown from animal studies [438]. Despite these, and other objections, work will continue in the search for a reliable method for the estimation of protein synthesis, catabolism and turnover in man. To the clinician such a method would provide information about nitrogen loss from the body resulting from malnutrition, postoperative trauma, burns or severe infection and perhaps more importantly an indication of the success or otherwise of the specific therapeutic regime implemented. 

Measurements of the levels of semm proteins such as albumin, transthyretin (also known as prealbumin), transferrin and retinol-binding protein are used as biochemical parameters in the assessment of protein energy malnutrition (Table 17-1). An ideal protein marker should have rapid turnover and present in sufficiently high concentrations in semm to be measured accurately. Transthyretin has these properties it is a sensitive indicator of protein deficiency and is effective in assessing improvement with refeeding. 

As can be expected, plasma proteins have a rapid turnover. Isotopic studies have shown that the half-life of a plasma protein molecule is about 15 days, and that plasma proteins are an important source of proteins in tissues. Animals put on a protein-free diet maintain a perfect nitrogen balance for 3 months when plasma is administered to them. During starvation, plasma proteins remain intact for a long time, in spite of rapid use of tissue proteins. These facts explain the salutary effects of plasma proteins administered for chronic malnutrition, or to surgical patients who cannot be fed. Since there is no correlation between the rate of plasma and tissue protein use in malnutrition, serum protein determination is of little value in estimating a patient s nutritional status. 

These data indicate that the turnover of RBP in vivo is quite rapid, with a fairly high body synthetic (production) rate for a protein of such low plasma concentration. This rapid turnover rate underlies the potential usefulness of RBP measurements in helping to assess the functional state of the liver in patients with hepatic disease, or the nutritional status of patients with borderline or actual malnutrition (see discussion below under clinical studies). It is of interest to compare the turnover of RBP with that of other plasma proteins. It has been pointed out (F. R. Smith et al., 1975) that the respective half-lives (in days) and synthetic rates (in milligrams per kilogram per day) in normal adult human subjects are approximately 0.5 and 5 for RBP 2-3 and 8-9 for TTR (Vahlquist et al., 1973 Socolow et al., 1965) and 14 and 200 for albumin (Beeken et al., 1962). 

Protein breakdown occurs at a more or less constant rate throughout the day, and an adult catabolizes and replaces some 3-A g of protein per kilogram body weight per day. Turnover is also important in growing children, who synthesize considerably more protein per day than the net increase in body protein. Even children recovering from severe protein-energy malnutrition (see Chapter 8), and increasing their body protein rapidly, still synthesize 2-3 times more protein per day than the net increase. 

Shetty PS, Jung RT, Watrasiewica KE, James WPT (1979) Rapid-turnover transport proteins an index of sub-clinical protein-energy malnutrition. Lancet 2 230-232... 

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