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Medical Department III, Division of Nephrology, Vienna Medical School, Vienna, Austria
Dear Sir:
Navarro et al (1) reported recently on amino acid losses during hemodialysis therapy without and during concomitant infusion of amino acids as intradialytic parenteral nutrition. The obligatory basal loss of amino acids without simultaneous amino acid infusion was 12.5 g/4 h. When 25.7 g amino acids was infused during hemodialysis, losses increased to 28.3 g (15.8 g more), so the net retention rate of amino acids was only 25.7 - 15.8 = 9.9 g, which is 38% of the amount infused. (These are my calculations; the authors give slightly different numbers, ie, a net uptake of 10.6 g).
These results are in variance with the results of previous investigations in this field and contradict the physiology of amino acid metabolism. Obviously, there is a basal amino acid loss (averaging 2–3 g/h, as was also confirmed by Navarro et al) and—if amino acids are given during hemodialysis as nutritional support—an additional loss because of the elevation of plasma amino acid concentrations during infusion. However, Wolfson et al (2) showed clearly that an amino acid infusion during hemodialysis only marginally increases amino acid elimination. In their study, basal amino acid loss (without additional amino acid infusion) was 8.2 g. The total loss of amino acids when 39.5 g amino acids was given during hemodialysis increased to only 12.6 g (4.4 g more); thus, the net uptake of amino acids infused was 39.5 – 4.4 = 35.1 g, ie, 89%. Recently, these findings were confirmed for high-flux polysulfone membranes by Berneis et al (3), who evaluated amino acid losses during intradialytic parenteral nutrition (3). During 4 h of hemodialysis, 48 g amino acids was infused continuously, but total amino acid loss into the dialysate was only 10.9 g, so 77.3% of the amino acids infused was retained. (Basal amino acids loss was not measured, so the net retention must have been even higher.)
These results were also to be expected. The endogenous clearance of amino acids is much higher than is the exogenous, dialytic clearance. In hemodialysis patients, mean endogenous amino acid clearance is 900 mL/min (4). This means that an amino acid infusion at the dosage used by Navrarro et al does not substantially increase plasma amino acid concentrations and thus can enhance amino acids losses during hemodialysis only moderately.
The only explanation for the excessive amino acid losses reported by Navarro et al is that amino acids were infused into the arterial line before the dialysis membrane, and this obviously is not appropriate.
Intradialytic parenteral nutrition has been shown to ameliorate several indicators of nutritional status, such as an increase in plasma protein concentrations, an increase in anthropometric variables, and an amelioration of immunocompetence (5). Nevertheless, it remains to be shown convincingly that this nutritional intervention can alter the course of disease and improve morbidity and mortality. One fact, however, is clear: the amino acids infused during hemodialysis are retained at a percentage of 75%.
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