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Department of Nephrology and Research Unit, Hospital Nuestra Señora de Candelaria, 38010 Santa Cruz de Tenerife, Tenerife, Spain, E-mail: jnavarro{at}hcan.rcanaria.es
Dear Sir:
We thank Druml for his interest in our study on amino acid losses during hemodialyisis with polyacrylonitrile membranes and the effect of intradialytic amino acid supplementation (1) and are glad to have the opportunity to respond to the issues he raised.
The first discrepancy arises in the calculation of the net retention of amino acids. In our study, the mean net uptake of amino acids was 10.6 g; if the calculations of Druml are used, this uptake is 9.9 g. This slight difference may be explained by the fact that our calculations were based on the net balance of individual amino acids, not the total amounts.
Second, as mentioned by Druml, in the study by Wolfson et al (2) the basal losses of amino acids (without supplementation) were 8.2 g, which increased to 12.6 g after amino acid administration. This increase of 4.4 g in amino acid losses, which represents 53.6% of basal losses, may be considered at least as relevant, but not marginal. Concerning amino acid balance in our study, hemodialysis with infusion of 25.7 g amino acids resulted in a net uptake of 10.6 g, which represents 44% of amino acids infused. What is the reason for the differences between this and the results of previous studies by Wolfson et al (2) and Berneis et al (3)?
Several factors must be taken into account. First, in the study by Wolfson et al, patients received an infusion of 400 mL amino acids and 400 mL of 50% glucose (200 g D-glucose), whereas in the study by Berneis et al amino acids were administered with glucose (150 g) and a fat emulsion (50 g). On the contrary, an important characteristic of our study was that amino acids were supplemented without glucose or other nutrients to avoid these confounding factors. It is possible that the infusion of glucose or lipids favors the cellular uptake of amino acids and therefore can explain the higher positive balance of amino acids observed in these previous studies.
Second, the characteristics of the dialysis session are important. Several aspects may affect the losses of amino acids during this procedure, such as blood flow, dialysate flow, intake of food during the session, and weight loss. Finally, a critical factor in the dialytic treatment is the dialysis membrane. Wolfson et al used a Travenol, Lundia, or Nova dialyzer; Berneis et al used a polysulfon dialyzer; and we used a polyacrylonitrile dialyzer. The classic and most commonly used membrane for hemodialysis is cuprophane, but new synthetic membranes such as polysulfone, polyacrylonitrile, and polymethylmethacrylate have become available during the past decades. Despite these membranes being considered as highly biocompatible, their characteristics depend on the specific membrane, not on class. Therefore, the amino acid losses need to be evaluated for each individual membrane because the membranes' intrinsic properties (eg, polarity, capacity for capture of amino acids, physiochemical characteristics, and bulk charges within the membrane) may modify important aspects of the dialytic procedure, including losses of amino acids and other nutrients.
Third, as is obvious, the amino acids were infused into the venous line after the dialyzer. Druml commented that amino acid infusion at a dosage such as that used by us would not substantially increase plasma amino acid concentrations. This conclusion is evident from Table 4 in our article. The table shows that dialysis with amino acid infusion results in a significant increase in the serum concentration of only 5 amino acids (leucine, asparagine, citrulline, cystine, and taurine) compared with basal values, whereas the plasma concentration of the other amino acids did not change significantly. This statement is specified in the discussion; furthermore, we highlighted that amino acid administration in our study prevented the reduction in the plasma amino acids observed after the dialytic procedure, with no significant modifications in the plasma concentrations of essential, nonessential, branched-chain, and total amino acids. Moreover, on the basis of these results, we hypothesized that an increase in the initial dosage of amino acids administered might result in a significant rise in plasma amino acid concentrations.
Finally, we agree with Druml that nutritional intervention can alter the course of disease in dialysis patients, with an improvement of morbidity and mortality. However, there are currently no studies designed to investigate the effect of different schedules of amino acid supplementation on the outcome of dialysis patients. Therefore, there is no evidence about the percentage of amino acids that must be retained after supplementation to achieve any beneficial effect. Thus, the value suggested by Druml (75%) is arbitrary.
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