Which optimal protein intake in maintenance dialysis patients?

Protein requirements have long been the focus of intensive research and discussion. Proteins are mandatory for normal growth and the maintenance of body composition and, because there is no possible storage (by contrast to lipids and carbohydrates), the proteins eaten in excess are catabolised through increased CO2 and urea production, with rapid excretion of urea by the kidney. At that point, when there is no kidney disease, the average daily intake of protein is of little relevance and, particularly for the western civilisation, remains far above normal requirements. Currently, this intake is approximately 1.3–1.4 g kg 1 body weight per day. Research has almost unanimously shown that an individual healthy adult in the noncatabolic state functions normally when ingesting 0.5– 0.6 g kg 1 body weight per day, as long as energy intake is adequate [>105–125 kJ (>25–30 kcal) kg 1 body weight per day]. At a population level, the recommended intake is approximately 0.8 g protein kg 1 per day (Rand et al., 2003). In subjects with chronic kidney disease (CKD), the reasoning has to be dramatically changed for several reasons (Fouque et al., 2011a). First, during the nondialysis stage (e.g. CKD stage 3–5 ND), when renal function decreases <60 mL min , and because urea is less excreted by the failing kidney, any attempt to decrease urea generation and its concomitant blood increase is worthwhile. For this reason, reducing protein intake during these CKD stages has been advocated for decades, and intakes of 0.6– 0.7 g kg 1 per day should be recommended (Bernhard et al., 2001; Fouque & Aparicio, 2007). When chronic dialysis treatment needs to be started (stage 5D), protein intake has to be increased in response to a more catabolic environment. The dialysis procedure itself induces superimposed protein breakdown and amino acid losses and patients more often face infection and surgery. Consequently, protein requirements are increased (Fouque et al., 2011b). Naylor et al. (2013), on behalf of the British Dietetic Association, addressed the important question of optimal protein intake during dialysis replacement by performing a formal systematic review. Taking information from previous guidelines, they updated the literature search from 2005 until 2009 for haemodialysis (HD), and from 1997 to 2009 for peritoneal dialysis (PD), uncovering 37 new publications. They used the most robust and up-to-date methodology to search for and grade evidence (AGREE, PICO, Cochrane Library, SIGN). However, the quality of retrieved studies (12 were kept for analysis in HD and 12 in PD) was moderate, including some cross-sectional or even retrospective reports. The search and analysis led Naylor et al. (2013) to propose intakes of 1.1 g protein kg 1 body weight per day in patients undergoing maintenance HD, and 1.0–1.2 g protein kg 1 body weight per day in PD patients (Naylor et al., 2013). How do these results compare with former guidelines? The K-DOQI guidelines in 2000 (Clinical practice guidelines for nutrition in chronic renal failure, 2000) recommended 1.2 g protein kg 1 per day in stable maintenance HD patients and 1.2–1.3 in PD patients. Seven years later, the European Best Practice Guidelines (Fouque et al., 2007), a working group of the European Renal Association, published guidelines that slightly decreased the required protein intake to 1.1 g protein kg 1 per day from food reports or 1.0 g protein kg 1 per day from normalised protein nitrogen appearance (nPNA), formerly called the normalised protein catabolic rate (nPCR). This 0.1 g kg 1 per day difference is important and often misunderstood. Indeed, as discussed by Naylor et al. (2013), during maintenance haemodialysis, simple formulas can provide a valid estimation of protein catabolism (or protein nitrogen appearance) (Fouque et al., 2007). However, this value is lower by approximately 0.1 g kg 1 per day compared to protein intake assessed by food reports as a result of non-urea insensible nitrogen losses. Thus, it is important to characterise which method is being used to reliably estimate patient’ protein intake because many professionals will use nPNA, which is more convenient to estimate. Thus, professionals in charge of maintenance dialysis patients can now rely on a larger set of clinical data, which identify protein requirements to be 1.1 g kg 1 per day in maintenance HD and 1.0–1.2 g kg 1 per day in PD patients. The confirmation of these results through larger randomised studies is an important future goal, although it will not be easy to achieve in practice. Such studies would need to last for many months (or years) to collect sufficient hard end-points such as mortality or hospitalisation, and should include hundreds of patients. In addition, they should ensure that the ranges of protein intake are sufficiently different between groups to avoid nutritional overlap. This is a commonly observed caveat because, despite