Common Nutritional Misconceptions in Dialysis Patients

The clinical importance of any disease state relies heavily on the sensitivity and specificity of the marker(s) that is used for its diagnosis. Serum albumin is the most extensively examined nutritional marker in almost all patient populations, probably because of its easy availability and strong association with hospitalization and death risk.Moreover, changes in serum albumin concentrations are accompanied by other markers of nutritional state in multiple studies with different patient populations, including maintenance hemodialysis (MHD) patients. On the other hand, certain unique characteristics associated with advanced CKD render the utility of serum albumin somewhat questionable (1). To understand the appropriate use ormisuse of serum albumin, one needs to understand its complex physiology. The concentration of serumalbumin is the net result of its synthesis, breakdown, volume of distribution, and exchange between intravascular and extravascular spaces, as well as losses (2,3). Albumin is synthesized in the liver and secreted into the bloodstream, where it is rapidly equilibrated and has a half-life of approximately 20 days. Approximately two-thirds of the albumin pool is in the extravascular space. The primary determinant of serum albumin concentration is by far the exchange between intravascularandextravascular spaces. The primary reason serum albumin is considered a nutritional marker is that inadequate dietary macronutrient intake is characterized by a decrease in the rate of albumin synthesis (4). Historical studies have shown that long-term protein and energy restriction in healthy subjects and patients with anorexia nervosa leads to significant decrease in serum albumin concentrations, albeit these changes are rather small, usually in the order of 0.2–0.4 g ⁄dl and relatively minuscule compared with changes in body weight and composition (5). Not surprisingly, the increase in serum albumin concentration in response to feeding is about the same in many studies, including those inMHDpatients (6). This relatively small change in serum albumin in response to feast or famine is not unexpected given its low turnover rate and large pool size and that its synthesis determines only a fraction of its serum concentrations. Indeed, serum albumin concentrations may actually increase during short-term fasting because of contraction of the intravascular space. Similarly, in the long-term, low protein intake and decreased protein synthesis may be compensated by decreased albumin breakdown and also a shift from the extravascular to the intravascular spaces, maintaining serum albumin concentrations. However, because the exchange between intravascular and extravascular albumin pools is so large, small variations in this exchange can cause significant alterations in serum albumin levels over time in certain conditions where low dietary nutrient is maintained as commonly observed inMHDpatients. Serum albumin concentrations decrease in situations of volume overload, which are highly prevalent inMHD patients. Serum albumin, as a negative acute-phase reactant, is also affected by conditions with acute-phase response, such as inflammation, infection, and trauma, which are quite common in ESRD patients. However, these conditions lead to prompt and usually substantial decreases in serum concentrations (>0.5 g ⁄dl) and may more closely reflect the degree of illness and inflammation, rather than the overall nutritional status (7). The primary driving force for those extensive changes appears to be volume shifts, rather than production or degradation of albumin (8). Indeed, evidence indicates that the fractional synthetic rate of albumin tend to increase under stressful conditions (9). Therefore, appropriate interpretation and use of serum albumin concentration in ESRDpatients requires detailed assessment of the patients’ clinical status including but not limited to a complete physical examination, consideration of concurrent morbidities, the quality of the renal replacement therapy, and the concurrent analysis of other available nutritional markers. From a practical point of view, serum albumin still remains as a simple but useful test that is readily available and should be considered as an indispensible marker for assessment of nutritional status and response to nutritional therapy in MHDpatients (10).