Arginine-supplemented enteral nutrition in critically ill diabetic and obese rats: A dose-ranging study evaluating nutritional status and macrophage function

Abstract

Objective

Critically ill diabetic and obese patients are at high risk of complications. Arginine availability is lowered in diabetes and in stress situations, yet arginine is necessary for immune response, mainly by its action through nitric oxide (NO). These facts argue for arginine-supplemented diets in critically ill patients. However, studies have raised concerns about possible adverse effects of such diets in intensive-care patients. We therefore analyzed the metabolic and immunologic effects of an arginine-enriched diet in stressed diabetic-obese rats.

Methods

Zucker Diabetic Fatty rats (fa/fa) were made endotoxemic by an intraperitoneal injection of lipopolysaccharide and then fed 4-d enteral nutrition enriched with arginine (ARG group) or a non-essential amino acid mix (NEAA group). The two groups each were subdivided into three subgroups: the ARG subgroups received 0.5 g (ARG0.5), 2 g (ARG2), and 5 g (ARG5) of arginine per kilogram daily, and the NEAA groups were made isonitrogenous with the corresponding ARG subgroups (NEAA0.5, NEAA2, and NEAA5). Plasma and urinary biomarkers were measured. Cytokine and NO production levels and inducible NO synthase and arginase protein levels were determined from peritoneal macrophages.

Results

The survival rate was lower in the ARG5 and NEAA5 subgroups than in all the other subgroups. The nitrogen balance was higher in the ARG5 group than in the NEAA5 group. Plasma triacylglycerol levels were lower in the ARG2 group than in the NEAA2 group. Interleukin-6, tumor necrosis factor-α, and NO production in the macrophages decreased and arginase-1 was upregulated in the ARG-treated rats.

Conclusions

In this model, mortality was increased by the nitrogen burden rather than by arginine per se. Arginine improved nitrogen balance and had an anti-inflammatory action on macrophages by regulating NO production, probably through arginase-1 expression.

Introduction

Obesity and type 2 diabetes have increased in populations worldwide, and medical teams in hospital intensive care units (ICUs) are thus increasingly faced with such patients. In the ICU context, pre-existing diseases such as diabetes and obesity are associated with protein wasting, a higher risk of complications, a higher risk of infectious episodes [1], [2], and ultimately increased mortality [3], [4], although this last point is still debated [1]. Optimized nutritional therapy therefore may improve the outcome of these patients.

In a search for a new nutritional strategy, we considered the evidence that obesity is associated with a state of chronic low-grade inflammation that contributes to the development of insulin resistance and diabetes in addition to a depressed immune status and an impaired metabolic response to stress [5], [6], [7]. There is no pharmacologically based approach to nutrition in these patients. We therefore examined the utility of “immune-enhancing diets” (IEDs) in this specific context of type 2 diabetes and obesity associated with sepsis. We especially focused on the potential beneficial effects of the pharmaconutrient arginine.

Arginine is a conditionally essential amino acid in stress situations because its endogenous synthesis may not be sufficient in highly catabolic conditions such as sepsis [8]. Studies have found that plasma arginine concentration is decreased in insulin-resistant and obese Zucker rats [5], [9] and in the general ICU population, which generally exhibits what is termed arginine deficiency syndrome [10] associated with various degrees of insulin resistance [11].

Arginine is at the crossroads of the inflammation pathway and the immune response by nitric oxide (NO) synthesis and has been shown to improve insulin sensitivity [12], to be essential for the immunologic response [13], [14], and to exert anti-inflammatory effects [15]. Arginine is also associated with a decrease in infectious complication rates and a shorter hospital stay in surgical patients [16]. Nevertheless, because arginine-enriched diets have also been associated with harmful effects in critically ill septic patients [17], [18], guidelines have recommended that IEDs, particularly those enriched with arginine, should not be used in ICU patients [19]. However, the conclusions are not based on evidence and have been criticized [20], [21]. Furthermore, recent data [22] have suggested that arginine may not be the IED component implicated, and we underline that there is still no dose-ranging study available.

Macrophages are one of the main actors of innate immunity and have been found to present functional impairments in experimental models of diabetes and obesity [23]. Macrophage activity is largely controlled by the action of NO, whose sole precursor is arginine by the NO synthase (NOS) pathway. In these cells, arginine can be metabolized by NOS into NO and citrulline or by arginase into ornithine and urea [24]. Thus, it appears that two major keys to macrophage immune function are arginine availability and arginine metabolism.

Given the lower arginine availability observed in diabetes [5], [9] and exacerbated in sepsis [15], our working hypothesis was that nutritional supplementation with arginine might improve systemic metabolism and innate immunity-related impairments in catabolic type 2 diabetic and obese rats. Clearly, diabetes associated with obesity and injury modifies protein metabolism and cell signaling in a way that renders the action of arginine-enriched diets unpredictable, thus justifying relevant experimentation. Our aim was to evaluate the effects of arginine-supplemented enteral nutrition compared with an isonitrogenous standard nutrition. We conducted a dose-ranging study including a high dosage of arginine to assess the potential adverse effects of excessive arginine intake [19]. For this purpose, we used a model of male Zucker Diabetic Fatty (ZDF) rats homozygous for non-functional leptin receptors (fa/fa) that develop type 2 diabetes and obesity [25].