As discussed above, results of studies suggest that marked amounts of dietary fructose may already be metabolized in small intestinal enterocytes also suggesting that fructose could either directly or through intermediates affect these cells. Indeed, employing an
ex vivo model of small intestinal everted tissue sacs, we recently showed that even in the absence of bacteria physiological concentrations of fructose (5 mM) may alter intestinal barrier function in as short as 30–60 min upon exposure [
84]. Results of these studies also suggest that alterations of the nitric oxide (NO) homeostasis may be critical herein [
84,
85]. Specifically, it has been shown by us and others that chronic intake of fructose is associated with an induction of iNOS and NO synthesis in the gut [
86,
87]. And while results of our own group also showed that iNOS knockout mice are not protected from the increased translocation of bacterial endotoxin into the portal blood stream [
86], targeting NO production in small intestinal tissue, e. g., through L-arginine or L-citrulline (also see below) has been shown to dampen fructose-induced intestinal barrier dysfunction [
84,
88]. Supporting the hypothesis that alterations of the NO homeostasis are critical in fructose-induced intestinal barrier dysfunction, an oral supplementation of L-arginine and L-citrulline, respectively, has been shown to attenuate the loss of intestinal barrier function (e.g., the loss of tight junction proteins and increased permeation of xylose) [
84,
88]. This was also associated with a lower translocation of endotoxin into the portal vein in various rodent models employing fructose-rich diets (30 % fructose in drinking water [
89]; 50 % wt/wt fructose in liquid diet [
90,
91]). The protective effects of the amino acids were associated with ‘normalization’ of arginase activity found to be markedly lower in small intestinal tissue of mice-fed fructose enriched diets and in
ex vivo models of small intestinal everted tissue sacs challenged with fructose [
84,
88]. These findings are in line with those of others reporting a critical role of arginase in inflammation in intestinal tissue [
92,
93]. L-arginine and L-citrulline have both been shown to be allosteric regulators of arginase activity. Interestingly, the supplementation of L-citrulline had no effect on the alteration of intestinal microbiota composition in small intestine inflicted by the feeding of a fructose-, fat- and cholesterol-rich diet. Furthermore, a treatment of animals with the arginase inhibitor nor-NOHA attenuated the protective effects of L-arginine and L-citrulline, respectively, being also associated with an attenuation of the protective effects of the two amino acids on the development of NAFLD [
84,
88]. Results of our own studies also suggest that the ‘normalization’ of arginase activity was associated with a decrease in MLCK protein [
84]. It has been shown that MLCK activity may be induced in cells treated with spermine [
94], the latter being a downstream substrate of arginase-mediated formation of ornithine. However, if the lower arginase activity found in small intestinal tissue exposed to fructose alters spermine bioavailability, and hereby, MLCK activity remains to be determined. Also, despite the results of studies of our own group suggesting that arginase activity is also lower in patients with steatosis, further studies are needed to determine if this reduction in arginase activity is related to an increased fructose intake and if this is causal in the increased bacterial endotoxin levels found in these patients [
84]. Also, the question how fructose alters NO homeostasis remains to be answered.