41 Open image in new window Open image in new window Our clinical study in patients with CD revealed an impairment in glutamine metabolism as evidenced by a systemic and mucosal glutamine deficiency and a low glutaminase activity in the ileum independent of inflammation. Because glutamine is the principal fuel for enterocytes (but not for colonocytes []), a substrate for GSH synthesis, and a re-quisite precursor for nucleic acid and protein biosynthesis, the disturbance in gut glutamine metabolism may render in particular the ileum less resistant to inflammatory tissue damage.

The enzyme activity of glutaminase, a critical determinant of glutamine utilization in the gut, was found to be higher in the normal ileum than in the normal colon, in line with previously published data [11, 33]. The decrease in glutaminase activity in the ileum of patients with CD indicates impaired gut glutamine utilization. Thus, the abnormally low mucosal glutamine levels in both the ileum and the colon in CD do not result from an accelerated glutaminase-dependent metabolism and rather suggest a decline in gut glutamine uptake, which also has been observed in sepsis [42], endotoxemia [42], and starvation [7]. In these conditions, the diminished glutamine uptake was found to be associated with a reduction in mucosal glutaminase activity. The local production of inflammatory mediators in CD may add to the deterioration in gut glutamine metabolism: IL-1β, which is abundantly present in both the noninflamed and the inflamed gut in CD [43], has been shown to decrease the mucosal glutamine uptake and glutaminase activity [44].

The low glutamine content in the noninflamed gut in CD may result, at least in part, from the decreased availability of plasma glutamine, because abnormally low systemic glutamine levels are associated with a decrease in the mucosal glutamine content [45, 46, 47]. The percentage ideal body weight is correlated with the plasma glutamine concentration; a weight loss of >10% is associated with a deterioration in gut glutamine extraction [47]. Low plasma glutamine levels in CD may be related to nutritional depletion rather than inflammation, because the mean body mass index of patients with CD included in this study was only 90% of age and gender-matched healthy controls and persisted at 93% 3 months after surgery in association with low plasma glutamine levels.

Glutamine-enriched feedings in normal rats stimulate the glutamine uptake by the gastrointestinal tract [46], improve the mucosal glutaminase activity [48, 49], increase the mitotic index of crypt epithelial cells, and enhance the tissue levels of energy-rich phosphates [27]. The addition of glutamine to a standard nutrient formula attenuates the mucosal injury induced by chemotherapy [19], reduces the bacterial translocation after radiation-induced enteritis [21, 22], and improves the translocation of endotoxin in experimental ulcerative colitis [23]. Our data obtained in CD, therefore, suggest that glutamine supplementation could be of therapeutic value. Indeed, enteral glutamine feeding in our rat model of ileitis abolishes the macroscopic inflammation, normalizes the body weight gain and bowel weight, enhances the intestinal glutaminase activity above control levels, and normalizes the glutathione redox status and GST activity.

We show that enteral glutamine supplementation in experimental ileitis enhances the intestinal GSH content, in line with previous findings that glutamine is a biosynthetic precursor of GSH in the gut [50, 51]. Moreover, individual mucosal glutamine levels are correlated with tGSH concentrations in our clinical study. Glutamine may thereby contribute to mucosal healing, because GSH is essential for the integrity and antioxidative capacity of the gut mucosa [52]. As shown, the normalization of intestinal GSH concentrations is related to an increase in enzyme activity of glutaminase, but not γGCS. We, therefore, suggest that the availability of glutamine, but not cystine (as has been found in lymphocytes [53]), is limiting for GSH synthesis in the gut in agreement with recent findings from our laboratory [32].

Normalization of the intestinal glutathione redox status after glutamine supplementation contrasts with the substantial lipid peroxidation in the ileum and colon (Fig. 6B). These conflicting results can be unified only by the explanation that glutamine also augments the inflammatory activity of neutrophils, which use glutamine at a high rate for energy production and nucleotide biosynthesis [54]. This is evidenced by a significantly enhanced MPO activity along the gastrointestinal tract in glutamine-fed animals (Fig. 6A). Because oral glutamine has been shown to attenuate leukocyte emigration in indomethacin-induced ileitis [55] and because glutamine delays spontaneous neutrophil apoptosis [56], we assume that the high intestinal MPO activity in glutamine-fed animals is the result of persistent activation of recently recruited neutrophils. In addition, glutamine augments the superoxide generation in neutrophils [57]. The enhanced oxidative stress in glutamine-fed animals is demonstrated by the potentiation of oxidative tissue injury in the uninvolved colon in the presence of a normal intestinal GSH content and GST activity.

After glutamine supplementation, the delicate balance between mucosal regeneration and immune-enhancement with oxidative tissue injury is in favor of mucosal regeneration in the ileum but may exacerbate tissue injury in the colon. It is reasonable to explain the differential effects of glutamine in the ileum and colon by the fact that glutamine is the primary fuel for enterocytes, whereas colonocytes mainly use butyrate [41]. Indeed, high-dose oral glutamine feeding worsened the chronic colonic injury in the trinitrobenzene sulfonic acid colitis model, and a lower dose of glutamine proofed to be ineffective [25]. Similarly, immune-enhancement by dietary nucleosides and nucleotides has been found to exacerbate experimental colitis [58]. Parenteral glutamine supplementation could not ameliorate the mucosal barrier function after induction of trinitrobenzene sulfonic acid colitis [59]. A randomized trial in 18 pediatric patients with CD with colonic involvement in 15 cases showed that a glutamine-enriched polymeric diet is less effective in improving the CDAI than a standard low-glutamine polymeric diet [30]. Oral glutamine feeding of patients with CD for 4 weeks failed to improve the intestinal permeability in two randomized studies [28, 29]. As opposed to systemic glutamine supplementation, topical application may mask circulating inflammatory cells from the immune-enhancing effect of glutamine and support mucosal healing in colitic disease, at least if administered before induction of colitis [24, 26, 27].

We show that enteral glutamine feeding involves both mucosal regeneration and immune-enhancement of inflammatory cells. Because of differing energy sources used by the ileum and colon, the net result of glutamine supplementation is beneficial in the treatment of inflammatory small-bowel disease but may be deleterious in colitis. With the knowledge obtained in this study, we conclude that CD confined to the small bowel benefits most from enteral glutamine feeding.