Of all the compounds discussed above glutamine is the most extensively applied in clinical and experimental amino acid supplementation, often in the form of the more soluble and stable dipeptides ala-nyl- and glycyl-glutamine. Glutamic acid and a-ketoglutarate are less ideally suited for use in feeding formulas because of poor inward transport of gluta-mic acid and poor solubility and stability of a-keto-glutarate. Moreover, glutamic acid has been related to the 'Chinese restaurant syndrome,' characterized by light-headiness and nausea after consumption of Chinese food containing glutamic acid for flavor improvement. However, scientific evidence is weak. Numerous experimental and clinical studies have suggested that glutamine supplementation has positive effects on immune function, intestinal mucosal integrity, nitrogen balance, and glutathione concentration in a wide variety of conditions. Nevertheless, the true benefit of glutamine supplementation is difficult to quantify in clinical practice. Its benefit has especially been claimed in the critically ill and surgical patients in whom clinical outcome is multi-factorial. Recent meta-analyses support the view that glutamine supplementation is safe and may reduce infectious morbidity and hospital stay in surgical patients. A positive effect of glutamine supplementation on morbidity and mortality in critical illness, trauma patients, and burn patients has been demonstrated in a few well-designed clinical trials. However, due to the paucity of such trials reliable meta-analyses are not possible in these latter patient categories. It has been demonstrated in some small clinical series that supplementation with ornithine a-ketoglutarate may improve wound healing in burn patients, benefiting from the combined actions of both a-ketoglutarate and ornithine (see sections on arginine and ornithine).
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