Glutamine is a conditionally essential amino acid that becomes essential during times of stress. Research shows that during times of stress, the production of glutamine is not sufficient to meet demands. In response, glutamine stores in muscle tissue are mobilised in order to maintain plasma levels of nitrogen, and this in turn causes a catabolic state in muscle. Chromic stress is therefore able to cause decreases in lean body tissue, and both low plasma glutamine levels and decreases in lean body tissue have been linked to increases in total mortality. Glutamine is one of the preferred respiratory fuels for lymphocytes. If plasma levels of glutamine remain depressed, immune function is compromised due to inhibition of immune cell proliferation. This is one of the mechanisms by which stress, which can include intense physical exercise, is able to depress the immune system.
Glutamine is a preferred respiratory fuel for not only lymphocytes, but also for enterocytes. Glutamine is metabolised in the gut to citrulline, ammonia and other amino acids. If the availability of glutamine falls, the barrier function of the enterocytes becomes compromised and absorptive and secretive functions altered. Supplemental glutamine has been shown to prevent the absorption of exotoxins associated with compromised glutamine status by providing the enterocytes with a source of glutamine. Glutamine is also used to synthesise ammonia in the kidney, and therefore plays a role in maintaining the acid base balance in the blood. The antioxidant glutathione required glutamine for synthesis, and low plasma glutamine therefore compromise host antioxidant defences. Low levels of glutathione are associated with an increase in mortality, and this may reflect the stress induced glutamine deficient catabolic status of the patient.
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