Glutamine
The consumption of high biological value proteins is an essential component for a healthy diet and for the optimal production of antibodies [141]. Proteins, or amino acids, deficiency is known to impair immune function and increase susceptibility to infectious diseases. In fact, some amino acids modulate both metabolism and immune functions [142]. Most reviews indicate an important role for amino acids in the immunity by regulating the activation of T lymphocytes, B lymphocytes, natural killer cells and macrophages; cellular redox state, gene expression and lymphocyte proliferation. Evidence shows that the dietary integration of specific amino acids in humans with malnutrition and infectious diseases improves the immune status, thus reducing morbidity and mortality [141]. Glutamine is the most abundant and versatile amino acid present in the body and its level in the immune cells is similar to, or even greater than, glucose in both health and disease conditions.
The biological activities of this nutrient are also associated with the reduced cellular potential of oxygen, which mainly depends on the ratio between reduced/oxidized glutathione [143]. In addition, glutamine is an essential nutrient for lymphocyte proliferation and cytokine production, macrophage phagocytes plus secretory activities and bacterial killing of neutrophils. In immune cells, glucose is mainly converted into lactate (glycolysis), while glutamine is converted into glutamate, aspartate and alanine undergoing a partial oxidation in carbon dioxide, in a process of glutaminolysis [142]. This unique conversion plays a crucial role in the effective functioning of immunity. Glutamine is necessary for the expression of a variety of immune system genes [144], in particular through the activation of proteins, such as the ERK and JNK kinases which are involved in the activation of transcription factors, including JNK and AP-1, finally promoting the transcription of genes that participate in cell proliferation. Besides, a sufficient level of glutamine is important to express the key markers of the cell surface of the lymphocytes and also various cytokines, for example, IL-6, IFN and TNF [143–145]. In healthy subjects with a balanced diet, glutamine supplementation does not increase the effectiveness of immune surveillance or prevent disease episodes, as reported by some reviews, but in some catabolic situations or in a low glutamine intake obtained from the diet, the amino acid supplementation could be required [142].