The nutritional cost of the immune response in pigs

In growing animals, microbial challenges are associated with a decrease in feed intake and growth. Experimental data show that the slowing down of growth cannot be explained by the sole decrease in ingestion, suggesting that the immune response must be costly in nutrients. Immune defenses include maintenance of resting sentinel leukocytes and mucous barriers. Upon immune activation, mucosal epithelia are often dramatically challenged, haematopoiesis of innate immune cells is increased, lymphocytes undergo clonal expansions, and leukocytes metabolism increases to produce various effector responses (immunoglobulin synthesis, oxidative burst…). The cost of these effector responses is uneasy to estimate. However, considering the marginal mass of the immune system regarding the whole body mass, it is doubtful that the immune system would be a significant consumer of nutritional resources, even upon severe immune activation. Many immune responses are accompanied by an acute phase response, which involves non immune tissue. It is characterized by the synthesis of acute phase proteins in the liver, fever, accelerated whole-body protein turnover, and high rates of hepatic gluconeogenesis. Clearly the acute phase response is a process that is both nutrient liberating (skeletal muscle catabolism) and nutrient consuming (acute phase protein synthesis, fever). Experimental data show that inflammation alters the metabolism of amino acids in pigs. Some of them, such as histidine, valine, threonine, arginine or tryptophan, displayed more severe and long lasting alterations, maybe related either to their role as a substrate for gluconeogenesis or ketogenesis, either as specific precursors / substrates for immune processes. Glucose metabolism is also severely affected. Based on this knowledge, various nutritional strategies have been tested, to adequately feed the immunologically challenged pig, limit inflammation and minimize growth deterioration. This presentation focuses on two of them: moderate feed restriction displayed mixed results, while tryptophan supplementation is more promising