Overview of the inflammatory response and its nutritional costs

Abstract Text:

Innate immune cells respond quickly to a potential pathogen due to the presence of a common set of receptors on all phagocytic cells that recognize broad categories of pathogens. Thus, a very large number of cells can recognize invading microbes and respond to them quickly. A consequence of this is pathogen clearance, usually by phagocytosis, followed by the release of inflammatory cytokines and chemokines that amplify the local infiltration of additional inflammatory cells and activate them. If the challenge is large or if it is accompanied by damage to host tissue, cytokines are released in sufficient amounts that they have endocrine-like effects throughout the body. This cytokine storm induces metabolic changes, including increased protein degradation and insulin resistance in skeletal muscle, which diverts nutrients from muscle and other tissues so that they become available for the increased demands of leucocytes and for the production of protective proteins. Importantly the liver transitions from maintaining homeostasis and supporting the nutritional demands of growth or reproduction to the production of protective proteins such as complement, mannan binding protein, and C-reactive protein that aid in the detection and neutralization of pathogens. This transition is accompanied by hepatic hypertrophy.  A study of the costs of a systemic inflammatory response in chickens to Salmonella that examined the amount of nutrients in 6 different leukocyte types in 5 different tissues (blood, spleen, bursa, thymus, bone marrow) and 12 protective proteins (acute phase proteins and immunoglobulins) found that the amount of essential amino acids in the protective proteins greatly exceed that in the cellular component of the immune system during both a normal and an inflammatory state.  The ideal balance of amino acids for the acute phase of an inflammatory response differs greatly from that needed for growth and there is a critical need for additional cysteine and threonine. Ongoing research indicates that higher metabolic rate, decreased intake of food, a mismatch between the nutrient balance needed for the inflammatory response relative to that in body tissues and less efficient digestion that accompany a robust inflammatory response are, together, even more costly than the direct use of nutrients by inflammatory cells and the liver. Together, these costs result in decreased productivity that cannot be completely reversed by supplying additional nutrients.

Keywords: inflammation, cytokines, nutrients