Can We Quantify the Impact of Inflammation and Immune Activation on Nutrient Use and Partitioning?

The costs of immune challenges – in feed efficiency, growth, and productivity – have long been of interest to animal scientists. These questions have been magnified in recent years by controversies around sub-therapeutic antibiotic use as well as novel feed additives intended to modulate the immune responses of animals. Unfortunately, clearly defining the true nutrient cost of immune activation is not a simple task. First, there is no such thing as a standard immune response – each trigger engages a different subset of immune tools, to varying degrees. Secondly, most real infections and model systems have pleiotropic impacts on physiology, including changes in feed intake and loss of steady state conditions, that confound some measurements of nutrient or energy flux. Nevertheless, research across a variety of species has solidified some general concepts about nutrient responses to immune activation. To begin with, nutrient use by immune cells escalates rapidly during inflammation, but because these cells compromise a relatively minor fraction of body mass, the impact on whole-body energetics is limited. Nevertheless, because of the fuel preferences of immune cells, this response may significantly impact availability of specific nutrients. The estimated increase in resting metabolic rate during immune activation ranges from 10 – 40%, depending on the severity of the insult. The majority of this energetic drain has been attributed to the acute phase response, including the fuel used to drive fever. This, too, may impact nutrient availability, including essential amino acid use for acute phase protein synthesis. Finally, inquiries into nutrient reallocation of non-lactating animals during immune activation have generally indicated that adipose tissue is sacrificed first, followed by reproductive functions, and finally lean tissue. Despite difficulties in dis-entangling overlapping response mechanisms, negative impacts of illness on feed intake and maintenance requirements are probably usually due to the immune response rather than pathogen per se. To the extent that these host immune responses can be suppressed without putting the animal at risk of succumbing to infection, there may be opportunities to enhance production efficiency of livestock.