Macronutrient-specific feeding behaviors after an immune
threat.
The goals of the second experiment (diet selection) were to: 1)
investigate how an immune threat influences feeding behavior and
macronutrient selection, 2) explore if social cues of disease can alter
feeding behavior and immune and endocrine responses relevant to disease
susceptibility, and 3) determine whether an immune threat and behavioral
shifts in feeding alter the gut microbiome. Based on research in
invertebrates, we predicted that birds given an immune challenge would
either increase protein intake or maintain consistent levels of protein
consumption while reducing lipid intake (Adamo 2008, Adamo et al. 2010,
Povey et al. 2013, Cotter et al. 2010). Conversely, we found that birds
given an immune challenge with LPS engaged in macronutrient-specific
sickness-induced anorexia by maintaining consumption of the high lipid
diet while significantly reducing consumption of the high protein diet.
However, we did not detect shifts in the microbiome that were driven by
LPS exposure. Consistent with sickness-induced anorexia, immune
challenged individuals lost weight but did not have any detectable
changes in furcular fat stores. Caloric restriction during illness can
improve host health and recovery in some cases (Cheng et al. 2017; Wang
et al. 2016); thus, the observed reduction in caloric intake in
LPS-challenged birds may be an adaptive response to an immune threat.
The finding that sickness-induced anorexia in LPS-injected birds was
driven by a macronutrient-specific reduction in protein intake is
interesting given the apparent importance of protein to immune function
and responding to and surviving infection (Lee et al. 2006, Povey et al.
2013). Prior research in insects suggests that individuals should
benefit from reducing lipid intake during infection, however we saw no
change in lipid intake in birds given an immune challenge. For example,
infected caterpillars assigned to a high-lipid diet have higher
mortality rates than infected individuals feeding on water or sucrose
(Adamo et al. 2007). Further, research in crickets identified a tradeoff
between immunity and lipid-transport, suggesting that reducing lipid
consumption can maximize immune responses (Adamo et al. 2010). It is
unknown whether a tradeoff between lipid-transport and immunity exists
in vertebrates (Demas and Nelson 2012). However, our finding that
LPS-challenged birds reduce protein consumption but not lipid
consumption challenges this idea and warrants further investigation in
avian and other vertebrate systems.
Although studies examining the relationship between infection and
dietary macronutrient preference are uncommon in vertebrates, one study
in mammals found a similar reduction in protein intake following LPS
immune-challenge. Specifically, rats injected with LPS voluntarily
decreased protein intake while lipid intake remained unchanged, however
this study also observed a significant increase in carbohydrate
consumption in LPS-treated individuals (Aubert et al. 1995). Coupled
with our finding that LPS-challenged birds selectively reduce protein
but not lipid intake, this suggests that reduced protein consumption may
be a common behavioral response to an immune threat in vertebrate
species, although the function of this shift in macronutrient preference
is still unclear. It is possible that a reduction in protein consumption
occurs in response to an immune challenge because protein is more likely
to contain iron than other macronutrients. Although iron is essential to
host immune function, it is also used by pathogens. Thus, limiting iron
intake could interfere with pathogen growth and help limit infection
(Soyano and Gómez 1999, Kluger and Rothernberg 1979). Further work is
needed to determine if reduced protein consumption during infection is
adaptive for hosts in terms of responding to and overcoming infection,
and whether these effects are mediated through shifts in micronutrient
intake such as iron.