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.