Effects of diet macronutrient content on avian feeding behavior,
physiology, and gut microbiota.
Diet macronutrient content did not affect feeding behavior, with birds
consuming similar amounts of food regardless of diet treatment.
Surprisingly, we did not find an effect of diet on complement activity
or baseline or stress-induced corticosterone concentrations. Contrary to
our results, several studies have found a relationship between diet and
various metrics of the complement pathway. In fish, alternative
complement activity (ACH50) is higher when fish were fed diets with 8%
or higher lipid levels when compared with fish fed a lipid-free control
diet (Lin and Shiau 2003). Similarly, in mice, a high-fat diet induces
complement activation and proinflammatory cytokine production (Doerner
et al. 2016). Previous work investigating the effects of dietary
macronutrients on stress physiology found that kittiwake chicks fed a
low-lipid diet had higher baseline and stress-induced concentrations of
corticosterone (Kitaysky et al. 2001). Conversely, but similar to our
results, corticosterone concentrations were unaffected by dietary
supplementation with anthropogenic items like bread in captive white
ibis (Cummings et al. 2020). It is possible that more dramatic shifts in
macronutrient ratios between diet treatments or having longer time
frames on the diets is required to detect large effects on host immune
and endocrine physiology.
While we did not find a direct effect of diet treatment on complement
activity or corticosterone concentrations, the gut microbiome is capable
of both influencing and responding to these physiological endpoints
(Williams et al. 2020), which can occur independently of changes in
diet, or possibly be indirectly influenced by diet through another
metric that was not measured in the present study. For example, higher
glucocorticoid levels are associated with lower bacterial diversity in
squirrels (Petrullo
et al. 2022) and
gulls (Noguera et al.
2018). Complement
activity has also been linked to changes in gut microbiota. Depletion of
gut commensal bacterial inhibits complement activation in mice (Yoshiya
et al. 2011), and in tilapia fish complement C3 is positively correlated
with the Shannon’s and Simpson’s index of gut microbiota (Zhu et al.
2020). In our study, complement activity was positively correlated with
observed richness and Shannon diversity on day 0, but this relationship
was disrupted on day 8. Baseline corticosterone concentrations did not
significantly affect microbial richness or diversity, while
stress-induced concentrations were generally associated with higher
Shannon diversity. However, the positive association between
stress-induced corticosterone and Shannon diversity was not apparent in
birds on a protein diet prior to treatment (day 0) or birds fed a high
fat diet post-treatment (day 8). For both complement and stress-induced
corticosterone, birds in the high fat diet treatment shifted from having
a positive association between physiology and gut microbiota alpha
diversity metrics at day 0 to a negative association between metrics
following 8 days on a high fat diet. This suggests that diet
macronutrient ratios can influence the relationship between gut
microbial communities and physiology and that the fat content of diet
might be important in altering these relationships.
While the diet treatments in experiment 1 (diet manipulation) did not
have major effects on overall gut microbiota composition, we found that
protein content can shift the abundance of particular taxa. While
community-level changes in gut microbiota can have important
implications for host health, smaller changes in specific taxa can also
affect host physiological processes (Hooper et al. 2012, Round and
Mazmanian 2009). Specifically, we found that birds fed a high protein
diet for 8 days had lower proportions of the phylum Campylobacterota.
Previous work in mice found that individuals fed a high fat diet have a
higher relative abundance of the genus Campylobacter (a member of
the Phylum Campylobacterota), and Campylobacter abundance was positively
correlated with serum lipid levels (Mu et al. 2020). The association
between a high fat diet and members of Campylobacterota might explain
why we observed a decrease in the abundance of Campylobacterota in birds
on the high protein diet but not birds fed the high fat diet or diet
with equal ratios of fat and protein.