Dietary fat quality did not affect oxidative status (H2)
Our study does not provide evidence to support H2 that migratory
songbirds fed diets composed of more 18:2n-6 are more susceptible to
oxidative damage and thus must increase antioxidant capacity compared to
when fed diets with less 18:2n-6. This hypothesis was informed by the
biochemistry and oxidative susceptibility of PUFA (Halliwell and
Gutteridge 2007, Skrip and McWilliams 2016) and the demonstrated
responsiveness of the antioxidant system in migratory birds
(Jenni-Eiermann et al. 2014, Skrip et al. 2015, Dick and Guglielmo
2019). Additionally, 18:2n-6 has been shown to stimulate antioxidant
enzymes in fish (Li et al. 2013, Zengi̇n and Yilmaz 2016) but not in rats
(Tou et al. 2011). Dietary long-chain n-3 or n-6 PUFA did not affect
oxidative damage or enzymatic antioxidants in the pectoralis of
Yellow-rumped warblers (Dick and Guglielmo 2019), and a companion study
(citation redacted for initial review) demonstrated that antioxidant
gene expression and enzyme activities (GPx, SOD, CAT) did not increase
in these same starlings consuming more 18:2n-6. The lack of change in
oxidative parameters among diets suggest that dietary 18:2n-6
composition did not oxidatively challenge birds even after flight
training. It is possible that a bird’s antioxidant system is equipped to
combat reactive species associated with consuming any amount of PUFA
which would explain the lack of differences among diet groups. For
example, White-throated sparrows (Zonotrichia albicollis ) fed
diets with more 18:2n-3 and 18:2n-6 PUFA had higher levels of d-ROMs,
but similar ratios of oxidative damage: antioxidant capacity (Alan and
McWilliams 2013), and Common blackbirds (Turdus merula ) caught
during migration had higher circulating total n-3 and n-6 PUFAs and had
higher non-enzymatic antioxidant capacity and similar damage levels
compared to resident birds (Eikenaar et al. 2017, Jensen et al. 2020).
Alternatively, Common blackbirds may instead mediate total unsaturation
levels in their diet and hence fat stores to minimize the oxidative
challenge of consuming more PUFA (Jensen et al. 2020) rather than
relying on their antioxidant system for protection. A companion study
revealed that increased 18:2n-6 elevated the eicosanoid hormone
prostacyclin which mediates potential downstream effects on energetics
and aerobic performance, and this could be another alternative
explanation for the lack of an effect of dietary PUFA on oxidative
parameters (citation redacted for initial review). In sum, lipid
peroxidation likely remains a relevant challenge for migratory songbirds
that rely on 18:2n-6 and other fats to fuel migratory flights (Pierce et
al. 2004, Pierce and McWilliams 2005, Price et al. 2008, Smith and
McWilliams 2010), and birds seem able to modulate their antioxidant
system in response to the oxidative challenges fats pose and thereby
successfully protect against damage. Whether or not these adjustments to
the antioxidant system have metabolic tradeoffs or tradeoffs with
immunity remains unknown (Costantini 2019). Thus, future studies that
compare the effect of different amounts of dietary 18:2n-6 on a
migratory bird’s physiological status, (e.g. oxidative, metabolic, and
immune statuses) will better elucidate the direct effects of dietary
18:2n-6 on the potential tradeoffs among physiological systems.