Metabolic partitioning of sucrose and seasonal changes in fat turnover rate in ruby-throated hummingbirds (Archilochus colubris)

Abstract

Hummingbirds fuel their high energy needs with the fructose and glucose in their nectar diets. These sugars are used both to fuel immediate energy needs and to build fat stores to fuel future fasting periods. Fasting hummingbirds can deplete energy stores in only hours and need to be continuously replacing these stores while feeding and foraging. Whether and how hummingbirds partition dietary fructose and glucose towards immediate oxidation versus fat storage is unknown. Using a chronic stable isotope tracer methodology, we examined whether glucose or fructose is preferentially used for de novo lipogenesis in ruby-throated hummingbirds (Archilochus colubris). Potential seasonal changes were correlated with variation in the overall daily energy expenditure. We fed ruby-throated hummingbirds sucrose-based diets enriched with 13 C on either the glucose or the fructose portion of the disaccharide for 5 days. Isotopic incorporation into fat stores was measured via the breath 13 C signature while fasting (oxidizing fat) during the winter and summer seasons. We found greater isotopic enrichment of fat stores when glucose was labelled compared with fructose, suggesting preference for glucose as a substrate for fatty acid synthesis. We also found a seasonal effect on fat turnover rate. Faster turnover rates occurred during the summer months, when birds maintained lower body mass, fat stores and exhibited higher daily nectar intake compared with winter. This demonstrates that fat turnover rate can substantially vary with changing energy expenditure and body composition; however, the partitioning of sucrose towards de novo fatty acid synthesis remains constant.

Glucose Transporter Expression and Regulation Following a Fast in the Ruby-throated Hummingbird, Archilochus colubris

“…Organs such as the brain are exceptionally 511 demanding of glucose (Tokushima et al, 2005) in gallus gallus chicks and likely other birds as 512 well. Further, lipogenic pathways of the hummingbird liver also shows a preference for glucose 513 over fructose (Dick et al, 2019). Finally, while hummingbirds have the capacity to hover 514 oxidising either glucose-only or fructose-only meals (Chen and Welch, 2014), their flight muscle 515 cells' maximal capacity for monosaccharide phosphorylation is twice as high for glucose 516 compared to fructose in tissue homogenates in vitro (Myrka and Welch, 2018).…”

Genomic insights into metabolic flux in ruby-throated hummingbirds

“…Four ruby-throated hummingbirds were fasted for 1 hour, following which they were placed in a 500 ml respirometry container and baseline fasting breath delta 13 C breath stable isotope signature and respiratory exchange ratio (RER) recording (see (Dick et al 2020) for respirometry and breath stable isotope set up). After 5 min the birds were then fed a 150 ml of a 20% sucrose solution with sucrose enriched with 13 C on all six carbons of the glucose (sucrose (glucose- 13 C6, 98%), Cambridge Isotope Laboratories, Tewksbury, MA, USA] or fructose [d-sucrose (fructose- 13 C6, 98%), Cambridge Isotope Laboratories] portion of the sucrose molecule.…”

“…The birds were then returned to their cages and repeated the process again 1 week later with the other sucrose solution, with 2 birds starting with fructose-enriched, and 2 birds starting with the glucose-enriched. RER was analyzed following (Dick et al 2020), and tracer oxidation rate analyzed following (M. D. McCue et al 2010), and were averaged for each minute over the course of 20 minutes. The time to and peak tracer oxidation rate was analyzed using a pair t-test.…”

Riding out the storm: depleted fat stores and elevated hematocrit in a small bodied endotherm exposed to severe weather

“…epinephrine) and glucocorticoids into the bloodstream, which mobilize energy stores and influence body composition (reviewed in Sapolsky et al, 2000 ; Wingfield et al, 2017 ). Fat is the component of body composition that provides the greatest amount of energy per unit mass ( Jenni and Jenni-Eiermann, 1998 ), and in birds, can be rapidly deposited and mobilized in response to short-term foraging excesses or deficits ( Dick et al, 2020 ). While fat deposition or mobilization can be regulated within minutes, body composition responses are typically evident within hours to days ( Seewagen and Guglielmo, 2010 ; Boyle et al, 2012 ; Krause et al, 2017 ).…”