What hummingbirds teach us about using energy
Hummingbird metabolism is a marvel of evolutionary engineering. These tiny birds can power all of their energetic hovering flight by burning glucose and fructose equally.
Now new research from the University of Toronto Scarborough shows they are equally adept at burning both glucose and fructose — the individual components of sugar — a unique trait among all vertebrates.
"Hummingbirds have an optimal fuel-use strategy that powers their high-energy lifestyle, maximizes fat storage, and minimizes unnecessary weight gain all at the same time," says Kenneth Welch, assistant professor of biological sciences at UTSC and an expert on hummingbirds.
His research appears in the journal Functional Ecology and is currently available online.
Welch and his graduate student Chris Chen, who is co-author on the research, fed hummingbirds separate enriched solutions of glucose and fructose while collecting exhaled breath samples. They found the birds were able to switch from burning glucose to fructose equally as well.
"What's very surprising is that unlike mammals such as humans, who can't rely on fructose to power much of their exercise metabolism, hummingbirds use it very well. In fact, they are very happy using it and can use it just as well as glucose," says Welch.
Hummingbirds require an incredible amount of energy to flap their wings 50 times or more per second in order to maintain hovering flight.
If a hummingbird were the size of a human, it would consume energy at a rate more than 10 times that of an Olympic marathon runner.
They are able to accomplish this by burning only the most recently ingested sugar in their muscles while avoiding the energy tax of first converting sugar into fat.
From an evolutionary perspective the findings make perfect sense, says Welch. Whereas humans evolved over time on a complex diet, hummingbirds evolved on a diet rich in sugar.
"Hummingbirds are able to move sugar from their blood to their muscles at very fast rates, but we don't yet fully understand how they are able to do this," he says.
Humans are not good at burning fructose because once ingested, much of it gets taken into the liver where it's turned into fat.
The prevalence of high fructose corn syrup found in products like soda pop is also strongly linked to a rise in obesity rates.
On the other hand because hummingbirds burn sugar so fast — if they were the size of an average person they would need to drink more than one can of soda every minute even though it's mostly made of high-fructose corn syrup.
"If we can gain insights on how hummingbirds cope with an extreme diet then maybe it can shed some light on what goes wrong in us when we have too much fructose in our diet," says Welch.
Hummingbirds have specialized on a diet consisting almost exclusively of a mixture of sucrose, glucose and fructose found in floral nectar. Previous studies have shown that hummingbirds can fuel energetically expensive hovering flight almost exclusively using recently ingested sucrose. However, the relative capacities for the direct utilization of glucose and fructose by hovering hummingbirds remain unknown.
13C-enriched solutions of glucose and fructose were fed to ruby-throated hummingbirds (Archilochus colubris) separately. Along with simultaneous measurements of gas exchange during hovering we collected exhaled breath samples using feeder-mask respirometry and analysed these to determine the isotopic signatures of exhaled carbon dioxide. We found that hovering hummingbirds transition from exclusively oxidizing endogenous fatty acids when fasted, to oxidizing newly ingested carbohydrates when given access to either glucose or fructose solutions. We then switched hummingbirds to the respective unlabelled solutions of glucose or fructose to estimate carbohydrate turnover kinetics.
During the period of availability of enriched solutions, the percentage of metabolism supported by exogenous sugar increased from 0% to near 100% in some individuals. On average, hummingbirds fuelled 81% and 88% of their metabolism during hovering flight with exogenous glucose and fructose, respectively.
The amount of energy ingested, fractional turnover of ingested sugars in the pool of actively metabolized substrates, amount oxidized, energy expended and proportion of hovering metabolism supported by each hexose were all similar between glucose and fructose.
By foraging frequently and fuelling hovering flight directly with ingested monosaccharides hummingbirds avoid the energetic tax associated with the cost of synthesis of fats from these sugars prior to their oxidation. Remarkably, hovering hummingbirds are able to utilize fructose and glucose equally, a physiological feat which no mammals are thought to match, and one that suggests novel physiological capacities for the oxidation of fructose by active muscle tissues in hummingbirds. The data presented here indicate hummingbirds enhance net energy intake though specialization of diet, behaviour, and, uniquely, metabolic physiology.
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