Birds Evolved Genetic Tricks to Thrive on Sugary Diets | Science News
For many species of birds, a diet rich in sugar – from nectar and fruit – doesn’t lead to the metabolic problems that plague humans consuming similar amounts. Researchers are now uncovering the genetic mechanisms that allow these avian sugar aficionados to thrive while sidestepping obesity, diabetes, and other related health issues. A study published on in the journal Science details how different bird lineages have independently evolved similar genetic “workarounds” to manage high-sugar intake.
The ability to process sugar efficiently isn’t universal in the bird world. While hummingbirds, honeyeaters, and parrots are well-known for their sweet diets, most other bird species cannot readily taste sweetness. This difference stems from evolutionary changes in taste receptors, as research published in July 2021 demonstrated that songbirds, for example, repurposed their umami receptors to detect sweet flavors. This adaptation allows them to identify and consume sugary food sources that contribute to their evolutionary success.
The human body responds to sugar with a cascade of hormonal signals, primarily involving insulin. Insulin prompts cells to absorb glucose from the bloodstream, but in birds, this system appears to function differently. Birds have fasting blood glucose levels 1.5 to two times higher than similarly sized mammals and exhibit relative insulin insensitivity. Unlike mammals, birds seem to lack the protein GLUT4, which is crucial for transporting glucose into cells. Birds can maintain high blood sugar levels without experiencing the negative consequences seen in humans.
“If [humans] are eating a lot of sugar, then a lot of bad things are happening to us: metabolic syndrome, obesity, type 2 diabetes,” explains Ekaterina Osipova, a genomicist at Harvard University, highlighting the stark contrast between human and avian physiology. “At the same time, there are birds that naturally solve this problem. They’re feeding on a lot of sugar, but nothing bad happens to them.”
To understand the genetic basis of this resilience, Osipova and her team analyzed the genomes of five sugar-feeding bird species – including parrots, honeyeaters, and hummingbirds – and compared them to four species with diets based on seeds, insects, or meat. They also examined the transcriptomes – the complete set of RNA transcripts – from various tissues in nectar-loving and non-nectar-loving birds. The analysis revealed thousands of genetic differences, with the majority occurring in regions that regulate gene expression rather than in the protein-coding genes themselves.
Approximately 600 genes were directly involved in sugar and fat metabolism, and remarkably, different bird groups – such as parrots and sunbirds – evolved similar genetic changes independently. Sixty-six protein-coding genes were altered in more than one of the high-sugar species, suggesting convergent evolution driven by dietary pressures.
A single gene, MLXIPL, stood out as being altered in all four sugar-feeding species studied. This gene produces a transcription factor called ChREBP, which acts as a cellular sugar sensor. When the researchers introduced hummingbird MLXIPL into human cells, they observed changes in how the cells responded to sugar, activating genes involved in carbohydrate metabolism. This suggests that MLXIPL plays a critical role in enabling birds to efficiently process high sugar loads.
The adaptations weren’t solely focused on metabolism. Researchers also found alterations related to blood pressure regulation. “What we have is a stunning example of evolutionary integration,” says Chang Zhang, a physiologist at Sichuan University in China. “It suggests that evolving to thrive on a diet of nectar and fruit isn’t just about processing the sugar itself.” The stickiness of sugar and the high water content of nectar and fruit place unique demands on blood circulation, requiring precise control of blood plasma consistency to prevent blockages.
The findings suggest that genes like MLXIPL could potentially serve as clinical targets for treating metabolic diseases in humans, though Osipova cautions that a single gene is unlikely to be a complete solution. She emphasizes that a combination of genetic adjustments – influencing sugar sensing, metabolism, and blood pressure control – is necessary to successfully navigate a high-sugar diet.
Interestingly, similar metabolic adaptations have been observed in some mammals. Research published in 2007 showed that long-tongued bats, like hummingbirds, rapidly metabolize sugar, extracting energy almost immediately upon consumption. This “little metabolic trick,” as described by John Speakman of the University of Aberdeen, was previously thought to be unique to birds. The high energy demands of these nectar-feeding bats, which can burn up to 60 percent of their energy reserves daily, necessitate this efficient sugar processing.
While the research offers valuable insights into the genetic mechanisms underlying sugar metabolism in birds and bats, it’s important to remember that these adaptations evolved over millennia. Applying these findings to human health requires further investigation and a nuanced understanding of the complex interplay between genetics, diet, and lifestyle.