For decades, textbooks have taught that lactate is little more than a metabolic dead end—a cellular waste product that accumulates during strenuous exercise, causing the familiar burn in overworked muscles. A new study presented at the Society for Experimental Biology’s annual conference in Florence, Italy, turns this dogma on its head, at least for birds. Researchers have discovered that avian red blood cells possess a remarkable ability to harness lactate as an emergency fuel to repair damaged hemoglobin and restore the blood’s oxygen-carrying capacity, a feat made possible by a cellular component that mammalian red blood cells deliberately discarded long ago: mitochondria.
Hemoglobin, the iron-rich protein that shuttles oxygen through the bloodstream, constantly faces a subtle but significant threat. Even under normal conditions, a small fraction of hemoglobin spontaneously oxidizes into a non-functional form called methemoglobin, which cannot bind oxygen. Left unchecked, this oxidative damage can cripple oxygen delivery to tissues. Mammals rely on a constellation of enzymatic systems to continuously convert methemoglobin back to functional hemoglobin, but these repair processes require a steady supply of the electron carrier NADH. Yi Yang, a doctoral candidate at the University of Auckland, and her colleagues wondered whether birds, with their notoriously high metabolic demands and oxygen-hungry lifestyles, might have evolved a more powerful rescue mechanism.
To test this, the team collected red blood cells from chickens and rats and exposed them to an oxidative challenge designed to push hemoglobin into its methemoglobin state. They then spiked the samples with various potential fuels, including glucose and lactate. The difference was striking. When lactate was present, chicken red blood cells converted methemoglobin back to functional hemoglobin three times faster than rat cells could manage. Probing deeper, the researchers found that the avian cells’ secret weapon lay in their retained mitochondria, the double-membraned power plants that mammalian red blood cells eject during maturation to streamline oxygen delivery.
The biochemical logic is elegant. Lactate is transformed by the enzyme lactate dehydrogenase (LDH) into pyruvate, simultaneously generating NADH. That freshly minted NADH can then be immediately deployed by methemoglobin reductase enzymes to repair damaged hemoglobin. However, this reaction is reversible and easily stalls if pyruvate accumulates, quickly sapping the supply of NADH. Here is where mitochondria change the game. In bird red blood cells, pyruvate is swiftly taken up by mitochondria and oxidized, pulling the entire reaction forward and allowing a sustained, high-flux supply of NADH. Mammalian red blood cells, stripped of their mitochondria during development, cannot clear pyruvate at the same rate, so their repair machinery sputters under heavy oxidative load.
This mitochondrial advantage became even clearer when Yang’s team measured oxygen consumption. Both chicken and rat red blood cells increased their apparent oxygen use after lactate was added, but the spike in avian cells was substantially larger. Part of the oxygen rush was driven directly by mitochondria burning pyruvate, while another portion reflected the rapid restoration of functional hemoglobin molecules that could once again bind oxygen. The team also identified an unexpected molecular specialization: chickens express a heart-type variant of LDH, an isoform kinetically optimized to convert lactate into pyruvate, whereas rats rely on a muscle-type form that operates less efficiently in this direction.
The findings challenge the long-held assumption that jettisoning mitochondria is an unequivocal upgrade for oxygen transport. Birds, which navigate extreme oxygen demands during flight, seem to have made an evolutionary trade-off. Keeping mitochondria makes their red blood cells slightly larger and bulkier, theoretically slowing oxygen diffusion, but it also equips them with an on-board metabolic workshop that can generate NADH on demand and rapidly resuscitate crippled hemoglobin during episodes of oxidative stress. “At first glance, retaining mitochondria might seem risky,” Yang explains, “but our findings show they aid methemoglobin reduction by consuming pyruvate.”
Beyond rewriting a chapter of comparative physiology, the research spotlights lactate’s emerging identity as far more than a waste product. Even in mammals, lactate shuttles between tissues as a versatile fuel and signaling molecule, but birds appear to have elevated this capacity into a dedicated cellular repair pathway. While studies in the 1970s had hinted that nucleated red blood cells might tap lactate, Yang’s work is the first to lay bare the precise mitochondrial mechanisms that safeguard avian oxygen supplies.
The implications ripple outward. If bird red blood cells can weaponize lactate in this way, what about other vertebrates that retain mitochondria in their circulating cells, such as reptiles, amphibians, and fish? Yang and her team are now turning their attention to these lineages, curious whether evolution has repeatedly converged on the same trick to defend against oxidative damage. In an era where metabolic flexibility is increasingly linked to resilience against disease, understanding how nature has wired these elegant recovery circuits could inspire new strategies to protect cells under stress—even in organisms that long ago traded mitochondrial power for a sleeker, leaner red blood cell.
Subject of Research: The role of lactate and mitochondria in protecting avian red blood cell hemoglobin from oxidative damage.
Article Title: How Bird Blood Cells Turn a “Waste Product” into a Rescue Fuel
News Publication Date: July 2025
Web References: Not available
References: Work presented at the Society for Experimental Biology Annual Conference, Florence, Italy.
Image Credits: Yi Yang, Hickey Lab, The University of Auckland
Keywords: avian red blood cells, lactate, mitochondria, methemoglobin, hemoglobin, oxidative stress, NADH, lactate dehydrogenase, metabolic repair, comparative physiology
