Ultramarathons push the human body to its limits, but new research from the University of Victoria suggests those limits may be shaped long before race day.
A study published in Frontiers in Ecology and Evolution, led by UVic biological anthropologist Alison Murray, found an athlete’s birth weight may influence how their kidneys respond to the strain of extreme endurance events.
“Humans are often described as naturally built for endurance, a trait thought to have evolved during hunter-gatherer periods when long-distance travel was essential for survival. But our new research suggests that extreme endurance events may push some internal systems closer to their limits than previously understood,” Murray said.
The research followed 44 runners, 15 women and 29 men, competing in either a 230-kilometre multi-stage race in Spain or a continuous race of up to 300 kilometres in Finland. Blood samples taken before and after each race tracked changes in creatinine, a marker of kidney function.
On average, creatinine levels rose by about 0.5 mg/dL in the hot-weather race and 0.2 mg/dL in the cold. Overall, 57 per cent of athletes recorded rises meeting clinical thresholds used to flag possible acute kidney injury.
After accounting for age, sex, distance, hydration and muscle damage, athletes at both the low and high ends of the birth weight spectrum showed larger increases in kidney stress.
Those in the middle fared better, producing a ‘U-shaped relationship’. The lowest predicted increases in creatinine were linked to a birth weight of about 3.8 kilograms, roughly eight pounds.
These findings reflect differences in renal functional reserve, the kidney’s capacity to handle stress, which is largely set before birth.
People born with lower birth weights tend to have fewer nephrons, while very high birth weights are also linked to altered kidney function. Both scenarios leave less room for the body to adapt under extreme physical demand.
Athletes racing in the heat showed larger increases in kidney stress markers and greater weight loss compared to those in the cold. However, birth weight remained a significant predictor of kidney response regardless of the environment.
While researchers caution the study is based on a relatively small group and self-reported birth weights, the results point to a deeper biological layer behind performance and recovery.
“Our study asks the question whether there is such a thing as ‘too much’ exercise, and how early-life biological factors may shape the body’s response to extreme physical demands,” added Murray.
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