Believing that the human body is limitless is a certain kind of hubris. The underlying premise of endurance athletes, particularly ultramarathon runners, is that biology can be pushed in any direction by willpower, training, and unadulterated conditioning. That notion is being subtly contested by recent research from the University of Victoria. And it’s coming from a place that very few people expected: the delivery room.
According to research published in Frontiers in Ecology and Evolution by biological anthropologist Alison Murray and her team, a person’s birth weight—the figure written on a hospital tag decades before they put on a running shoe—may influence how their kidneys react to the harsh demands of ultramarathon racing. It’s the kind of outcome that causes you to pause and reevaluate issues you thought were resolved.
In order to measure serum creatinine, a trustworthy indicator of kidney stress, the study monitored 44 ultramarathon runners competing in both hot and cold environments. Blood samples were taken both before and after each race. Researchers discovered a U-shaped relationship: athletes who were born at either the lower or higher ends of the birth weight spectrum had higher post-race spikes in creatinine. Those who weighed closer to eight pounds, or about 3.8 kilograms, at birth fared much better. Extreme runners’ kidneys had to work harder and didn’t always recover properly.
It’s important to consider the true demands that ultramarathons place on the body. These aren’t your typical 26-mile races. Many are staged in intense heat, where dehydration exacerbates every physiological strain imaginable, and many stretch to 50, 100, or even more miles. The entire time, the kidneys, which filter waste and control fluid balance, are silently performing massive amounts of work. The prevalence of kidney injury in endurance athletes is higher than most people realize, and sports medicine is finding it more difficult to overlook it.
The suggestion that this vulnerability isn’t related to fitness is what makes Murray’s work truly fascinating, if a little unsettling. You can manage your hydration, optimize your diet, and train for years. However, that baseline might accompany you onto the racecourse if your kidneys were formed during fetal development in a way that made them structurally weaker. Birth weight has long been linked to the risk of diabetes, cardiovascular disease, and chronic illnesses. It seems like a significant continuation of an already serious discussion to include kidney function under severe physical stress on that list.
The sports science community seems to have focused a lot of attention on what athletes can do to their bodies during training, but less on what those bodies came with. Murray’s framing draws on the evolutionary history of humans to suggest that endurance was a survival strategy developed over millennia of long-distance travel and persistent hunting. Renal capacity may have always played a role in that calculation, subtly limiting the distance that even the most determined human could truly run.
It’s genuinely unclear if this affects how athletes, coaches, or race organizers view preparation. It’s not exactly a useful pre-race checklist to know your birth weight and map it against kidney risk. However, the science is posing an important question: how much of our endurance performance is predetermined before we walk for the first time? Perhaps more importantly, is it possible for some bodies to be overexerted due to biology rather than personal preference?
For a culture that views more as superior, that final thought lingers in a way that feels almost inconvenient. Perhaps it ought to.
