An endurance limit that surfaces in punishing races may begin at birth

A new study is raising questions about whether human endurance has biological limits shaped long before adulthood—possibly beginning at birth. Researchers are examining whether birth weight, a known risk factor for disease later in life, may also influence how the body responds to extreme endurance exercise.

Study explores kidneys and endurance

The study, published in Frontiers in Ecology and Evolution and led by University of Victoria biological anthropologist Alison Murray, is the first to investigate a potential link between birth weight and kidney stress in ultramarathon athletes.

“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,” says Murray.

How ultramarathons strain the kidneys

Kidney health, in particular, is emerging as a critical factor in endurance sports, according to researchers. Events such as ultramarathons—races exceeding 26.2 miles and often held in high temperatures—place significant strain on the kidneys’ filtration system and may lead to cellular damage.

Murray’s study focuses on whether an individual’s birth weight plays a role in how the kidneys respond to this stress. Birth weight is already recognized as an independent risk factor for health outcomes later in life, and the research suggests it may also be linked to kidney resilience under extreme physical conditions.

Birth weight and race-day biomarkers

Participants in the study underwent blood testing before and after ultramarathon races to measure biomarkers associated with kidney function and potential damage.

In Murray’s group of ultrarunners, the optimal birth weight for minimal kidney damage was approximately eight pounds. Individuals at either the lower or higher end of the birth weight range may face a greater risk of kidney stress or damage during extreme endurance events.

“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,” adds Murray.