Your adult race destiny may be determined at birth.

Ultra-endurance events have surged in popularity, with more than 150,000 North Americans racing ultramarathons (a distance longer than the standard 26.2-mile marathon) last year.

And while these hardcore events can prematurely age vital parts of the body, one factor may shape how the body responds to extreme endurance exercise long before adulthood.

Scientists believe humans were made to run long distances, citing evolution during the hunter-gatherer days.

However, endurance events place a large amount of strain on the kidneys, which could negatively affect their filtration system and lead to cellular damage.

A study published in Frontiers in Ecology and Evolution has now found a potential link between birth weight and kidney health in ultramarathon athletes.

As birth weight is a known risk factor for disease later in life, the research found it could also play a role in kidney resilience under extreme physical conditions.

44 ultramarathon athletes underwent blood testing to measure kidney function and potential damage before and after their races.

The ideal birth weight for minimal damage? 8 pounds.

This weight was most likely to reduce the chances of acute kidney injury, a temporary reduction in kidney function.

Those who weighed less or more at birth could have a greater risk of kidney stress or damage during extreme endurance races, as organ development was likely impaired during growth in the uterus.

During exercise, blood flow is redirected to the muscles and less to the kidneys, which, over time, lose the ability to filter waste and excess fluid from the blood.

These current findings, as well as previous research, seem to indicate that ultra-endurance events may be doing more harm than good for the body.

“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,” study lead and University of Victoria biological anthropologist Alison Murray said in a press release.

Previous studies have shown that ultramarathoners experience a breakdown of normal red blood cells during races, which can lead to anemia and other issues.

This is most likely due to the blood cells becoming less flexible after a long race, a crucial step as they need to bend to pass through tiny vessels and transport oxygen, nutrients and waste throughout the body.

Runners’ red blood cells showed both molecular and physical damage, from the pressure of blood surging through the body during exercise, inflammation and oxidative stress, or low levels of antioxidants.

Another study also found that extreme endurance efforts cause the brain to temporarily break down its own insulation to use as an emergency energy source.