An NYC Marathon runner died suddenly at 24. Her family says the cause was rhabdomyolysis, a rare risk even for fit athletes.

• A New York City Marathon runner died after collapsing at the finish line near Mile 24.
• Her family said the cause of death was rhabdomyolysis, a rare side effect of hard exercise.
• Rhabdo can afflict even experienced athletes and cause life-threatening symptoms if left untreated.

A 24-year-old woman who died after the New York City Marathon on Sunday was suffering from a rare side effect of exercise called rhabdomyolysis, her family said.

Rachel Apa, an experienced D1 field hockey athlete who spent months training for the race, collapsed near mile 24 in Central Park and was taken by ambulance to the hospital, her father told The New York Times.

The New York Road Runners, the group that organizes the marathon, and Apa's family did not immediately respond to a request for comment from Business Insider.

Rhabdomyolysis, known as rhabdo, is a risk of extensive strenuous exercise caused by muscle tissue breaking down and overloading the kidneys.

Without prompt medical treatment, it can be life-threatening. Here's what to know about rhabdo, including the symptoms to look out for and how to prevent it.

What is rhabdomyolysis and what causes it?

Rhabdomyolysis can occur after severe muscle injury or intense exertion. As damaged muscle tissue breaks down, it releases compounds into the bloodstream that the kidneys typically filter out.

In rare cases, those byproducts can overwhelm the kidneys and lead to kidney damage or acute kidney failure.

About 26,000 cases of rhabdo are reported in the US each year, research suggests.

Anyone can suffer from rhabdo, but factors like hot environments and dehydration can increase the risk, according to Cleveland Clinic.

While it's generally not common, cases of rhabdo are often caused by high-intensity exercise or long endurance sessions without rest. For instance, research has previously linked cases of rhabdo to marathons (particularly during high temperatures) and vigorous spin classes.

It's also increasingly a concern for the military, which is working to spot rhabdo in troops more quickly.

What are the symptoms of rhabdo?

Symptoms of rhabdo include fatigue, muscle cramps or unexpected pain, and dark-colored urine, which can show up hours or days after exercise, according to the CDC.

Rhabdo can be diagnosed with a blood test and treated with IV fluids to help flush out the waste products.

To prevent rhabdo, avoid pushing too hard during exercise, take breaks as needed, and drink plenty of water, especially in hot, humid weather.