Study: is the Boston Marathon harder on your kidneys?

Marathon running is hard on your body. Case in point: according to the Yale School of Medicine, 82 per cent of runners at the 2015 Eversource Hartford Marathon showed acute kidney injury (AKI) (don’t worry — this can easily be reversed with water and electrolytes). Recently, a group of researchers studied participants at the Boston Marathon and found that the coveted race might incite more kidney damage than other marathons.

Boston Marathon study finds younger, faster runners are at greater risk for heatstroke

Why are marathons hard on your kidneys?

An acute kidney injury is a reversible condition in which your kidneys cannot filter waste from your blood as efficiently as they normally do. When this happens, your kidneys aren’t able to properly balance fluids and electrolytes, including sodium, calcium and potassium. An AKI can happen for a number of reasons, and some runners may be surprised to find out that marathon running is one of them.

Researchers are still working to understand exactly why marathon running is hard on your kidneys, but the main theory is due to sweat. When you run a marathon, your core body temperature rises and you sweat to cool yourself down, losing both water and salt in the process. This appears to trigger both a hormonal and inflammatory response that injures the kidneys.

This theory makes sense, considering that marathoners who end up with an AKI at the end of their race tend to sweat far more than those who don’t experience kidney damage. The main indicator of an AKI is high levels of creatinine in the urine, and the issue can usually be resolved by taking in extra fluids.

The Boston Marathon

The researchers of the Boston Marathon study, published in the journal Frontiers in Physiology, wanted to investigate the effects of running the Boston Marathon on acute kidney injury biomarkers. The Boston Marathon is different from other marathons for a number of reasons. First, you have to qualify for it, which means likely those who are racing are more competitive and will be pushing themselves harder. The course is also very challenging with lots of downhill, which is tougher on your body. Finally, given the time of year, the race is often warmer than many other marathons, so participants will be sweating more.

“The combination of more competitive athletes, faster finish times, warm and humid weather and downhill running may increase the amount of muscle damage while limiting filtration rate, potentially increasing kidney stress,” the researcher hypothesized.

65 participants were recruited for the study. They gave urine and blood samples before the race, immediately after and 24 hours later, and the researchers analyzed several biomarkers for kidney damage, including urine creatinine levels.

The results indicated that the Boston Marathon did appear to cause greater levels of kidney damage when compared to data from other marathons. Importantly, biomarkers for AKIs were elevated after the marathon, but persisted 24 hours later, indicating that participants weren’t rehydrating properly. Female participants also appeared to experience more renal stress than their male counterparts, but the researchers were unable to pinpoint a reason. One possible reason is female marathoners have a harder time hydrating during the race, possibly due to stomach issues.

“Other studies on marathon runners have demonstrated apparent kidney stress post-marathon but that biomarkers are reduced to near baseline levels 24 hours after marathon completion,” the researchers conclude. “Our results differed, suggesting the characteristics unique to the Boston Marathon – increased heat strain, a negative elevation profile in the first 10 miles, and a more elite field of runners resulting in faster finish times – attributed to unresolved renal stress at 24-h post-marathon.”

What happens to your body after a marathon?

More research is needed to understand the sex differences in hydration and kidney stress, but in the meantime, this research highlights the importance of proper hydration before, during and particularly after marathon running.