The nation's biggest marathon is coming up April 17, luring runners from all over the world to Boston to challenge their body and spirit. And it is quite a challenge as a new study supports what other investigations have shown: marathon runners risk serious health consequences with the exertion and loss of fluids that take place over the hours spent running the course.

Sherry Mansour, DO, from Yale University School of Medicine, led a team of researchers in the study. They evaluated 22 marathon runners with an average age of 44 and a distribution of nine men and 13 women. They found that 82 percent of the runners had an increase of biomarkers indicative of kidney tubular injury.

The study was undertaken to determine the impact of known changes of blood flow of as much as 25 percent diminishment with intense physical exertion. With exertion, the heart pumps to address the oxygen demand, and the blood flow is biased toward vital organs or skeletal muscles with diminished flow to other tissue, such as the kidneys.

"The kidney responds to the physical stress of marathon running as if it's injured, in a way that's similar to what happens in hospitalized patients when the kidney is affected by medical and surgical complications," said one of the contributing authors, Dr. Chirag Parikh. "We need to investigate this further. Research has shown there are also changes in heart function associated with marathon running. Our study adds to the story — even the kidney responds to marathon-related stress."

There have been previous reports of kidney biomarkers of stress with marathon runners.

Kyung-A Shin, Ph.D., from Chungnam National University in South Korea, and colleagues evaluated 15 athletes matched for physical and demographic attributes and evaluated functions after each participant had run a marathon, a 100-kilometer race and a 308-kilometer race. They found that the biomarkers of kidney stress were elevated after a marathon, but not the other race events.

Acute kidney failure during a marathon is rare. When kidney failure occurs, it is usually because of a combination of factors such as heat stress, dehydration, latent myopathy, nonsteroidal anti-inflammatory or other drug/analgesic use, and viral/bacterial infection.

More study is warranted to determine whether the acute, but not permanent, impact of the stress related to marathon running is harmful and whether the repetitive injury to kidneys with participation in multiple marathon events may result in permanent harm.

Maintaining optimum hydration before and during a running event is important to prevent any kidney harm. How much fluids are needed depends on the body size, the race pace, heat, humidity and sweat rate.

Marathon races have fluids at aid stations throughout the course, and it can be advisable to take in fluids at each station. The temptation to skip the few seconds it takes at each station to hydrate in order to achieve a better overall course time may be costly to kidney health.