There is no denying that exercise is good for the body.

Research has shown that it helps to stave off multiple illnesses including type 2 diabetes, cardiovascular disease, and some cancers.

Above and beyond physical activity’s ability to reduce disease risk, it also strengthens muscles, improves lung function, and has a range of psychological benefits.

However, as with most things in life, it may be possible to have too much of a good thing.

More vigorous activities can have negative health consequences.

As exercise goes, running a marathon is a particularly intense pastime – running 26.2 miles is not something to be taken lightly.

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Are marathons bad for you?

The number of people participating in marathons has risen rapidly over recent years.

In fact, over the past 35 years, the number of people in the United States participating in marathons has increased 20-fold.

Because of this surge in interest, research into the benefits and potential dangers has also seen a sharp rise. Previous studies looking at marathon running and its impact on health have uncovered a number of potential health risks associated with the heart.

For instance, a study published in 2012 concluded that endurance training, including marathons, caused dysfunction in the right ventricle of the heart. Similarly, marathon- runners have been shown to have a fivefold increased risk of atrial fibrillation.

Some researchers theorize that running marathons might also damage the kidneys.

Earlier work has shown that particularly vigorous activities – including harvesting sugar cane, mining work, and military training – can damage the kidneys.

However, until now, marathon running’s impact on kidney health has not been studied.

A recent study set out to specifically examine the impact of running marathons on kidney health. The findings were published today in the American Journal of Kidney Diseases.

Dr. Chirag Parikh, who is a professor of medicine, led a team of researchers from Yale University. Participants were all runners from the 2015 Hartford Marathon, which takes place in Connecticut.

Before and after the race, the scientists took blood and urine samples, which they analyzed for markers of kidney injury.

Specifically, they checked for protein in the urine and creatinine levels in blood serum - a marker of reduced kidney function. They also examined kidney cells under a microscope.

In all, 82 percent of the runners in the study showed stage 1 acute kidney injury.

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The dangers of kidney injury

Acute kidney injury, as described by the National Kidney Foundation, is a “sudden episode of kidney failure or kidney damage that happens within a few hours or a few days.”

It causes waste products to build up in the blood, making it hard for kidneys to maintain the correct balance of fluids in the body.

Parikh explains the results: “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.”

Why does marathon running damage the kidneys in this way?

The researchers believe that there could be a number of factors involved, including dehydration, a rise in core temperature, and a drop in blood flow to the kidneys.

Although the drop in kidney function was significant, they also found that within two days, the kidney injury had resolved. However, more work needs to be done to understand whether this type of strenuous activity could cause long-term damage over time.

In conclusion, Parikh said: “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."

It is important to note that exercise, in general, is a healthy life choice. However, evidence is mounting that excessive, particularly strenuous activity should be undertaken with caution.

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