An overweight diabetic patient, whom we advised to do moderate to high-intensity exercises at least four times a week to lose weight, sent me an urgent e-mail last week concerning an article he read in this paper, stating that intense physical activity like marathon running may cause kidney injury.

The article was based on a study done by researchers at Yale University.

Being diabetic for more than 10 years, our patient already has a mild kidney problem (diabetic nephropathy) and he was worried it might be aggravated by the moderate to high-intensity exercise regimen we prescribed for him.

I immediately tried to get a complete text of the study, which was published in the March 28 issue of the American Journal of Kidney Disease. It was conducted by a group of researchers headed by Dr. Chirag Parikh at the Department of Medicine in Yale. The main author of the study is Sherry Mansour, a young doctor of osteopathic medicine.

They’re involved in the translational research program of the university—meaning, they engage in research that aims to “translate” findings of their research on relevant medical problems and public health issues into medical practice and meaningful health outcomes.

For this study, the group was concerned about any possible effects on the body of marathon running, since more than a million people worldwide are participating in this activity regularly. The subjects in the study are from the participants of the Hartford Marathon, which consists of a full 26.2-mile (42-km) marathon, and a 13.1-mile (21-km) half marathon.

Specifically, the researchers looked at kidney function by measuring blood levels of serum creatinine, which is supposed to reflect the waste products of the body’s metabolism, and other blood and urine indicators of kidney injury.

Proper context

Eight out of 10 marathoners showed indicators of kidney injury which were just transient, and cleared after 48 hours. This was described as marathon-induced acute kidney injury, and the findings raise the question: Does this also apply to other repeated strenuous activity over time, which was what our patient was worried about?

We have to put things in the proper context.

First, moderate- to high-intensity exercise, which we commonly recommend to our overweight patients and even to lean patients who want to maintain their cardiovascular fitness, can never approximate the degree of intense physical stress from a 42k or even a 21k marathon. It’s nowhere close to it.

In the study, the increase in serum creatinine only indicates that the body’s metabolism was put in high gear; hence, there was an increase in the level of waste products  which would take the kidneys a little more time to clear. It’s like after New Year’s Eve—you see more debris the following day, which may take a couple of days or longer to clean up, but after that, everything goes back to normal. Unless one participates in a 21k or 42k marathon daily, this should not be a problem.

Second, with proper preventive measures, especially adequate hydration—which should ideally replace not only the water lost, but also the electrolytes and minerals that are lost with the profuse sweating—this should not be a problem. With dehydration, which may happen even with ordinary exercise, the kidneys will have a difficult time cleaning up all the waste products in it.

Feeling thirsty is a compensatory mechanism to prevent dehydration. For as long as one feels thirsty, it means he or she is on the dehydrated side and more fluids should be taken. Ideally, when exercising, one should not wait to get thirsty.

A good advice is to drink about 15 to 20 ounces of water one to two hours before working out or exercising, and to drink another eight ounces 15 minutes before. During exercise, one should sip water every 15 minutes, especially if the weather makes one sweat buckets.

Lastly, although this is a well-designed study, one cannot draw any definitive conclusions from it. At most, one can consider it an initial pilot study which should be validated by a more extensive and more stringently designed research. There were only 22 subjects in the study, and the study duration was only two days.

Even the authors admit the study’s limitations and propose a more extensive study of longer duration, so we can really find out the long-term effects of intense activity not only on marathon runners, but in other athletes exposed to rigorous physical exertion.

Meanwhile, let’s all go back to the gym and not worry about the findings of this recent study.