The Third Kidney

Here I am back from the semiannual vacation with my husband, brother, and sister-in-law. It was sad to realize this was our last cruise, but some of our bodies just can’t handle that anymore. It looks like mine may be one of them since I’m in bed feeling not so great. How was I ever going to be able to write a blog for Monday, I wondered.

And then I remembered that I’d met someone with an idea so old that it’s new again and he’d promised a guest blog for this week.  And there it was, right in my mailbox. I’d met Raymond Keller, Jr. DO at the American Association of Kidney Patients I attended recently. He had an intriguing idea, one I thought should be shared with you.

Take it away, Raymond…

First and foremost, please do not consider any of the following as medical advice. Consult your doctor before making any changes to your medical treatment plan.

I’m not the first person to suggest the skin as a “Third Kidney,” but like many others I did independently conceive the idea. For the origin story you can read a recent interview done by the American Association of Kidney Patients. The premise of the Third Kidney is that skin, through the sweat glands, can excrete water, potassium, and urea in amounts that would be clinically useful to patients with chronic kidney disease especially those on dialysis. Before we get into the Third Kidney, let’s take a brief look into the history of dialysis itself.

Willem Johan Kolff is credited with being the inventor of dialysis. He pieced together things that could be found in a contemporary house to create the first dialyzer. The original dialysis membrane was a sausage casing. Crude, but effective. Belding Hibbard Scribner would come to create the “Scribner shunt” which allowed repeated use of the same vascular access. Once long term vascular access was obtained, long term hemodialysis became a reality.

Now let’s get down to the details about how sweating can help dialysis patients. While there are many potential compounds that can build in the body with renal failure, urea, water, and potassium are of particular importance. Let’s take a moment to explore the consequences of each and how sweat therapy can help.

Water is essential to life. So essential, we search for evidence of it on other planets to decide whether life could exist. To most dialysis patients water is a constant enemy. It is the reason they have to spend more than two hours on dialysis per day – to reach their dry weight. The evidence for keeping fluid off is part of the reason why people that do dialysis more than 3 days a week have better outcomes.

As anyone who lives between the Arctic and Antarctic Circle has likely experienced, sweating removes water from your body. Sweating is so interrelated with being human that almost every culture in human history has a tradition of inducing it. The Finns are perhaps the most well-known with their saunas. The Russians have banas, the Turks have hammams, and the Native Americans have sweat lodges. While everyone is different, it is not unreasonable to expect that a 45 minute sauna session could remove between 500-1000mL of fluid from the body. Higher losses are possible with training. To put that into context, a 4 hour dialysis session typically removes 2000mL and removing more than 400mL per hour can cause symptoms of hypotension. Sweating out fluid is a natural process, which is why it can reduce the ultrafiltration required.

In the table 1 below (adapted from it is very obvious how likely it is for people to develop symptoms from removing fluid from the blood stream rather than the skin. This is especially important when we consider that the skin is where most excess fluid is stored, which is why dialysis patients get puffy.

Now on to potassium. Even though it is a vital nutrient, it has a dark side. Potassium chloride is one of the typically used compounds in lethal injections because it causes the heart to stop beating. As it builds up in the blood of a patient with renal failure it can have the same effect. Similar to fluid overload, keeping potassium levels at an appropriate level are a major reason daily dialysis patients do better than thrice weekly patients. Fortunately, potassium is excreted in sweat at 2-3 times the level it is found in the blood stream. During a regular sauna session the clinically relevant amount of potassium, in upwards of 4.6 grams, can be removed from the body.

And urea? Urea is a controversial molecule is the dialysis community, yet a relatively simple molecule that our bodies use to detoxify ammonia and remove nitrogenous waste from our bodies. We used to think that it freely diffused across cell membranes, like water. But seminal work by my mentor Jeff Sands, MD showed that there are molecular transporters for urea. In the dialysis community, urea rebound is proof that urea is not freely diffusible.

There has been much debate about the toxicity of urea. Regardless of whether urea is toxic, and at what levels it is, blood urea nitrogen is one way we monitor the adequacy of dialysis. Urea is excreted in sweat at about 2-3 times its presence in serum. Understanding how sweat affects the blood urea nitrogen levels will be important in coordinating the combination of sweat therapies with dialysis.

How does all of this relate to SlowItDownCKD? There is value to researching whether sweat based therapies like sauna can be used to reduce the dependence on dialysis. Given the above facts it is useful to ask the question of whether sweat based therapies can reduce the number of days per week or number of hours per day of dialysis. There is also the potential for sweat based therapies to push off dialysis for patients with CKD. Third Kidney currently has IRB (institutional review board, also known as an independent ethics committee) approval to do safety trials with Harvard Medical School professors. After a safety trial, the next step would be a study in patients that have chronic kidney disease.

When it comes to sweat based therapies for CKD I’ll leave you with a few thoughts:

  1. No rational person would say that sweating vis-a-vis exercise is a bad idea for CKD patients.
  2. If fluid balance was better achieved by sweating hours, or even days of dialysis, might be avoided.
  3. If potassium is lost in sweat it would allow people to liberalize their potassium intake, opening up a culinary panoply.

If you are interested in learning more about how sweat based therapies may be beneficial in patients with chronic kidney disease and the research that Third Kidney is doing, you can visit us at

Wow! Just wow. This is – as we used to say in college decades ago – mind blowing. It’s so simple, yet so complex. With many thanks for this new/old information, I’ll say good bye for now.

Until next week,

Keep living your life!