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by Elaine K Howley

August 11, 2021

Temperature, pressure, and immersion diuresis are all reasons why you might need to pee when you swim

Open water swimming holds certain advantages over pool swimming in a number of areas, perhaps the most convenient of these being not having to exit the water to answer the call of nature. (You really shouldn’t in pools.) Indeed, for most open water swimmers, taking a leak is part and parcel of every swim—sometimes multiple times—and we invest a good bit of time and energy into perfecting the ability to “go” on the go.

But if you’ve noticed that you feel the urge to pee more often when swimming in open water, or swimming in general, it might not be a figment of your imagination. In fact, there’s a term for this condition: immersion diuresis.

An Increased Need to Pee

On average, healthy adults typically pee four to six times a day, and the output can range widely from three cups to three quarts of urine depending on a variety of factors.

But diuresis arises when the kidneys filter more bodily fluid than they should, resulting in increased urine production and more frequent trips to the bathroom. This increased need to pee can result from a number of conditions and situations.

For example, if you drink a lot of water or another liquid, you’ll feel an increased need to urinate. This type of diuresis aims to keep your body’s water level in balance.

But for people with certain health conditions, no change in liquid intake is involved in generating that increased urine output. Perhaps the most well-known of these conditions is diabetes; an early sign of the disease is more frequent urination and a higher output of urine. (That urine often smells sweet or fruity because the body is trying to dump excess blood glucose via the urine.)

Certain medications can also increase the need to pee. Diuretics, for example, are called water pills because they help the body expel excess fluid and sodium. They’re used to treat conditions including heart failure, high blood pressure, and chronic kidney disease. They can very effectively reduce swelling and restore the body’s fluid balance.

Tea, coffee, and other caffeinated drinks can also have a diuretic effect, as can very salty foods.

Immersion Diuresis

Although all of these things—and a bunch of other medical conditions, medications, and other factors—could be contributing to an increased urge to pee, for open water swimmers, often the most likely explanation is the water itself. In fact, two aspects of that water may be at work when you feel like you always need to pee in the sea: temperature and pressure.

Water Temperature

Water that’s significantly colder than body temperature can trigger immersion diuresis. That’s because when you’re immersed in cold water, the body constricts blood vessels—particularly the tiny capillaries and other small vessels in your hands and feet, and shunts the warm blood there into the core to keep your internal organs warm and humming along.

But all that blood creates a bigger volume of fluid in the core of your body, which raises your blood pressure. This signals the kidneys that the body’s fluid balance is out of whack, and they step up and start filtering out more fluid. That can create a powerful need to pee.

Water Pressure

If you’ve ever gone scuba diving—or even just tried to get to the bottom of the diving well to retrieve a lost pair of goggles—then you know well the weight of a volume of water.

Being immersed in water creates hydrostatic pressure on your entire body, and your tender inner ear is often the first body part to remind you that that even just a few feet of water can create a substantial increase in pressure.

As you submerge in water, hydrostatic pressure drives up your blood pressure a bit, enough to trigger your kidneys to respond by stepping up their filtration game and increase urine output.

A raft of studies conducted in the 1960s and ’70s indicated that you need to be fully immersed in the water for the water pressure to lead to urination. In other words, that childhood slumber party prank of putting a sleeping person’s hand in a bowl of warm water to make that person wet their bed is not grounded in any scientific fact.

But for swimmers, full body immersion really can increase the amount of water, salts, and urea (a waste product found in urine) that your body releases.

When you combine cold temps and this hydrostatic pressure, that’s a recipe for making noticeably more pee.

What to Do About Immersion Diuresis

For most open water swimmers, the short-term increase in urination caused by swimming-related immersion diuresis does not pose a significant health issue. Just be sure to drink plenty of fluids afterward to restore the water you’ve used while exercising and to replace whatever other fluids may have been left behind as warm spots in the lake.

You may also want to consider adding an electrolyte drink or tablet to your regimen, as you’re excreting salt and electrolytes every time you pee. Those compounds need to be replaced to help restore balance to your fluid management system.

You may notice that you feel thirstier after an open water swim than you do after a pool swim, and that could in part be related to immersion diuresis. Listen to your thirst, and aim to stay well-hydrated before, during, and after all workouts.

If you’re bothered by an increased need to urinate and there’s no clear explanation, be sure to talk to your health care provider to rule out an underlying health condition. And if you have heart or kidney failure, diabetes, another health condition, or are taking a medication that can cause diuresis, check in with your doctor before you start any new fitness protocol, including adding open water swimming to the mix.


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