TV adventurer Bear Grylls has built a global reputation through his often unconventional and sometimes extreme survival feats to stay hydrated.

He has squeezed moisture from elephant dung, sipped the contents of camel intestines, downed yak eyeball juice and, perhaps most famously, drank his own urine.

If you’ve seen Grylls gulp down a mouthful of his own urine on camera, you might conclude it’s a legitimate survival hack. After all, Grylls used to be in the SAS.

In one episode, he tells viewers urinating on the ground would be wasting fluids, drinking your own urine is “safe”, and grimaces while taking a warm, salty mouthful.

Let’s see if this is fact or fiction. https://www.youtube.com/embed/4U_xmfSwYSw?wmode=transparent&start=0 Was Bear Grylls right? Can you really rehydrate by drinking your own pee?

Your urine is like a bin

Fluids make up about 60% of your body’s total weight. To maintain the correct balance of substances in this internal environment, your kidneys will continuously filter about 180 litres of blood fluid (plasma) every day.

Thankfully, we don’t pee out 180L of urine, because our kidneys “throw back” or reabsorb about 99% of what they filter back into the bloodstream.

The best way to imagine this process is by picturing a messy garage. If you tried to pick through the chaos and remove only the unwanted items, you’d be there all day. A more efficient method is to empty everything onto the driveway, keep what matters, toss the rest. Your kidneys use the same strategy.

They ignore the large cells and proteins, and filter the plasma portion of blood, which essentially empties the entire garage. They then selectively return the useful substances back to the bloodstream. What’s left behind becomes urine, the physiological bin.

Its final contents depend on a few factors, including your hydration status, metabolic activity and recent diet (including medications and supplements).

Typically, urine is about 95% water. The rest is:

• urea (about 2%, a byproduct of breaking down protein, which we’ll come back to shortly)
• creatinine (about 0.1%, a by-product of muscle metabolism)
• salts and proteins.

So does urine hydrate you? Is it safe?

The answer … yes and no. The answer isn’t always clear-cut because, as we saw above, what’s in your urine depends on what was in the garage.

If you are well hydrated and healthy, your urine will likely appear clear to straw-coloured, meaning it is mostly water (but will still contain urea, salts and other waste products). A drink of this “first pass” urine will indeed provide you with some degree of hydration.

But in a Grylls-type survival setting, you’d be losing water from your body via other means. For instance you’d lose about 450 millilitres a day via skin sweating and about 300mL a day via water vapour in your breath. If you were in a hot, humid environment, these volumes would increase significantly.

As a result, your kidneys would need to work harder to hold onto precious water and keep it in your blood. This will further concentrate the waste products, and what ends up in the bin will be pretty toxic to your body.

So by drinking urine in a survival setting, you’d be consuming higher concentrations of waste products, including urea, that your body explicitly intended to remove.

By drinking urine with higher concentrations of waste products (and/or if your kidneys are impaired), urea and other metabolic waste products can accumulate in your body. This can become toxic to cells, particularly those in the nervous system.

This can lead to symptoms such as vomiting, muscle cramps, itching and changes in consciousness. Without treatment, this toxic state (known as uraemia) can be life-threatening.

Is your urine sterile?

Toxins aren’t the only issue.

While urine leaving the kidneys is likely sterile, the rest of the urinary tract (bladder and urethra) isn’t. Our bodies are full of resident bacteria that maintain our health and support daily functions – when they stay in their usual place.

So when urine passes through the bladder and urethra, it can collect these bacteria. If you drink that urine, you are re-introducing those bacteria into parts of the body where they don’t belong – mainly the gastrointestinal tract.

In healthy conditions, stomach acid often kills many of these bacteria. But in a survival situation where dehydration, heat stress or poor nutrition can compromise the gut lining, the risk of those bacteria crossing into the bloodstream increases. This can set the stage for life-threatening infections.

That’s the last thing you need while lost in the bush.

In a nutshell

Please don’t rely on drinking your own urine if you’re lost in the bush. It’s basically the equivalent of drinking from the bin.

Matthew Barton, Senior Lecturer, School of Nursing and Midwifery, Griffith University and Michael Todorovic, Associate Professor of Medicine, Bond University

This article is republished from The Conversation under a Creative Commons license. Read the original article.