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Why does drinking alcohol cause dehydration?

Our bodies are amazing organisms, especially when it comes to processing alcohol. Dr Karl explains why, if you're drinking booze, what goes in is not equal to what comes out.

We humans have been making and drinking alcohol for thousands of years.

It's a strange liquid. We can use it as a fuel or germ-killer, use it to preserve human heads or other body parts in jars for years on end, or to strip oil stains from the garage floor.

And yet, in small quantities we use alcohol as a social lubricant.

But over time, too much alcohol can set off diabetes and malnutrition, and diseases of the central nervous system and the liver.

A short-term side-effect is excessive urination. In Shakespeare's play, Macbeth, the porter says that alcohol promotes "nosepainting, sleep and urine".

But even today we still don't fully understand how alcohol causes this excessive urination.

After all, beer is about 95 per cent water and only five per cent alcohol. And the liver converts that five per cent of alcohol into roughly the same mass of water and some carbon dioxide.

So if you drink 200 millilitres of beer, the end result is 200 millilitres of water. But you don't urinate just 200 millilitres of urine. No! You urinate a total of about 320 millilitres of urine.

So in general, each shot of alcohol makes you urinate an extra 120 millilitres of urine on top of your normal urine output.

Where does that extra 120 millilitres come from?

To understand what's going on, you need a bit of background knowledge.

First, the body pays special attention to alcohol. It's a small molecule and gets very quickly through the walls of the gut into the bloodstream and then to the brain.

Second, if you weigh 60 kilograms, you generate about 60 millilitres of urine each hour. And for 80 kilograms, about 80 millilitres per hour, and so on.

Third, we humans seem to prefer to drink our alcohol in 10 gram lumps. Ten grams of alcohol is about 12.5 millilitres (but you can call it 10 mL and still be fairly accurate). So each glass of beer, wine, or spirits has about 10 grams of alcohol.

Fourth, alcohol interferes with the mechanism that regulates the water levels in our body.

So now, a little anatomy and physiology. In your brain is a small gland called the pituitary gland. It is divided into two sections: the front; and the back.

The back section is called the posterior pituitary. One of the hormones made by the posterior pituitary gland is called vasopressin, or anti-diuretic hormone (ADH). Diuresis is a fancy word meaning urination.

Now suppose that you are really dehydrated. So the volume of water in your body is low. But you still have just as many salts floating in this reduced volume of water.

So these salts are now more concentrated in the reduced volume of water that you have when you are dehydrated.

Your body has detectors that can sense both the saltiness of your water, and the volume of the water. If these detectors reckon that you are dehydrated, they send a signal to the posterior pituitary gland, which starts pumping out ADH. The job of ADH is to stop you urinating, so you hang on to your precious water. You reduce your normal rate of making urine.

Alcohol does the opposite. It reduces how much ADH you make, so it increases how much urine you produce. Each shot of alcohol that you drink forces your kidneys to generate an extra 120 millilitres of urine on top of the normal 60–80 millilitres per hour.

"Aha!", you cleverly think to yourself. "Why don't I just drink lots of water to compensate for the extra 120 millilitres?"

Unfortunately, it's not that simple. You'll hang on to only about half or a third of the extra water you drink. Most of it will go out in your urine, and you'll still end up dehydrated at the end of a night of drinking.

Mind you, you'll be a bit better off than if you didn't drink any extra water at all, but you'll still be dehydrated.

Could this extra urination caused by alcohol consumption be the origin of that old Aussie expression, 'a night on the piss'?

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