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Why can't you stop going to the toilet after a couple of drinks?

If you've ever had a long night out at the pub you will have come across the strange phenomenon called 'breaking the seal'. Dr Karl explains the physiology of beer and bodily fluids.

If you've ever had a long night out at the pub you will have come across the strange phenomenon called 'breaking the seal'. Yes, indeed, we're talking bodily fluids.

Your bladder does hold up well until around the third drink and then you just have to go. In other words: break the magical seal and for the rest of the night you have to keep popping back to visit your new best friend the toilet.

Of course there is no special, magical seal keeping your urine inside your bladder apart from your normal urinary sphincter. But alcohol interferes with the ability of your body to maintain proper hydration. In other words, for every 200 millilitres of beer you drink, your kidneys generate 320 millilitres of urine.

First, a little physiology. As far as water is concerned, your body is a dynamic powerhouse. In each 24-hour day about 50,000 litres of water crosses the many, many membranes in your body. And practically all of it — yes, 50 tonnes — crosses right back again.

Only a few litres of water actually leave your body as urine, water vapour from your mouth, sweat and so on. Your body has several mechanisms to help keep itself properly hydrated. Hydration can be measured with a probably unfamiliar unit called milliosmoles per kilogram of water (mOsm/kg water).

Your brain and kidneys work together on a minute-by-minute basis to keep your bodily fluids in a very narrow range between 280 and 296 milliosmoles per kilogram of water. The fine tuning mechanisms begin to kick in around 284.7 — long before you actually begin to feel thirsty. Thirst itself happens around 294 — still within the normal range. And dehydration begins around 302 milliosmoles per kilogram of water.

Dehydration is quite serious — you can die of it. So your body has several mechanisms to keep you well hydrated. One of these involves the hormone which confusingly has two common names: vasopressin or ADH — antidiuretic hormone.

Diuretic just means related to urination. So the main job of ADH is to stop you from urinating. Suppose you're lost in the desert and you run out of water. Your brain makes lots of ADH and so you virtually stop making urine — that's a good survival tactic.

But suppose you drink alcohol… The awkward side-effect of drinking alcohol is that your brain makes hardly any ADH and so your kidneys start making lots of urine.

The other factor related to breaking the seal is simply volume. If you're heading for a big night out you're probably not going to drink half a dozen big glasses of water, but you might easily drink half a dozen big glasses of beer.

So, let's compare your bladder to a dried out sponge. Start dripping water on to the sponge and for quite a while it will just soak it up and store it. But eventually the sponge will become totally saturated with water. Add just one more drop of water on top of the sponge and now a single drop of water will form on the bottom of the sponge and begin to form a continuous trickle.

Your bladder has the capacity of around 300 - 400 millilitres. You can throw down the first and second beers pretty quickly but remember, the alcohol massively reduces your production of ADH. So, for every 200 millilitres of beer you drink, your kidneys generate 320 millilitres of urine.

Of course, by the time you get up to a litre of beer you're getting a little bit dehydrated and your kidneys will cut back a little. But overall the volume of urine you generate is still greater than the volume of water contained in the beer, so you get slightly dehydrated.

Now, this is weird. Normally when you get dehydrated you produce very little urine. And the urine you do generate is very dark in colour. But, because you are voluntarily taking a powerful drug called alcohol, your protective systems get out of whack. So instead of generating less urine as you get dehydrated, you generate more.

But what if you drink lots of water to compensate for the extra volume of urine you generate? Well, your body will hang on to only about 30 - 50 per cent of the extra water. The rest just goes down the toilet bowl. You end up better off than not having drunk the extra water, but you'll still be dehydrated.

The moral is quite straightforward: you don't buy beer, you only rent it. And it's very unlikely you'll ever regret not drinking too much …

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