Why drinking more water can leave you drier

The large intestine absorbs salt and around 80% of the liquid still present in waste before passing a formed stool. If you have an ileostomy or an internal pouch, the large intestine is no longer connected or has been removed, and cannot do that. The small intestine does absorb liquid, but not in the same way.

This is where the usual advice stops working. Drinking lots of water alone may, on occasions, upset the balance of minerals and increase the risk of dehydration.

The mechanism

Water and salt move through the wall of the intestine by osmosis and diffusion — from where they are more concentrated to where they are less concentrated, to even out the difference. Put simply: if you drink plain water, which has little or no salt or sugar in it, sodium moves out of the body’s cells, through the bowel wall, into the bowel. That evens out the two sides — and then the salt is lost when it is excreted.

When to think about it

The situations where the balance is most likely to tip: when output is very watery or loose, in hot weather, during physical exercise, if you are vomiting, or if you already feel dehydrated and extra thirsty.

And a reassuring counterweight: if your output is a porridge-type consistency and you are not feeling thirsty, you are probably drinking adequately.

Glucose matters here too. Sugar is required to help the body absorb salt — without it, the body cannot move sodium from the bowel into its cells, and the water will not be absorbed as quickly. That is why rehydration drinks are sugar-salt solutions rather than salty water.

What the colon used to do

It helps to know which job went missing. The whole route: after swallowing, food and drink travel down the oesophagus into the stomach, where strong digestive enzymes break down what you have eaten before moving it into the small intestine. The small intestine digests and absorbs the nutrients. What remains moves into the large intestine — and that is the stage that changed.

Water is not a minor character in this. On average around two-thirds of your body is water, and it is busy: helping digestion, flushing waste products out, and keeping skin and joints healthy. If the water content falls, it upsets the balance of minerals — sugar and salt — that the body needs to function.