Why plain water is not enough

Giving a child with diarrhoea plenty of water is the obvious thing to do, and it is not sufficient. The reason is a genuinely elegant piece of biology.

A child has had loose motions all day. Somebody sensibly keeps her drinking water, glass after glass. By evening she is weak, dizzy, and her mouth is still dry.

Nothing careless has happened here. The obvious replacement was made, and it was not enough.

What is actually being lost

Diarrhoea drains salts as well as water. And the body can only absorb and hold on to water when the salt balance is right.

Give plain water into that situation and a good deal of it can pass straight through while the dehydration carries on. The glass is going in. It is not staying in.

The part that surprises people

An ORS drink is water, salt and sugar. Most people assume the sugar is there for energy, or to make it drinkable.

It is neither. Glucose and sodium are carried into the gut wall together, using the same transport mechanism. The sugar is the ferry that brings the salt across. Without it, much less of the salt is absorbed, and without the salt, the water does not stay.

So all three ingredients are doing a job, and the one that looks decorative is the one making the whole thing work.

Getting it right

Packets cost a few rupees and every chemist has them. Use one, mixed exactly as the packet says, in the stated volume of clean water. The proportions matter, so this is not a recipe to eyeball.

See a doctor for a young child, for blood in the stool, for persistent vomiting, for signs of severe dehydration, or if it simply is not improving.

The answer

Show the answer

In the puzzle, what is missing is: salt and sugar in the water, making it an ORS drink.

The distractors are the tempting ones: more plain water, milk, a fizzy drink for energy. Each is what somebody in a real house would reach for.

Talking about it afterwards

• "If she is drinking so much water, where is it going?"
• "Why would sugar help salt get in? What is it doing?"
• "When is the obvious answer not quite the whole answer?"

The reasoning habit here is a good one: replacing the obviously missing thing is not always enough. Sometimes you have to find what makes the replacement actually work.

Source

• WHO — Diarrhoeal disease

General information for parents, not medical advice. For anything specific to your child, ask your doctor.

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