Isotonic Drinks, Goldilocks’ Porridge and Tooth Rot

You can pay a lot of money for isotonic drink, but why drink it, and can you get the same effect much more cheaply?

After a run on a hot day, you obviously need to get fluid back on board.  Water is great, we’ve got by on water for millions of years, and haven’t suddenly evolved an inability to absorb water.  However, when you need to get lots of fluid on board, after lots of sweating, whether it’s playing on the beach, a long bike ride or long hot training run, we can improve on water.

There are two issues: ‘absorption’ and ‘retention’.  Plain water is absorbed reasonably well, but, as you’ve probably experienced, if you drink a lot of cold water quickly, you just end up with ‘soggy belly’ – water sloshing around in your stomach waiting to be absorbed.

Once water has been absorbed (best done by drinking room temperature water little and often), it dilutes the blood.  However, it’s not actually the blood which needs the water, but all the cells around your body.  Unfortunately, diluting the blood means that the kidneys decide to get rid of the excess fluid before the cells around the body get their turn.

So water is reasonably well absorbed, but not retained very well.

‘Isotonic’ drinks contain carbohydrate (usually sugar) as well as salt, and get around this in two ways.  Firstly, they improve on the absorption: Small amounts of carbohydrate actually help fluid get absorbed by the stomach and intestines.  This was known in ancient times in the east, where rice water was a traditional cure for diarrhoea.  However, it wasn’t until quite recently that we in the west discovered that dehydration resulting from diseases such as Cholera (and, topically, Ebola) are best treated with rehydration fluids containing sugar as well as salt.

So having sugar (or more complex carbohydrate), as well as a little salt, in your drink helps absorption.  And the salt also helps retention: Once the fluid is absorbed, the salt ensures that the kidneys don’t think of the blood as ‘dilute’, and the extra fluid is retained to be pumped around your body, ready for all those thirsty cells.

How much sugar and salt?  Well the quantity of sugar is key.  Like Goldilocks’ porridge, there is a ‘just right’ quantity of sugar for enhanced absorption: 5 to 8 grams of sugar in 100ml of fluid.  Too much, and the body thinks of the drink as ‘food’, and holds it in the stomach for digestion.  Too little, and there is no enhanced absorption.

Ordinary fruit juice and Coca Cola are too strong to be absorbed quickly, but if you dilute them half and half with water, they come down to the magic level.  Mmmm, warm flat diluted coke, just the thing for the thirsty marathon runner!

Isotonic drinks do have a bit of a reputation for rotting your teeth.  Traditionally, they used ‘fructose’, the sugar in fruit, a favourite of the bacteria in tooth plaque.  However, these are less common now.  Ok, so the replacement glucose and sucrose is not great, just like sweets are not great for your teeth.

So an easy recipe for cheap isotonic drink:  Dilute 1 part ‘High Juice’ with 6 parts water, and add a little salt at a rate of 1/2 teaspoon per litre of drink.  This is based on Tesco’s Blackcurrant High Juice (AVOID the ‘No Added Sugar’ version, you DO need the sugar).  Other versions of ‘High Juice’ may contain more sugar, and need to be diluted more; the Tesco’s version has 36.7g/100ml.

The after training drinks in the club house, made up using our calibrated jugs, give you a drink at just this strength (unless your name is Jullian), but we don’t go as far as adding the salt.  Something for the future?

More sophiticated versions use a variety of salts to match what you sweat out (including potassium, magnesium and zinc), but unless you’re really in for the long haul this simple recipe should be good enough.