Your stomach and gut works a little like a sieve. If you pour water into it, the water passes straight through without any problems. If you pour a milkshake into the same sieve, it will pass through, but will take a little more time and will slowly drip. If you throw solid food into a sieve, it stays exactly where it is. The only way to pass solid food through a sieve would be to mix it up with water and make a thin enough solution, which could then start to drip through. The solution which enters your stomach, is therefore very important in terms of performance. During endurance events, we eat and drink to get energy, but if the food sits in your stomach, then you aren’t actually getting any energy into your bloodstream. Not only are you receiving less energy, you are also likely to get some kind of stomach problems, because you’re clogged up!!
Isotonic is just the tonic
Isotonic refers to a solution which is a similar concentration to fluids in the body. Solutions of 7% are generally referred to as isotonic, this means that 7g of carbohydrate in 100ml of water is isotonic. You can count grams and millilitres as the same thing, so the calculation is simple, 100ml / 7g = 7%. Drinks bottles generally come in 2 different sizes, 500ml and 750ml so based on the 100ml / 7g rule, the calculations would be as follows:
500ml water + 35g carbohydrate = Isotonic (35 / 500 = 0.07)
750ml water + 52.5g carbohydrate = Isotonic (52.5 / 750 = 0.07)
Some solutions are less concentrated than isotonic fluids. For example, water has no carbohydrate in it and no calories, this is classed as hypotonic (hypo = low / less than). Solutions which are more concentrated than isotonic fluids, are classed as hypertonic (hyper = high / more than). An example of a hypertonic solution would be a smoothie.
That’s fine for drinks but what about solid food?
Many athletes choose to eat solid food during their event. As stated above, anything which is above 7% solution is hypertonic. Therefore, all energy bars and solid food is hypertonic. This means that if you wish to absorb solid food effectively, you must add sufficient water to make a 7% solution. For example, a standard energy bar is approximately 50-60g in total weight. We said earlier that 7g in 100ml of fluid would be a 7% solution, so that means you would have to drink 7-800ml of water with each energy bar to make at isotonic solution as 56g is 7% of 800ml (56 / 800 = 0.07).
In ultra running events, there’s often solid food such as sandwiches at feed stations, so get into the habit of estimating the portion size, e.g. what does 60g of cheese sandwich look like! Eating sandwiches, pasta and cake can very quickly result in a large mass of food gathering in your stomach. As for gels, they work the same way. A single gel contains 20-30g of carbohydrate (you need to read the packet). A gel with 21g would require 300ml to make a 7% solution.
Why is solution an issue?
Taking energy bars, gels and other solid food provides energy, but you have to take a lot of fluid to create an isotonic solution in your stomach. If you fail to take sufficient fluid you will have a thick ‘hypertonic’ solution in your stomach which may not digest and may well lead to stomach problems. Drinking enough fluid is critical for both energy supply and avoiding stomach upsets and bloating. All of the above calculations are based on 7% solutions, but in our experience, you can probably go to 10% without major issue (50g of carbohydrate for every 500ml of water).
What about the 60g per hour rule
The original rule for sports science was always based on taking no more than 60g per hour of carbohydrate, as the body cannot absorb any more than this. Eating too much may lead to food gathering in the stomach and leading to feelings of bloating or sickness. The carbohydrate ‘maltodextrin’ seems particularly prone to doing this and all carbohydrate drinks and gels tend to consist of maltodextrin (pretty much every energy drink on the market is the same, it’s flavoured maltodextrin). Recently, athletes have used more advanced products such as Maurten energy to take much higher amounts during events. We should also not underestimate the benefits of ‘stomach training. In most cases, athletes use nowhere near 60g per hour in training (they often use nothing at all), then they are surprised when they suffer gastric issues, taking 60g per hour or more on event day.
In summary, make sure you drink enough fluid to create a 10% solution or less. Try the more advanced products if you do struggle with gastric issues and always practice multiple times in advance.
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Regards
The Endurance Coach