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When it comes to endurance sports, carbohydrate or sports drinks are essential for maintaining intensive performance over a long period of time. Here are the best tips on how to train your gastrointestinal tract for optimal intake.

First, some theory: carbohydrates are absorbed in the intestine via special carbohydrate transporters. To do this, two- or multi-chain carbohydrates must first be broken down by enzymes before being absorbed by the body as individual particles such as glucose, fructose or galactose. A distinction is made between two different transporters: 

• One that transports glucose and galactose… 
• …and another that transports fructose

The absorption capacity of the glucose transporters is limited to around 66 grammes per hour of exercise. If you want to consume more than 60 grammes of carbohydrates per hour during intense exercise or in competition, you must also add fructose. 

Simply imagine it like two tunnel systems: if you add more than 66 grammes of glucose per hour, a traffic jam will occur in front of one of the tunnels. To be able to transport a larger quantity, you have to use the "fructose tunnel" and thus "avoid" the traffic jam. 

Predominantly "passive” fluid intake 

Fluid, i.e. water, is not actively transported into the cell interior, but diffuses "passively" through the cell wall. The osmotic pressure or osmolality, i.e. the number of active particles in the fluid, is decisive. If this is below the osmolality of the body fluid (e.g. blood), a sports drink is called hypotonic. If the osmolality is identical to that of the blood, it is called an isotonic drink.

The fluid, that is, the water, can be absorbed by the body more easily if a drink is hypotonic. However, it is relatively difficult to create a sports drink with a high amount of carbohydrates and low osmolality. For this reason, many sports drinks are 6% carbohydrate solutions, so they contain 60 grammes of carbohydrates per litre of fluid. This ensures a hypotonic state. When the amount of carbohydrates is increased, it becomes far more difficult to keep the drink hypotonic or isotonic because the number of active particles in the water is greater. This means a higher osmolality and the drink becomes increasingly isotonic or even hypertonic.

However, sports drinks that have an 8% carbohydrate solution and still have a low osmolality are now on the market. In order to avoid increasing the number of active particles, long-chain and branched carbohydrates or starch are increasingly used. The omission of additional substances (e.g. vitamins, flavourings, etc.) can also help to keep the osmolality low despite a high carbohydrate content. From practical experience, such drinks are very well tolerated, but you should test which drink is best for you personally. 

Practice intake during training

When it comes to consuming large amounts of carbohydrates during exercise – for example, 90 grammes per hour – it is not only the drink that is important, but also the training and carbohydrate intake. If you never add carbohydrates during training or only small amounts at a time, don’t be surprised if problems arise when you consume larger quantities during a competition. 

What’s the best approach? Firstly, you can train the gastric emptying rate. This works primarily through the volume being supplied. The emptying rate can be increased within a few days. This means that more volume is transferred from the stomach to the intestine more quickly. This can also reduce existing stomach problems – especially when trained. In concrete terms, this means gradually increasing the volume of fluid – i.e. the amount of drink – during training. This can be crucial, especially at hot temperatures!

Another point concerns the trainability of carbohydrate intake in the intestine. The absorption capacity can be improved by increasing the number and activity of the carbohydrate channels over a long period of time. However, larger amounts of carbohydrates must also be consumed during training. This should be at least the same amount (or even a bit more) as you'll eventually consume in the competition. The intensity of the workout is also an important piece of the puzzle. Essentially, the greater the intensity, the more difficult it is to absorb and digest the amounts consumed. It's therefore worth practising competition food consumption even at an increased intensity. 

Conclusion

If you rarely or never use carbohydrates in training and want to improve carbohydrate intake in competition, it’s advisable to practice this during training. You should start with small quantities and gradually increase the amount until you reach your target. As a result, you'll minimise or even prevent gastrointestinal problems during intensive exercise. The right choice of sports drink, the correct mixing ratio as well as personal compatibility are crucial to success. 

Joëlle Flück (born 1986) is a sports and movement scientist, nutrition expert and president of the Swiss Sports Nutrition Society (SSNS), the professional society for sports nutrition in Switzerland. The enthusiastic runner has a marathon best time of 2:44:07, run at the 2024 Zurich Marathon.