Fluid intake during a marathon

Sometimes a figure is given along with this advice: at least 1 litre of fluid per hour should be ingested during a long distance race. However, ingesting such a large volume is unnecessary and may even be dangerous.

The common belief is that it is essential to avoid becoming dehydrated during a marathon, as this is a very dangerous condition. However, dehydration is seldom a problem. Nevertheless, runners force themselves to drink as much as possible. In the majority of runners this will result in an excess fluid intake and they will stop and urinate during the race.

Hyponatraemia

In a small percentage of runners, however, the fluid will accumulate and a condition called hyponatraemia will develop. This is when the concentration of sodium in the body becomes very low due to the extra fluid and is therefore also known as water intoxication.

Slower runners, and runners who stop and walk frequently during the race, are the most likely to develop hyponatraemia because their sweat rates are fairly low and they have ample time to ingest sufficiently large volumes of fluid to potentially cause the development of the condition. Ironically, hyponatraemia is more dangerous than dehydration.

Handy Tips

There are various ways to combine both fluid ingestion and carbohydrate ingestion.

• One is to ingest approximately 600 ml per hour of undiluted “Coke”, which is a 10% carbohydrate drink or “Powerade” during the race, therefore supplying about 60g of carbohydrate per hour. These drinks will be supplied at the seconding tables.
• A second regimen to use if you do not tolerate undiluted Coke or Powerade very well, is to ingest a mixture of Coke and water. Since this will not supply sufficient carbohydrate, supplementation with additional carbohydrate in the form of corn syrup, or equivalent, will be necessary.
• Thirdly, if only water is ingested, then a large amount of additional carbohydrate must be supplied from other sources to give a total carbohydrate ingestion of around 70g per hour. Typically, one sachet of corn syrup will provide approximately 20 g of carbohydrate.

Whichever fluid replacement regimen is followed, it is important to test it before the race.

By Exercise Physiologist, Dr Andrew Bosch - Head of Running Division of the Discovery Health High Performance Centre of the Sports Science Institute of South Africa.