Q+A: I drank plenty. Why this marathon cramp?

Q I was disappointed to get cramp at the 15-mile mark during a recent marathon. In the 48 hours before the race, I consumed plenty of carbohydrates and drank approximately four to five litres of water. During the race I drank plenty of water and carbohydrate drink at each of the aid stations. I’m mystified by this cramp. Did I do something wrong?

A First of all, drinking four to five litres of fluid in the 48 hours before a marathon is about right. A person generally needs a millilitre of fluid for every calorie they burn. As the two days prior to the race would have seen light (if any) activity, your output was most likely 4200-5000 calories.

But as many foods contain liquid, you don’t need to get all of your fluid from water alone, as you did. It would actually have been better to have consumed sports drinks, thus taking in electrolytes to ensure the muscle cells were hydrated and balanced with salts and minerals. What I’m getting at is that you can get cramp as a result of very slight fluctuations in your electrolyte balance. Because you drank water and sports drinks copiously at each aid station, you may have actually diluted your electrolytes by drinking too much water and not enough electrolytes. This would render the solutions ineffective at holding water in your body.

You don’t mention your urine output during the race, but I suspect this was quite high. Most runners can only absorb 600-900ml of liquid per hour at most. You could have easily exceeded this if you were drinking ‘scared’, and not keeping track of your intake. Next time, drink only fluid-replacement drinks (about 600-750ml per hour) and not water. And practise drinking these amounts in your pre-race training runs.

Joe Beer, sports scientist and level-two triathlon coach