sports nutrition, triathlon training

People taking part in prolonged endurance exercise, such as triathlon training, don’t need supplements to maintain normal levels of  nutrition (ie. sodium in the blood) and prevent the life-threatening sodium deficiency condition known as hyponatraemia. That’s the reassuring conclusion of a study of 413 triathletes who completed the 2001 Cape Town Ironman triathlon in South Africa.

This study set out to test the prevailing wisdom that athletes need to ingest 20-40mmol per litre of sodium during exercise to preserve their normal blood sodium concentration and prevent hyponatraemia, particularly during ultradistance events, when total losses of sodium in sweat might be as high as 400-650mmol.

A total of 145 triathletes were randomly assigned to either an experimental or control group for the event, which comprised a 3.8k swim, 180k cycle ride and 42.2k run. Subjects in each group were given 40 tablets to take ad lib during the race, with a suggested range of one to four per hour. The tablets given to those in the experimental group contained 620mg of table salt while those given to the controls were filled with starch (placebo). Food and fluid intake – water or sports drink – were also allowed on an ad lib basis.

After the race, the researchers compared the blood sodium levels and various other parameters of these two groups and of the remaining 299 triathletes who had taken neither supplements nor placebo during the race.
Subjects in the sodium group consumed a mean of 14.7 tablets during the race, giving them an additional 156mmol of sodium. The placebo group took 15.8 of their dummy tablets. Nevertheless, there were no significant differences between the three groups in the following measures:

• finishing time;
• sodium concentration before and after the race;
• weight before and after the race;
• temperature and blood pressure after the race;
• perceived effort, muscle soreness and mental wellbeing.

Only one athlete – in the placebo group – developed dangerous hyponatraemia during the race. He was the only athlete to show a substantial weight gain during the race and his problem was put down to drinking too much water.

‘We can reasonably conclude,’ say the researchers, ‘…that additional [sodium] supplementation is unnecessary during prolonged endurance exercise to maintain the serum [sodium] within the normal range.’ According to the scientists, this may be because athletes may sweat less or lose less sodium in their sweat than is currently believed, or that during states of acute sodium loss, additional sodium may be released from body stores to compensate for these losses until sodium supplies can be replenished at the next meal.

Br J Sports Med 2006;40:255-259