CNN  — 

Ultramarathoners and other endurance runners take note: You can’t rely on electrolyte sports drinks and supplements to keep essential salt levels in balance and prevent illness during and after these grueling races, according to a new study.

Researchers from Stanford University evaluated 266 athletes who took part in RacingThePlanet – an extreme sporting event that involved running 155 miles over seven days across rough terrain in different deserts around the world. Dr. Grant Lipman, the lead author of the study, said the findings were applicable to other sports.

The study, which published Tuesday in the Clinical Journal of Sports Medicine, focused on hypernatremia, which occurs when sodium levels are too high and is associated with dehydration, and exercise-associated hyponatremia, or EAH, which is caused by a drop in sodium levels.

Exercise-associated hyponatremia can lead to altered mental status, seizures, pulmonary edema and even death.

Their study showed that hot weather increased the rates of these types of illnesses, but use of sodium supplements did not prevent EAH.

“In the past, athletes were told to make sure they’re taking electrolyte supplements and drinking as much water as they can,” said Lipman, a professor of emergency medicine at Stanford and director of Stanford Wilderness Medicine.

“It was generally thought that that would prevent things like muscle cramping, electrolyte imbalances and dizziness. But there is currently no evidence to show this is true.”

The 61 women and 205 men in the study ran in one of five different races held in 2017 and 2018 in South America, Namibia, Chile and Mongolia. Ninety-eight of the runners competed in temperatures that averaged over 93 degrees Fahrenheit (33 degree Celsius).

Some runners took salt tablets every hour, while others diluted electrolytes in a water bottle, Lipman said.

“There are multiple different methods. However, most electrolyte strategies end up with a drink that has a lower sodium concentration than what is found in the body. This is why drinking too much electrolyte solutions can result in EAH. “

Competitors take part in the second stage of the 2nd Half Marathon Des Sables Ica Desert-Peru, in Paracas, Peru, on December 3, 2019.

Data was collected from athletes at the beginning and end of a 50-mile race, undertaken on the fifth day of the competition.

Prior to the race, the participants had reported what electrolyte supplements they planned to use, how often they planned to take them and what their drinking strategy was – whether they planned to drink at regular intervals or just when they got thirsty. They also reported their previous training programs. And they weighed in before the race.

At the finish line, before hydrating or resting, researchers weighed them again and asked how closely they followed their plans for drinking and taking supplements. A blood sample was also taken to measure sodium levels.

Analysis of the data showed that 41 of the athletes had sodium imbalances by the end of the race: 11 were found to have EAH due to too little sodium, and 30 were dehydrated, with too much sodium in their blood.

Some 88% of the sodium imbalances recorded occurred during the hot races, the study’s authors said, which suggested that heat and hydration levels were better predictors of sodium imbalances than either the manner or type of electrolyte supplements taken.

“Electrolyte supplements are promoted as preventing nausea and cramping caused by low salt levels, but this is a false paradigm,” said Lipman. “They’ve never been shown to prevent illness or even improve performance, and if diluted with too much water, can be dangerous.”

Further analysis of the data also showed that participants with EAH had, on average, shorter training programs, weighed more and took five to six hours longer to complete the race, the authors said.

Get CNN Health's weekly newsletter

Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

Lipman said the takeaway from the study is to listen to your body, no matter what sport you are participating in, and particularly pay attention in the heat. While the study focused on endurance running, he said the same illnesses had been found in people participating in rugby, soccer and rowing.

“Drink when you are thirsty. Don’t have a schedule on time or mileage,” he said.

“Just [drinking] electrolytes aren’t going to protect you from high or low salt levels you need to eat salty food as well,” he added.