Like many runners, Brian Tolsma worries that once he starts feeling thirsty during a hard workout, he's in trouble: His body has started to break down.
That's why Tolsma will force down as much water as possible on the day before Sunday's Bank of America Chicago Marathon. During the 26.2-mile journey, he plans to be even more methodical: "I'll likely stop at all 20 water stations, taking in at least a cup of fluid at each one, whether I'm thirsty or not," said Tolsma, 34.
Drinking "ahead of thirst" is a common hydration strategy that was widely encouraged for years. But many experts in exercise science advocate a simpler and surprisingly controversial method: Trust in your thirst, and drink water when the urge hits.
The supposed dangers of dehydration — such as heat illness and cramps — have been overblown, these scientists allege, a problem some blame on the sports drink industry. They say water loss is a natural consequence of exercise and is far less dangerous than overconsumption, which in extreme cases can cause serious illness and death.
Ingesting too many fluids can lower the blood's sodium levels enough that cells start to swell, a potentially dangerous condition called hyponatremia. When 43-year-old runner Kelly Barrett died after the 1998 Chicago Marathon, a doctor who treated her said swelling in her brain caused her to go into cardiac arrest. Relatives said Barrett had been drinking vast amounts of water.
Among marathon runners, the incidence of hyponatremia may be as high as 13 percent, according to one published study that looked at Boston Marathon participants.
By contrast, most marathoners are probably somewhat dehydrated, but that doesn't matter much, scientists say. Dehydration is unlikely to cause problems unless a thirsty person is deprived of fluids for a long time — not likely during a marathon where unlimited water is available.
Athletes also may think that aggressive hydration will help them avoid heat illness and cramping. But experts say hydration plays only a minor role in the root cause of heat illness — body temperature — and cramping is likely caused by neuromuscular fatigue. "Hitting the wall," meanwhile, happens when muscle cells run out of glycogen, which is the fuel stored inside the cell.
"Drinking to thirst is the ideal situation," said Dr. George Chiampas, medical director of the Chicago Marathon. "The message, especially when running a marathon, is not always to drink more, but to make sure you drink enough according to your individual needs and the conditions on race day."
With guidelines and philosophies on hydration changing dramatically over the past two decades, confusion still exists over what, how much and when to drink during exercise.
Back in the 1970s, runners sometimes passed up fluids altogether, for fear it would slow them down or make them look weak, said Dr. Tim Noakes, a professor of exercise and sports science at the University of Cape Town in South Africa.
Attitudes began changing in the 1980s, as ads for sports drinks warned about the dangers of dehydration. Gatorade also founded the Sports Science Institute in Barrington to conduct research on exercise science, hydration and sports nutrition.
By 1996, the American College of Sports Medicine was advising athletes to drink early and as much as they could tolerate in order to replace the sweat lost during exercise.
As the dangers of overhydration came to light, however, the organization changed its position in 2007. Like many sports associations and nutritionists, it now recommends customized fluid replacement programs based on an athlete's sweat rate. Even that method overestimates the amount of fluids needed, according to some experts.
Today many runners remain baffled about how to drink during exercise, and their choices often reflect advertising messages rather than the scientific consensus, said Dr. James Winger, a sports medicine physician and hydration researcher at Loyola University Medical Center.
Nearly half of recreational runners may be drinking too much fluid during races, Winger reported in the British Journal of Sports Medicine in 2010.
That might not be surprising, given the dizzying number of thirst-quenching options that have flooded the marketplace, including enhanced waters, sports and energy drinks, endurance formulas, and pills or powders designed to be mixed with water.
The marketing and research power of the sport drink industry "has convinced the public that the products are a requirement for both sport performance and health," said exercise scientist Paul Laursen, a researcher and performance physiologist at the Sports Performance Research Institute New Zealand.
Asker Jeukendrup, global senior director of the Gatorade Sports Science Institute, said sports drinks can play an important role when performance is important and the exercise lasts more than 30 minutes.
"There is no one-size-fits-all answer," he wrote in an email. "For athletes, there are situations in which drinking to thirst is appropriate and others in which thirst is not a reliable enough mechanism because thirst is affected by so many other factors. The key is to be educated, know your individual needs and go into your race with a plan."