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."
Sports nutritionist Matt Fitzgerald, author of "The New Rules of Marathon and Half-Marathon Nutrition," credits the sports drink industry with raising runners' awareness of the value of hydration.
But the industry "went too far, wrongly teaching runners that normal exercise dehydration was dangerous and they couldn't trust their own thirst," he said. "The pendulum is swinging back, but it still has a ways to go."
When a person exercises and gets hot, the body tries to cool off by sweating, which pulls water out of the bloodstream. As the blood becomes increasingly concentrated with sodium and other electrolytes, a hormone is released that prompts the kidneys to salvage pure water from urine and put it back into circulation.
Continuing to exercise will eventually cause thirst, and drinking fluids brings the electrolyte concentration back down. Whether thirst provides a timely enough signal is the subject of a lively debate among both athletes and researchers.
"No one who is sweating profusely should rely on thirst as a guide for how much to drink, especially marathon runners," said Bob Murray, director of the Gatorade Sports Science Institute from 1985 to 2008. Research has shown that "the body's thirst mechanism is slow to recognize dehydration," according to Murray, now working as a consultant.
Gatorade's educational materials say a benefit of sports drinks is that they stimulate people to drink more. Water quenches thirst before a person has fully replaced lost body fluids, the materials say.
"People will voluntarily drink more sports drink during exercise than they will with plain water," Murray said.
But Laursen, who works with New Zealand's elite Olympic-level athletes, said the thirst mechanism works perfectly and it isn't necessary to immediately replace 100 percent of fluids lost through sweat.
"Dehydration is a natural consequence of exercise, but we need to remember that the water returns when we enjoy our post-race recovery, or at dinner time and while we sleep at night," Laursen said.
Laursen's latest research, published this month in the British Journal of Sports Medicine, found no differences in performance among 10 cyclists who were randomly assigned different levels of hydration.
One common misperception, experts say, is that the salt in sports drinks can help prevent hyponatremia. But just like water, sports drinks can dilute sodium levels in the blood if they are drunk to excess.
Athletes also don't necessarily need to replace the electrolytes lost during exercise, Laursen said.
"The body gets rid of electrolytes because they are in excess in the body," he said. "Electrolytes consumed during exercise don't do anything, so it's pointless to consume them. You'll replace all lost electrolytes in your post-race meals."
Winger said sports drinks can be helpful after about an hour of exercising not because of the electrolytes but because of their carbs — the sugar content.
The one thing most experts and experienced runners agree on is that it's best to go into a race with a plan.
Bill Fitzgerald, who will be running his 65th marathon Sunday, figured out his needs through trial and error. He said he generally sticks with water for the first half of the race and switches to Gatorade in the second half.
"I know after 90 minutes of exercise, I need to get some glucose, whether it's in the form of a liquid, a gummy bear or half of a windmill cookie," said Fitzgerald, of Prospect Heights. He generally doesn't wait to drink until he's thirsty, but he also drinks sparingly and spends more time rehydrating after the race than during.
But runners also should be cautious once the race has finished. The Chicago Marathon's website warns about the dangers of overhydrating but also recommends that runners consume "125 percent to 150 percent of fluids lost during the race." Chiampas said the advice isn't meant to be carried out immediately.
Winger, who will be volunteering at a Chicago Marathon medical tent in Pilsen, said it's "outright dangerous" to replace so much fluid; many cases of hyponatremia are caused by overhydration after the race.
"Thirst is an evolutionarily ancient system that works quite well," he said. "Runners are used to listening to their bodies with regard to injury. They should utilize the same strategy here."