Drink when you're thirsty. Stop drinking when your thirst is quenched. Obey that one rule and there is no risk of dehydration.

"We are designed to drink enough water to keep ourselves from being thirsty and that does the job just fine," said Dr. Stanley Goldfarb, a University of Pennsylvania kidney specialist who is frequently quoted in the news as a voice of reason about our hydration fixation.

"Do you need to spend the extra money, do you need to carry around bottles of water all day to make sure that you are never thirsty, to keep staying hydrated beyond any sense that you have any desire to drink? There is absolutely no evidence that is beneficial to you," he said.

But responding to the repeated fears about the risk of mild dehydration is keeping the professor busy.

Brock University

Dr. Stephen Cheung, left, tests the effect of hydration on athletes' performance at his kinesiology lab at Brock University. (Kelly Crowe/CBC)

"The very fact that we continue to talk about this is a little bit distressing," he said. "I'm not sure who is keeping this alive."

Just last week, a national newspaper column warned that drinking too little water — even while sitting at a desk — can impair performance, according to one scientific study.

A few weeks earlier, news headlines declared that driving while dehydrated could be as bad as driving drunk, citing another study.

Both studies were funded, in part, by the bottled water industry, as is much of the research on the health of human hydration.

"Industries tend to sponsor studies with the hope that it's going to produce an outcome that is going to lead them to be able to make claims for benefits," Dr. Goldfarb said.

No effect on performance

Stephen Cheung is an elite cyclist and a scientist.

"It becomes a blurry line," he said. "Is the public message really based on good evidence, or is it based on commercial concerns?"

So he went looking for the evidence.

At his kinesiology lab at Brock University, he had 11 cyclists ride stationary bikes under various states of dehydration, and kept them from knowing how much fluid they were getting, using an intravenous line that he kept hidden, under a bag, out of their view.

The subjects were asked to ride 90 minutes at a normal pace, and then ride 20 minutes as fast as they could.  

 "We did a few tricks. We warmed up the saline bag to body temperature so they couldn't feel whether there was a sudden cool rush of fluid into their arm," Cheung said.

Sometimes he allowed them to rinse their mouth, to avoid the sensation of thirst. He is satisfied that during the tests, the subjects did not know whether they were dehydrated or not.

The results? Dehydration had no effect on their performance. The 20-minute ride was just as fast regardless of their state of hydration.

"It didn't matter at all," Cheung said. "What we found is any of these conditions, whether you were thirsty, whether you were hydrated or dehydrated, there was no effect."

"So I would say to athletes, get that out of your head. You can perform well even if you are a little bit dehydrated."

Here is Goldfarb's response to some of the enduring hydration myths. 

Myth 1: If you wait for thirst, it's too late

"Do you have to eat before you are hungry, or something terrible is going to happen?" he said. "Everybody knows you become hungry and then you eat and then you are fine. It's the same way with drinking. You become thirsty, and then you drink, and then you are fine."

Myth 2: Drinking more water flushes more toxins from the body

The kidneys are good at flushing toxins using the water provided through normal thirst. More water just adds more volume, it doesn't change the amount of waste that is flushed from the body.

"You don't increase the clearance of toxins from the body you simply excrete those toxins in a larger volume, which simply means more episodes of urination," Goldfarb said.

Myth 3: Checking the colour of urine is a good way to monitor hydration

The colour of urine is influenced by many factors. 

"It is true that as the urine becomes more concentrated it will become slightly darker, because you do put out substances in the urine that do have a yellowish colour, as everyone is familiar with," he said. "You can be healthy with urine that's a little bit dark."

Myth 4: It's healthy to drink lots of water, whether you're thirsty or not

"For a healthy individual, there really is no evidence that drinking extra amounts of water on some regular basis will produce any health benefits. This has been studied many times," Goldfarb said.

"It will not reduce your risk of cardiovascular disease. That has been studied over long periods of time. There is no evidence it will improve your kidney function. That has been studied."

He added: "The evidence is just not available to show that any of these manoeuvres of drinking extra amounts of water will be beneficial."

Myth 5: Mild dehydration can impair thinking

It comes down to a question of degree.

"A little bit of dehydration to the point where you are a little bit thirsty has been carefully studied, and does not alter reaction time," Goldfarb said. 

Bottom line: for healthy people doing normal things under everyday conditions, nature has already provided the perfect tool, precisely calibrated to replace the fluids that are lost through exertion, perspiration, urination and other excretion.

It's called "thirst." Use it, and you can stop sweating about hydration.