From campaigns like “Is it In You?” to “Hydrate or Die”, we’ve all been taught by a multi-billion-dollar drinks industry that dehydration is deadly. What we haven’t been taught, because there’s no money in it, is that the best rule of thumb is only to drink when you’re thirsty.

That’s the conclusion of a scientific study just completed by Dr. James Winger of Loyola University Medical Center and several colleagues, who recently interviewed, weighed, poked, and prodded marathon runners to find out how they drank, when they drank, how much they drank — and what harmed or helped performance.

The critical takeaway: We all drink way too much water (and sports drinks), putting ourselves at risk of worse, not better athletic performance.

Yes, there’s lots of conventional wisdom around drinking, from the need to pound eight glasses of water a day to chugging a bottle an hour for cyclists. To which Dr. Winger responds: There’s no science behind those whatsoever. Zero. And the idea that you’re restoring electrolytes is absolute quackery, he says, because the amount of minerals and salts in these drinks is far too diluted to make a difference.

“The bottle an hour is just a convenient rule of thumb — in the past there have been calls to replace a liter of water for every kg of weight that you lose, but we know that even this can lead to overhydration,” Winger says.

And overhydration leads to reduced performance because your blood has to soak up some of the excess water in an attempt to equalize your body’s salinity. Then your cells begin to swell, causing all sorts of distress, from gastric to dizziness, soreness, and lots of other symptoms that do nothing to make you faster. In very severe instances you can wind up with major GI distress, vomiting, and the like. Go way overboard and there’s a risk of death, because cellular swelling in the brain — hyponatremia encephalopathy — can cause coma or worse.

Exercise-associated hyponatremia (EAS) isn’t rampant, but it’s far more common than symptoms of dehydration, and even mild forms will make you very uncomfortable.

A lot of what the study revealed is how popular media and advertising has skewed our perspective on hydration. The study points out that athletes now view dehydration as a “disease,” when it appears that our bodies are very well set up to perform slightly better when we’re somewhat dehydrated.

Here’s what else you should know:

LOSE WATER WEIGHT, GO FASTER.
“Fifty-five percent of our respondents indicated that drinking less correlates with a slower running performance. This is incorrect,” says Winger. “Marathon running, ultrarunning, triathlon, and cycling all show similar linear relationship between weight loss (which indicates less fluid intake) and performance (race time).” Dr. Winger points to a joint South African-French study that shows that mild dehydration, which is completely normal during exercise, corresponds to faster marathon event times across ages and skill sets.

SO I SHOULDN’T DRINK AT ALL?
Winger’s study doesn’t suggest abstaining from drinking during exercise, but concludes that if you drink when you’re thirsty rather than before, you’ll maintain perfectly adequate hydration. In fact, more experienced, and often faster runners in the study did just that and suffered much lower incidents of GI and other distress. Winger cautions that if it’s very hot, you should drink more but that your body will tell you this through thirst, and he says you cannot over-tax this mechanism. It’s ancient, animal, and simply paying attention to it is all the guidance you need.

ELECTROLYTE LOSS ISN’T A RISK, AND WON’T SLOW YOU DOWN.
If you don’t overhydrate, you simply don’t need to worry about electrolyte imbalances caused by major efforts. “There is no need to replace minerals during exercise, because the loss of minerals has no deleterious effect on the body,” Winger asserts, while noting that this applies to athletes who drink when they are thirsty, and don’t drink constantly. As for taking salt tabs during a race to offset over drinking, Winger says this is also unwise: “During a 26-mile marathon, there is no role during or after the race for oral supplementation of salt.” Winger says the one quick fix for someone who has overhydrated during training or racing is to hit an aid tent or E.R. and get “highly concentrated IV fluids (not normal IV fluids) that will then raise the blood’s concentration.”

WHY DO I FEEL BETTER WHEN I DOWN A RECOVERY DRINK AFTER EXERCISE?
For a somewhat sneaky reason that sports drink makers have taken advantage of, which is that you’re being washed by endorphins when you stop an exercise, and at the same time, you aren’t being taxed by the effort, so you feel better, period. Plus, says Winger, you are thirsty, and quenching that thirst naturally makes us feel better. But lastly, a sports drink nearly always, “…adds carbohydrates. And that’s the preferred fuel for almost every tissue in the body. It may also contain protein, and if that’s the case it supports muscles, too.”

WHAT ABOUT FOOD DURING EXERCISE?
Winger says that if a sports drink has carbs it’s perfectly reasonable to dose with it, especially for workouts lasting longer than an hour, but to be careful that this doesn’t make you over-drink. Gels or an energy bar obviously contain higher concentrations of fuel and make it easier to avoid drinking more than necessary.