How to stay hydrated during a workout: Add a pinch of salt

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If you’re heading out for a long run and want to bring extra water with you, you can strap a fuel belt around your waist to carry a few mini-bottles. Or there’s a simpler approach, according to a series of recent studies: Just drink some salty water before you step out the door.

This tactic, known as "hyperhydration," enables athletes to temporarily store as much as one litre of extra fluid in their body for use during subsequent exercise. It's a neat trick – but whether it actually helps you to go farther or faster remains unclear, as researchers re-evaluate long-held assumptions about the links between thirst, dehydration, and endurance.

Hyperhydration seems like an obvious idea, and many of us try it instinctively by swigging some extra water before exercise. The problem, as thousands of runners have discovered over the years while standing in crowded race corrals, is that your kidneys excel at getting rid of excess fluid. Drink more than you need, and if you're not already dehydrated, you'll quickly feel the urge to pee it out.

There are several ways around this. One is to drink just a minute or two before starting, leaving your body no time to respond. (During hard exercise, the urge to urinate is partly suppressed.)

Another approach is to add glycerol, a sweet, gooey sugar-alcohol that traps extra water in your body. American marathoner Steve Spence used glycerol in blistering conditions at the 1991 World Championships in Tokyo, earning a silver medal. But glycerol was banned by the World Anti-Doping Agency in 2010 because it can also be used as a drug-masking agent, which left athletes searching for an alternative.

Salt works because your body tries to maintain a delicate balance between sodium and water levels. Extra sodium suppresses the signals that would otherwise tell your kidneys to excrete fluid.

In results presented at the American College of Sports Medicine conference in May, Université de Sherbrooke physiologist Dr. Eric Goulet and his colleagues compared the effects of salt, glycerol and plain water when volunteers drank about 1.9 litres of a liquid over the course of an hour. Two hours after they finished drinking, the salt group had retained 1.1 litres compared with 0.8 litres in the glycerol group. The plain-water group retained just 75 millilitres.

Of course, there are downsides to drinking salt water, as every shipwrecked mariner knows. Japanese researchers tested three different salt concentrations to find the best tradeoff between retaining fluid and minimizing gastrointestinal problems, such as diarrhea. The results, published last month in the European Journal of Applied Physiology, showed a sweet spot at about seven grams of table salt (just over a teaspoon) per litre, which is very similar to the concentration in the Sherbrooke study.

There are also health issues to consider: Goulet cautions that anyone with uncontrolled high blood pressure shouldn't try hyperhydrating.

This still leaves the central question unanswered, he acknowledges. "Whether performance can be increased by hyperhydration is not yet clear," he says.

In an earlier study published in 2012, the Sherbrooke researchers found no performance improvement in an 18-kilometre treadmill run in six highly trained volunteers after hyperhydration. One possible explanation is that the volunteers were also allowed to drink during the run, making their extra fluid stores less important.

A more subtle factor – and a topic of intense debate among physiologists – is the difference between the sensation of thirst as perceived by your brain, and the actual hydration status of your body.

For example, a study published last year by researchers in Australia and New Zealand tested cyclists in hot conditions under varying degrees of dehydration. But instead of allowing the cyclists to drink, their hydration status was controlled by infusing saline directly into their veins through an intravenous drip. Even when the cyclists were dehydrated by 3 per cent of their body weight, their performance was unaffected, contradicting decades of warnings that dehydration of more than 2 per cent slows you down.

Why the difference? When you drink, the sensation of fluids passing down your throat and into your stomach is part of what slakes your thirst. Some researchers now believe that being thirsty, rather than being dehydrated, is what slows you down. So even if hyperhydrating augments your water stores, you may still miss the sensation of sipping.

Still, there are plenty of situations where fluids aren't readily available, such as long swims or remote trail runs. In those cases, having extra fluid in your stomach may be better than no fluid at all. Otherwise, the best advice is still the oldest: Drink when you're thirsty, and don't when you're not.

Hydration dos and don’ts

• Do: Maintain consistent hydration throughout the day, checking that your urine is pale or clear.
• Don’t: Force yourself to drink when you’re not thirsty (unless you’re about to start an activity where you won’t be able to drink).
• Do: Plan ahead to make sure you’ll have fluids available when you need them.
• Don’t: Try to load up on plain water to “get ahead” of thirst.
• Do: Allow a week for your body to adjust to exercising in hot weather.
• Don’t: Ignore persistent thirst. Your body is telling you something important.