Endurance athletes who drink coconut water after their workouts could save their money.

Pop makers in the U.S. market coconut water — the liquid from a young coconut — as a refreshing drink to rehydrate after hitting the gym. (Coconut milk, by contrast, is the liquid from the meat of the fruit.)

"They are certainly safe, but if you are counting on them for serious rehydration, you should either pick them very carefully or look elsewhere," said Dr. Todd Cooperman of consumerlab.com, which investigates health claims of products such as vitamins.

"If you have water with some pretzels, you will get more sodium than with some of these products."

Replenishing electrolytes like sodium after a serious workout is important because the body loses salt as it sweats, Cooperman said.

"Coconut water is not necessarily a better choice than regular water, especially if you are trying to save money."

There is little research on the effectiveness of coconut water in rehydration following an intense workout. (iStock)

The drinks can cost about three dollars for 250 millilitres, or one cup.

The medical literature includes little research on its effectiveness. A few small studies have concluded that some of the beverages work just as well as commercial sports drinks for rehydrating after intense exercise, depending on the sodium content.

But not all coconut waters are created equal, registered dietitian Mary Bamford of Toronto agreed.

Sugar and salts

"Make sure there's no added sugar in your coconut water," Bamford advised. "Some of them are only 10 per cent coconut water."

Compared with sports drinks, some coconut waters are low on carbohydrates but have about one-10th the sodium and 10 times the potassium.

"It is not enough sodium and too much potassium" to compensate for lost sweat, Bamford said.

Among a group of runners in downtown Toronto, one man said he turned to coconut water after finding that commercial sports drinks disagreed with him.

"It doesn't turn my stomach and it feels good," Alex Sproll said. "I think the main thing is that it's more natural."

Coconut water gets into the system faster than eating potassium-rich bananas, a previous craze among runners, said Ian Barnaby. He said he drinks it after runs lasting longer than an hour.

For people who don't work out for that long — anything less than about 40 minutes of intense activity — Bamford suggested sticking with calorie-free water and limiting coconut water intake to no more than a cup a day.

With files from CBC's Melanie Nagy