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The research confirmed a maxim of runners around the world: It’s not how much you drink that matters; it’s what’s in the bottle.Sandra Mu/Getty Images

If the first rule of hydration is that what goes in must come out, then the second rule is that not all drinks are created equal. Your body (and bladder) can tell the difference between, say, coffee and Gatorade.

Do those differences really matter? Scientists have spent plenty of time studying the individual factors that can influence a drink's hydrating power, such as the diuretic properties of caffeine and alcohol or the fluid-retaining power of electrolytes, such as sodium. But a new study is one of the first to explore how all those factors combine in the actual drinks we consume in daily life – and the results aren't quite what the scientists expected.

The study, from researchers at Loughborough University, Bangor University and the University of Stirling, in Britain, measured the hydrating power of 13 different drinks in order to create a standardized "beverage hydration index." The results are published in the American Journal of Clinical Nutrition.

To compare the drinks, 72 volunteers drank a litre of fluid, then measured how much urine they produced over the next four hours. Each volunteer returned to the laboratory on four different occasions to test different drinks, including water at least once.

To account for individual differences in bladder size and other traits, the hydration index was calculated by comparing the amount of urine excreted after a given drink to the amount of urine after drinking water. A drink with an index value of 1.2, for example, means that on average people retained 20 per cent more fluid after two hours than they would have with water. (The index was calculated using two-hour urine values since that was considered a realistic between-drink interval in the real world.)

Four of the 13 drinks produced statistically significant increases in fluid retention, with beverage hydration index values close to 1.5: oral rehydration solution, orange juice, skim milk and full-fat milk.

The oral rehydration solution is no surprise, since it's specially formulated with salt and sugar to enhance absorption of fluid; it's often used to combat dehydration from diarrhea. Similarly, orange juice has lots of sugar and potassium, an electrolyte.

Milk's hydration power is perhaps a little more unexpected – but it shouldn't be, says Nicole Springle, the sports nutrition lead for the Canadian Sport Institute Ontario: "We know that having electrolytes helps with the absorption and retention of water, and having added nutrients and calories seems to make a difference as well, so in something like milk, you're getting a little of everything."

Because of its protein and carbohydrate content, milk has received plenty of attention (and marketing) in recent years as a postexercise recovery drink. The new results are a reminder that milk can also be a reasonable pre-exercise option, Springle says, maximizing hydration while minimizing the urge to urinate.

Perhaps the biggest surprise was that none of the drinks, including coffee, tea and beer, performed significantly worse than water on the hydration index. (The other drinks were sparkling water, cola, diet cola, sports drink and cold tea.)

Caffeine is a well-known diuretic, but it generally doesn't have measurable effects in doses below about 300 mg. The coffee and tea used in the study had 212 mg and 179 mg, respectively, of caffeine per litre, so it's unlikely to be an issue except for very heavy users. That said, Springle adds, individual responses vary greatly, so even modest doses of caffeine may send some people to the restroom.

For the beer, a Carling lager, the diuretic effect of alcohol was likely balanced by the drink's retention-promoting calories. A previous study did find that regular beer promotes more urination than alcohol-free beer, and it's probable that in greater doses the diuretic effects of alcohol would become even more pronounced.

"If [alcohol] is in moderation, it's not an issue," Springle tells athletes. And if it's not in moderation, and an important training session or competition is coming up, "they'll have bigger issues to worry about than dehydration."

Of course, hydration is only part of the equation in deciding what to drink. Part of the problem with chocolate milk as a postexercise recovery drink, and with sports drinks (or even fruit juices) more generally, is that not everyone needs the extra calories. A cup of partly skimmed chocolate milk has 150 calories; so does a 591 ml bottle of Gatorade.

Besides, there's another way of getting the calories and electrolytes that will help you retain fluid after a workout, Springle points out: Eat food. If you drink water and eat a snack – a banana and a tuna sandwich, say – you'll be getting the nutritional benefits of real food as well as keeping yourself hydrated.

Ultimately, the best drink choice will depend on your situation: a long-haul trucker, say, has different constraints compared with an office worker with a desk around the corner from the restroom. Either way, the beverage hydration index is a reminder that it's not just how much you drink that matters; it's what's in the bottle.

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