A FiveThirtyEight article, You Don’t Need Sports Drinks To Stay Hydrated challenges that sports drinks help prevent dehydration or increase exercise and performance better than water.

It claims that the studies demonstrating sports drinks are effective have been poorly done:

“As it turns out, if you apply evidence-based methods, 40 years of sports drinks research does not seemingly add up to much,”

In particular, it argues that moderate amounts of water is just as good:

One small study of cyclists and triathletes found that it didn’t really matter whether they drank plain water, a sports drink or a milk-based beverage after an hour of hard exercise. As long as they drank some liquids along with a meal, they restored their fluid levels just fine.

Is there sufficient scientific evidence that sports drinks will help with dehydration or increase performance during strenuous exercise compared to drinking the same quantity of regular water?

Related Questions:

Neither address the difference between the effectiveness of sports drink compared to water, so I don't consider this to be a duplicate.

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    There is a difference between "an hour of hard exercise" and six hours cycling on the road. No study that considers only brief periods of exercise is going to be particularly useful. – Daniel R Hicks Mar 7 at 1:03
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    This seems to be missing the sports drink claim. AFAIK, sports drinks claim that they are safer hydrating. They include both water and electrolytes (salts). So drinking a sports drink is less likely to overhydrate you than distilled water. If you read the 538 article, they describe overhydration as a more likely problem than dehydration. E.g. "She’d drunk so much fluid that her blood sodium had become dangerously diluted." But sports drinks include sodium as well as water. So if she had been drinking sports drinks, that wouldn't have happened. – Brythan Mar 7 at 16:53
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    @Brythan In the article there's a reference to a very old advertisement from 1970 or before that claimed "Gatorade was absorbed 12 times faster than water". But it seems silly to write a full article to go after a 50 year old marketing claim so I think this is bit of a strawman and/or a poorly titled article as much of it does, as you point out, explain why Brawndo's got Electrolytes. – JimmyJames Mar 7 at 22:23
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    @JimmyJames Not just any electrolytes. The electrolytes your body craves. – Acccumulation Mar 11 at 0:07
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    One question after looking at that is: mkay, adding sodium and glucose enhances water absorption in the jejunum by X%. Does this absorption speed difference matter half an hour later after ingesting the liquid at whole body level? No answer to that in there. – Fizz Mar 21 at 11:03

It's difficult to find either primary research or reviews in this area whose authors haven't received grants from companies that make such sport drinks or more medically oriented rehydration solutions.

Taking the 2007 Position Stand of the American College of Sports Medicine (ACSM) to "as good as it gets" in terms of existing reviews in this area, their overall conclusion is that

During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water along under certain circumstances. After exercise, the goal is to replace fluid and electrolyte deficits. The speed with which rehydration is needed and the magnitude of fluid/electrolyte deficits will determine if an aggressive replacement program is merited.

And to pick some detail:

The composition of the consumed fluids can be important. The Institute of Medicine provided general guidance for composition of "sports beverages" for persons performing prolonged physical activity in hot weather (73). They recommend that these types of fluid replacement beverages might contain ~20-30 meq·L−1 sodium (chloride as the anion), ~2-5 meq·L−1 potassium and ~5-10% carbohydrate (73). The need for these different components (carbohydrate and electrolytes) will depend on the specific exercise task (e.g., intensity and duration) and weather conditions. The sodium and potassium are to help replace sweat electrolyte losses, while sodium also helps to stimulate thirst, and carbohydrate provides energy. These components also can be consumed by nonfluid sources such as gels, energy bars, and other foods.

You can already guess from this that the extra stuff in sports drinks, except for sodium (which stimulates thirst) might not be directly useful in increasing hydration but may help with other issues affecting endurance and performance, e.g. carbohydrates provide energy replacement. The Position Stand actually has a lot of material on that. So your question (focusing on hydration alone) may be too narrow in this sense.

Another issues discussed at length in the Stand (with empirical evidence mostly from marathons) is hyponatremia: drinking water (without added salt) may dilute blood sodium concentration too much in some circumstances. So again it may be too narrow to ask if you get enough water replaced.

And a 1994 paper partly derived from US Army experiences, outlined some cases when they thought more than water is needed:

carbohydrate-electrolyte replacement fluids may be necessary in some, but not all, military field situations. The greatest need for carbohydrate-electrolyte replacement fluids is experienced by soldiers who (1) lose more than 8 liters of sweat per day; (2) are not heat acclimatized (e.g., during the initial 8 days of field living); (3) are performing a prolonged, continuous exercise bout (>60 min); (4) skip meals, have meals interrupted, or encounter anorexia because of a hot environment; (5) experience a caloric deficit of >1,000 kcal per day; or (6) are ill with diarrheal disease. The fluid-electrolyte needs of soldiers are specific to the intensity, frequency, and duration of the exercise involved, as well as the environmental stress encountered. This information is not presented to imply that many different solutions are required, but rather that the best use of such fluids can be recognized with proper soldier training (i.e., when to use them) and simple instructions for field implementation. It appears that carbohydrate-electrolyte replacement fluids, like weapons, should be available to the soldier for use when needed.

The US Army might not have found such drinks to be cost-effective though. Their 2003 revised hydration guidelines (Kolka et al.) managed to reduce the experimental incidence of hyponatremia simply by reducing the water intake (and did that without an increase in dehydration).

Finally Tim Noakes has a fairly critical review (2010) of the ACSM standards (including the 2007 one) and of the older (1980s) US Army standards, particularly as it concerns the amount of fluid intake recommended, which is excessive in his view. Noakes' review doesn't cover much the add-ons provided by the sports drinks, but he is quite critical of influence that the sports drinks industry has had on research. He says for instance that hyponatremia is largely a manufactured problem by standards suggesting that athletes drink "ahead of the thirst", that is more than they feel necessary (ad libitum). He does concede that the sports drinks carbohydrates' add-on do provide some performance benefits, e.g.

[Dr. Cade's Gatorade] simple studies at the time [1971] did not show that fluid ingestion was necessary to prevent heat stroke during exercise. However, the ingestion of the carbohydrate-containing solution clearly improved performance in a 7-mile run/walk, at that time a novel finding.

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