A bevy of new hydration products on the market claim to have mixed better concoctions for quicker and more effective hydration. Today's buzzword is osmolarity, a term that refers to the total concentration of dissolved particles in a liquid. Products that say they have lower osmolarity advertise that they provide faster hydration with less stomach discomfort. But is the difference really enough to make you rethink your hydration strategy?

A MATTER OF CONCENTRATION

Water travels around your body via osmosis: Water molecules squeeze through cell membranes, moving from areas of low osmolarity to areas of high osmolarity. Stacy Sims, who holds a doctorate in exercise physiology and sports nutrition, says any drink that has a higher osmolarity than the slightly salty fluid that permeates your body (including your blood) will pull water out of your blood and into your intestines. If you ingest something that's concentrated, like fruit juice, it can't be absorbed quickly. So until it is diluted to a lower osmolarity, it sits in your stomach and sloshes around, which can lead to nausea, stomach pain and gastrointestinal distress.

Trditional sports drinks like Gatorade and Powerade have osmolarities about 25 percent higher than blood's. Sims created Osmo, a line of sports drinks with an osmolarity close to that of blood. SOS Rehydrate takes a similar tack, with an osmolarity slightly lower than Osmo's. Other drinks tout their low osmolarity but do not provide specific measures.

Skratch Labs Exercise Hydration Mix, a sports drink formulated by Allen Lim, who holds a doctorate in exercise science, takes a more aggressive approach, with an osmolarity less than half that of a traditional sports drink. "The lower osmolarity of Skratch creates a very large driving force," or osmotic pressure, forcing water into the body quicker, he says.

Osmolarity is similar to tonicity--long used to divide sports drinks into categories: isotonic (matching the body's levels of sugars and minerals), hypertonic (greater concentrations) and hypotonic (lesser concentrations). Isotonic and hypotonic drinks, like Gookinaid ERG (popular among marathoners in the late '70s and early '80s and still available as Vitalyte), have made claims similar to today's low-osmolarity drinks--that they are absorbed faster and hydrate more effectively.

ENERGY INCLUDED?

The aim of a traditional sports drink is to provide three things: water, electrolytes and carbohydrates (usually in the form of sugar) to fuel performance. The new sports drink companies employ a wide range of strategies in their formulations.

Skratch and Generation UCAN aim for low osmolarity and sufficient carbs to fuel training sessions lasting longer than an hour. Skratch's sugar content is not much lower than Gatorade's or Powerade's, while UCAN delivers more calories using complex carbohydrates but no sugar.

Though Osmo contains sugar, it's not there for energy, Sims says. She explains that the sugar in the drink exists solely to help pull water into your body: As your body absorbs sodium and sugar from your intestines, water molecules are absorbed alongside.

What about refueling in a marathon or during your weekly long run? Sims recommends liquids be used for hydration and small amounts of food be used for fuel. Downing a pure-carbohydrate liquid like an energy gel pulls water into your stomach to dilute it, she says, impairing fluid absorption. Real food like an energy bar is processed much more slowly and does not upset the osmotic balance of the stomach.

Yet dense, energy-rich food can be hard to digest. The faster you run, the more difficult it becomes to refuel with solid foods, says Cathy Fieseler, a sports medicine practitioner, ultra-runner and chair of the Road Runners Club of America sports medicine committee. While Fieseler says slower runners or ultramarathoners might be able to handle bars and other solids, she recommends that faster marathoners use either carbohydrate-based sports drinks or gels washed down with a carb-free solution, such as a low-calorie sports drink or just water.

SOS Rehydrate contains even fewer carbs than Osmo. Blanca Lizaola, clinical research coordinator at the University of California-San Francisco Medical Center and one of the founders of SOS, says the rationale behind its sugar content mirrors that of Osmo. Unlike some of the other drinks, however, SOS uses stevia, a natural sugar substitute, to sweeten.

Two other drinks, Nuun and ZYM, contain no sugar whatsoever, using sorbitol to provide flavor and bind the tablets together. At only eight calories per 16-ounce serving, they are low-calorie options that can be mixed with other fuel sources if needed. "We don't think a one-size-fits-all mentality works," says Kevin Rutherford, CEO of Nuun. "We'll keep you hydrated, and you can choose where to get your carbs and proteins."

SALT AND MINERALS

Sports drinks have always claimed to replace electrolytes, along with providing energy. In addition to their lower osmolarity, new sports drinks emphasize their enhanced electrolyte content, designed to replace what's lost in sweat and prevent hyponatremia, a dangerous drop in blood sodium levels that can occur when runners drink water to excess.

SOS Rehydrate takes the most ambitious approach to electrolyte supply, providing three times as much sodium and six times as much potassium as Gatorade. Other drinks provide around 50 percent more sodium than traditional sports drinks and about the same amount of potassium. To compete with the higher electrolyte content of newer sports drinks, Gatorade offers Gatorade Endurance, a separate formulation tailored for what they call "endurance athletes" in "prolonged training and racing." It provides the same sugar content as regular Gatorade but has more sodium and potassium than any of the other drinks, except for SOS.

Several of these next-generation sports drinks also include magnesium and calcium in small amounts. Sims notes that calcium and potassium play a role in muscular contraction, and Lizaola claims that many people are magnesium deficient. Lim is more equivocal about the importance of these minor electrolytes. "I'm not totally clear if this is an advantage or not," he says, when asked about the benefits of including magnesium and calcium in a sports drink. Though he considered removing them from the Skratch formula to lower the osmolarity even more, he left the magnesium and calcium in, following his philosophy of replacing what's lost in sweat.

NO CLEAR ADVANTAGE

While the creators of the new drinks have impressive credentials, some doctors and exercise physiologists are skeptical of the purported benefits. Among them is Timothy Noakes, an ultramarathoner, doctor and exercise physiologist at the University of Cape Town in South Africa. He's the author of Waterlogged: The Serious Problem of Overhydration in Endurance Sports. Noakes contends that electrolytes are unnecessary during exercise. Although sodium and other minor electrolytes are lost in sweat, the relative amount of sodium lost is less than the amount of water lost. So in athletes who do not drink anything at all during a long run, blood sodium concentrations actually go up, not down. This also means that a sports drink with sodium in it won't prevent hyponatremia.

Fieseler concurs with Noakes when it comes to electrolytes. She points to a 2004 Journal of Sports Sciences article by Edward Coyle, a physiologist who recommends ingesting sports drinks with sodium only when you're exercising for much longer than two hours or if you have unusually large salt losses during exercise--something colloquially known as being a "salty sweater." Noakes, however, points to research showing that people who lose a lot of salt in their sweat already have an excess of sodium in their diet; their bodies are trying to get rid of sodium by sweating it out.

When it comes to other electrolytes like potassium, calcium and magnesium, Coyle also found no compelling case for including them in sports fluids. Noakes, Coyle and Fieseler agree on this: You shouldn't base your choice of sports drink on the presence of minor electrolytes, nutrients or vitamins.

And what of osmolarity? Noakes balks at the suggestion that a low-osmolarity sports drink is absorbed any faster than plain water or a higher osmolarity solution. By the time any ingested fluid gets into your intestines, your body has already equalized its osmolarity with secretions from the pancreas and extra water stored in the gut, he says. Carbohydrates do speed the absorption of water in your intestines, but all of this is "irrelevant," he says, to runners in training or competition, because their ability to absorb fluids isn't a limiting factor. When told to drink to thirst, athletes consume only about 13 to 27 ounces of liquid per hour, a volume that's easily absorbed by the intestines over a wide range of carbohydrate concentrations.

Fieseler is also skeptical about whether relatively small differences in osmolarity have a meaningful impact on how well a sports drink is tolerated. She maintains that carbohydrate concentration is the main determinant of how long a drink sits in your stomach. But Feiseler says individuals might have differing abilities to tolerate various sports drinks and carbohydrate solutions. A 2011 study by researchers at Massey University in New Zealand, published in the International Journal of Sport Nutrition and Exercise Metabolism, demonstrated that sports drinks of all concentrations were absorbed slightly better than water, but also reported differing levels of "gut comfort," with the hypotonic (low osmolarity) drink sitting easiest on the stomach.

Many elite runners experiment with a variety of drinks to figure out what works. Matt Gabrielson, a former professional marathoner, says that he tolerated most fluids well but struggled to find something that actually tasted good 20 miles into a marathon. He settled on Cytomax, a sugar and complex-carb blend, while training partner Jason Lehmkuhle, a 2:12 marathoner, prefers a half-Gatorade, half-water mixture.

PERSONAL CHOICES

Choosing the ideal sports drink is largely a matter of personal preference. From a performance standpoint, carbs are useful when your race distance stretches beyond an hour. You can get them either from a drink with carbohydrates or from something more substantive like gels or energy bars.

The new low-osmolarity drinks might be worth a try if you've had stomach issues with traditional sports drinks but need something more than water. Regardless of the osmolarity, their lower-carb concentrations should make them easier to digest. Low-calorie sports drinks like Nuun or ZYM seem best suited for ultra-length runs when you're getting carbs from other sources.

The evidence is not clear to say that electrolyte-rich and low-osmolarity sports drinks are a game changer, but they do broaden your options when you're figuring out how to hydrate and fuel.

Going Down Easy

Osmolarity, a measure of the concentration of particles dissolved in a liquid, varies greatly among sports drinks, as does what they provide beyond hydration.

Data for a 16 oz. serving, *Company did not provide osmolarity information, ** Contains sugar subsitutes.

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