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Not for Marathoners Only: What Are the Latest Recommendations for Fluid and Carbohydrate Consumption While Running?


Some running topics just never go away. We're always seeking the best and newest answers on these. How much should I drink in a race? How many carbs should I attempt to take in?

The new director of the Gatorade Sports Science Institute, Asker Jeukendrup, has been writing about these old chestnuts in several new journal papers. The information that follows comes from an article in the Journal of Sports Sciences, and summarizes expert opinion from a recent IOC Consensus group meeting on endurance sports nutrition.

How much should you drink in a marathon? Expert opinion now says it's commonplace and entirely okay to finish a marathon about two to three percent lighter than when you started. For the purposes of this blog, I'll use the middle ground–minus 2.5 percent–and assume that you are a 150-pound runner finishing the marathon in four hours.

Since 2.5 percent of 150 pounds is 3.75 lbs (60 ounces), this represents fluid that you don't have to replace during a marathon. Divide by four hours, and it means that you don't have to drink 15 ounces/hour (that you might once have aimed to drink if you were planning to finish the marathon with zero weight loss.)

You can use a similar 2.5 percent approach for your own body weight, be it 100 pounds or 250. The resulting computation will give you an amount that you don't have to drink, because you're no longer aiming to maintain body weight. You still have to figure out how much to drink, which is primarily dependent on your weight, the weather, and your pace.

For the roughly 150-pounder, there's an easier way to get to "the bottom line." The 2007 "Exercise and Fluid Replacement" Position Stand of the American College of Sports Medicine has a table for a 150-pound marathoner finishing in about four hours on a 64 degrees Fahrenheit day. If this runner drinks roughly 14 ounces of fluids an hour, he/she will likely finish about 2.3 percent dehydrated–that is, within the 2.5 percent guideline stated above.

Here are some other runners and drinking amounts from the same table that will result in an acceptable 2 to 3 percent dehydration at the finish. Obvious messages: The more you weigh, the more you have to drink. The faster you run, the more you have to drink. Important corollary: If you are small and slow, you should not drink very much per hour, or you increase risk of hyponatremia. (Remember, these all refer to a 64 F day. Temperature and humidity are huge factors in both directions, high and low.)

Runner weight Finish time Drink rate Finish % wt loss
110 2:46 13 oz/hr -2.6%
110 3:19 13 oz/hr -2.0%
154 2:46 20 oz/hr -2.6%
154 3:19 20 oz/hr -2.0%
198 3:19 27 oz/hr -2.0%
198 4:57 13 oz/hr -2.6%

Next, how many carbs do you have to stuff down when you run? Jeukendrup produced a neat chart in his recent article. I've summarized it below. Note that, like others, he doesn't think you need to worry much about carbs if you're only exercising for 30 to 75 minutes. The "mouth rinse" approach  is interesting, though, and hasn't been included in past charts of this type.

Jeukendrup is referring to experiments where runners have been told to "swish and spit" a sports drink, but not to actually swallow. A number of these trials have shown improved performance versus a no-drinking condition. Apparently, the mouth sends a message to the brain which says, "Three cheers, sugar is on the way." The brain is tricked, and the muscles too. As a result, you can run faster. These trials have not been extended for periods much beyond an hour. So Jeukendrup believes this is a good strategy if you're exercising for 30 to 75 minutes, but wouldn't work for a full marathon.

Jeukendrup is also a fan of pushing your carb-intake limits. He says his research supports this. (See my interview with him a few weeks ago.) That's why the final row in the below chart shows a carb-intake level up to 360 calories per hour. This exceeds the now typical recommendation of 30 to 60 grams (120 to 240 calories) per hour. I'm guessing (and this is pure speculation) that Gatorade will soon introduce new products to help you push the carb-intake barrier.

Race Duration Carbs
0-30 mins 0
30-75 0, mouth rinse
1-2 hrs up to 120 cals/hr
2-3 hrs up to 240 cals/hr
2.5+ hrs up to 360 cals/hr

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Dehydration and Hyponatremia: How Common? How Dangerous?


Yesterday Gretchen Reynolds and I blogged more or less simultaneously about fluid consumption issues while running, particularly while marathoning. Gretchen's blog appeared here on the New York Times website; and mine here at RunnersWorld.com.

Gretchen wrote primarily about studies showing that some people drink too much in marathons, and become hyponatremic (too much water in blood, too little salt). But she also mentioned that some runners don't understand the importance of hydration, and perhaps don't drink enough.

I wrote a more technical piece illustrating hydration strategies that would result in a two- to three-percent weight loss during a marathon. This is now accepted as a generally safe level of dehydration in a marathon. (Note: Not all body-weight loss in a marathon is water loss. Some is tissue loss.)

Now would be a good time to step back and ask the more general questions. What percent of marathoners get hyponatremic? What percent get dehydrated? What are the effects of each?

Hyponatremia is a classified medical diagnosis that has been fairly extensively studied in endurance events in the last decade. Results have shown anywhere from .1 percent of endurance athletes to 50 percent (in a 100-miler) are hyponatremic at the finish. The most famous study, involving the 2002 Boston Marathon, found an incidence of 13 percent. Many runners, including me, were stunned by the reports of gross overdrinking.

However, it's important to note that you can be hyponatremic without being sick or in danger, and this is in fact the case for the vast majority of cases mentioned above. Having modest hyponatremia (sodium <135 mE/L) is like having modestly high blood pressure. It's not good, but you're still alive and probably don't even know you have it.

Very severe hyponatremia (<130 mE/L; or, far worse, <125 mE/L) can be life threatening. Indeed, there have been eight recorded deaths from hyponatremia in endurance athletes in the last several decades. Most of these severe cases are caused by extremely high fluid-drinking rates during the event. (However, there are other genetic-type causes as well, and it's even possible to be hyponatremic and dehydrated at the same time.)

Given the low to modest percentage of hyponatremia cases in marathons, it's obvious that the vast majority of us finish at the same weight we started or (probably) somewhat dehydrated. But dehydration doesn't exist as a medical condition. It's not dangerous in the short-term (a day or two). It's completely normal for humans to get dehydrated during the day as we rush around taking care of our various responsibilities. Then at night, when we relax, we notice that we're thirsty and start drinking. End of dehydration.

Of course, a marathon exists in a compressed time period, of say two to five hours, and our heavy sweating rate makes it possible to get dehydrated quite quickly. Then what happens? Basically nothing. We simply slow down a little as our blood grows thicker and the heart has to pump harder to push it around. What about death by heatstroke? Some would argue this, but the direct link appears to be quite weak. Heatstroke can be deadly in rare cases, but it's not at all clear that it's caused by dehydration.

How about muscle cramps? Again, the link to dehydration is weak. How about bonking? Nope. The link is much more likely to be low carbs than dehydration.

Now, I can virtually guarantee that if you run a marathon, you will reach a point where your body just doesn't want to go any farther. I can also virtually guarantee that the cause is not dehydration. It's much more likely to be low carbs and/or muscle fatigue, neither of which will be helped by drinking water.

In summary, what I'm trying to say here is that just about everyone has some degree of hyponatremia or dehydration at the end of a marathon. And 99 percent of the time, nothing happens. Of the two, hyponatremia is the more serious. If your stomach is sloshing ominously and the rings on your fingers are getting tight, stop running and seek medical attention.

A final note: There are three primary causes of death during a marathon. All begin with an "H." They are, in order of incidence, heart attack, hyponatremia, heatstroke. You can't really prevent a heart attack or heatstroke; they happen too quickly. You can usually prevent hyponatremia by being careful not to drink too much. So be careful.

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