3 Things to Know about Fueling for a Marathon

Nathan Freeburg Marathon Training, Matathon, Our Best!, Race Day, Race Fueling

In theory, fueling for the marathon is simple: consume enough calories to power yourself through several hours of running without throwing up. Easy, right?

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During my first marathon, I did a terrible job at fueling. I experimented with different things during training, but on race day, for some inexplicable reason, I threw it all out the window. Mix in a nervous tummy and a poor pasta decision the night before, and I had myself a lot of time in the porta potties.

7 marathons later, I’m beginning to figure it out. Below I share what I’ve learned about how much fuel you actually need during the race, the best fuel sources, and how to make sure it all goes down smooth on race day.

How Much Fuel Do you Need During a Marathon?

Most of us have enough fuel (glycogen) stored in our bodies to run about 20 miles (read why here). After that, we bonk. Theoretically, this means we only need to fuel for those last 6.2 miles of the marathon.

Since body weight is used to determine caloric needs when running, we will start there for our calculations. John Davis summarized a really great (and complicated) study, and noted that we burn roughly .73 calories per pound per mile. 25% of calories burned come from fat, (which we can tap into for fuel) and the rest from carbohydrates. This reduces the number of calories burned per mile to .55 per pound of body weight.

I weigh 165 lbs, so I’m burning roughly 91 calories per mile (165 X .55) from carbs. This means I’ll need to ingest 564 calories to race a marathon without bonking.

For simplicity’s sake, we only absorb about 60 grams of carbohydrates per hour. There are 4 calories per gram of carbohydrate, so the maximum caloric intake one can have in an hour is 240 (4 X 60). My 564 calories should then be spread out over no less than 2 hours and 21 minutes to prevent stomach issues and take advantage of absorption rate.

To figure out your own numbers, do this:

(Body Weight X .55) X 6.2 = number of calories needed to fuel your marathon. Take that number and divide by 240 (max consumption rate per hour). This will give you a good idea on the amount of time needed to consume your fuel for maximum efficiency.

These carbohydrates can come from any source, but there are a few that are ideally suited to running a marathon.

Best Products to Use for Fuel During a Marathon

In a 2011 article, “Carbohydrates for training and competition” researchers note the best fuel sources are those that deliver “multiple transportable carbohydrates (glucose:fructose mixtures) [that] will achieve high rates of oxidation of carbohydrate consumed during exercise.” You probably avoid lots of glucose and fructose in everyday life (sugar), but for running long distances, it gets absorbed the best and transferred into energy the fastest. Researchers also seem to suggest that commercial products such as gels, blocks, and liquids all work with about the same efficiency.

I’ve experimented with many of these products, and with real foods such as dates, and once even a PB&J. For me, I think gels work best because they are easy to transport, and easy to consume. After trying countless brands and flavors, I settled on PowerBar PowerGel (tangerine flavor). It tastes fine, isn’t too thick and contains 110 calories with 27g of carbs.

The point here is that the specific product, brand, or flavor probably doesn’t matter as much as finding one that agrees with your stomach.

How and When to Take in Calories

Below is one way to consume my 564 calories during a 4 hour marathon. Due to the absorption rate, I can only consume 141 per hour.

1 gel every 45 minutes with water (110 calories) = 4 times total.
4 oz of sports drink at 2 aid stations per hour (56 calories) = 8 times total.

This totals 166 calories per hour, and perhaps is a bit much, but works for this example.

Note: A heavier and/or faster marathoner will need more calories in a shorter period of time.

You could also use only the sports drink provided along the course. Sports drinks contain about 14g of carbs and 56 calories in an 8 oz serving. Grabbing two cups per aid station would yield about 8 oz per stop (assuming you get most of it in your mouth). Marathons have roughly 15 aid stations per course, and thus would result in 840 calories total if you were to take 8 oz of sports drink at every station. Plenty!

You’ll want to consume more of your calories early in the race, but not too much so as to make yourself sick. If you space it all out perfectly evenly, you may not be able to get anything down later in the race. It’s more art than science, and practice makes perfect.

Practicing your Marathon Fueling Strategy

After you have determined how many calories/carbs you’ll need, start experimenting with different brands, flavors, and kinds of fuel sources. Use each and every long run to test what you’ll do on race day. There’s really no shortcut to this process.

An easy way to do this is to buy yourself a clunky water belt like this one, and fill half the bottles with water and half with your favorite sports drink. Drink a little sports drink every few miles, and take a gel every 45 minutes with water. On the next run, just use sports drink, and on another, just water and gels. Pay close attention to how you feel after each run, and adjust as necessary. (Tip: It’s best to sip a little beverage often, rather than a lot at once.)

Since a belt can be clunky, run a half marathon during your training. Not only is a shorter distance race a great way to test your fitness, but you can also test your fueling strategy through a supported long run. Bring your own gel, blocks, etc. and rely on the aid stations at the event.