When to Take Gels in a Marathon

Where the Hills Are on Your Course

Contrary to popular belief, a hilly course doesn’t change how much glycogen you need or how much you burn.

What the hills on your course do change about your fueling plan is where you should take your gels.

Using the Boston Marathon as an example, CarbRate's energy model shows that the Boston course actually requires 1.5% less energy than a flat marathon, because its net downhill outweighs the Newton hills.

Even for the hilliest course we've mapped, the Marine Corps Marathon, the energy model shows that it’s only about 0.5% harder than a flat course.

So, what needs to change in your fueling plan isn’t how many gels you take, but where you take them. Here is how you should plan when to take your gels on a hilly course…

Don’t take your gels on any climb steeper than about 3 percent. Instead, plan to take them on the flat before it or the flat after it.

Your stomach slows down digestion as your effort increases running uphill. This happens because blood moves from your gut to your legs, so a gel taken on a hard climb sits there instead of being absorbed.

Don’t take gels on long, steep descents. Below about 7 percent your quads are doing the braking and everything in your stomach is bouncing, so wait for the course to flatten before you take anything.

If you want a fueling plan with your gel and drink timing customized to the course, CarbRate does this for you. Start building your plan here.

How Much Water You Take With Each Gel

Research has shown that the optimal carbohydrate concentration is 6 to 8 percent carbohydrate to water.

That means you'll want about 100 mL (3.5 oz) of water for every 8 grams of carbohydrate you take in. A typical gel contains 25 g of carbohydrate, so you'll want to aim for about 300 mL (10 oz) of water with each gel.

A good estimate is that each cup of water will provide you 100 mL (3.5 oz) of water.

That's because volunteers typically fill cups about half way to prevent water from splashing and you'll lose a good amount as you run with the cup and what spills as you drink.

That means each gel needs about 3 cups of water, so plan when you take your gels to be when you have water available to wash it down, like at an aid station.

Where the Aid Stations Are on Your Course

The common advice is to take your gels every 30 to 45 minutes, but that might not coincide with when you hit an aid station.

If you’re not carrying your own water with you, you’ll need those aid stations to wash down your gel.

So your fueling plan should never be a random number, it should be plotted to the aid stations on the course.