Building A Road Marathon Fueling Plan

High-carb fueling is all the rage in ultras, triathlons, and cycling. But I’d argue that fueling is most important at the marathon distance.

In road marathons, athletes are trying to push as close to LT2 as possible, straddling the metabolic line between an effort that is sustainable for the distance and an effort that isn’t. The sustainability of an effort incorporates a number of different factors, like fatigue resistance, muscular resilience, and duration. But most of the variables can be incorporated into a simple question: What is an athlete’s running economy? I.e., How much energy does it take for a given output, and can they replace those carbs all the way to the finish line before hitting empty?

Running economy matters because it determines how much substrate an athlete is using. At very high effort levels, like in a 10k, athletes burn through glycogen reserves rapidly, but the events are short enough that it probably doesn’t require fueling (though some elite athletes are starting to take bottles even in events as short as 30 minutes). At very low effort levels below LT1, like a 100-miler, athletes can rely on lipid oxidation, allowing them to sustain lower carb totals if they really want to (though I don’t understand why they would sentence themselves to low efforts when high-carb fueling lets them push harder). In a marathon, meanwhile, every athlete is balancing the maximum amount of high-intensity glycogen they can burn, using lower-intensity fat to make up the remainder. 

In other words: fuel more, run harder, go faster.

Running economy (mixed with individual-specific metabolic factors) determines how many carbs and how much fat an athlete burns at a given pace. Go too hard and burn through too much glycogen, and that leads to the dreaded bonk. Consuming more carbs allows athletes to not only avoid bonking, but to push harder and truly race the marathon distance.

For athletes finishing a marathon in 2-3 hours, they can burn through a way higher proportion of glycogen, since they can rely on their stores when starting the race to go way above the glycogen burn rates that they can replace. For athletes finishing in 5+ hours, the effort needs to be dialed back because the glycogen clock will tick to zero if they aren’t careful, since the glycogen they start with needs to be metered out more safely. Thus, athletes going faster with higher output likely need to consume more carbs to reach their potential. But almost anyone can benefit from high-carb to perform closer to the ceiling set by their running economy (and to recover faster post-race). 

As a starting point, we recommend athletes aim for 75 grams of carbs per hour. That is the opening of the high-carb fueling funnel, where any athlete can probably benefit with minimal GI risk as long as they practice it. 

If that works in training and athletes see the benefits (which are usually gobsmackingly amazing), the high-carb funnel narrows for most athletes around 90 grams of carbs per hour. That is an ideal goal for very fast athletes who are pushing high efforts, and any athlete who finds major training benefits.

The high-carb funnel gets very, very narrow above 90 grams an hour in a road marathon, with it usually requiring faster paces, practice, and some GI talent. Aiming for 105-120 grams per hour is a good ceiling until there is more data on athletes excelling with higher totals (road cycling, ultras, and triathlons have more anecdotal evidence of levels above that working, likely due to road marathons having higher sustained intensity and higher impact forces).

In addition to carb intake, it’s essential to dial in hydration and caffeine. Here’s a hot take from the carb man himself: hydration is even more important than fueling.

You can finish a race under-fueled, but under hydrate and you may be in the med tent.

Unlike carb intake, though, hydration needs vary wildly even among similarly situated athletes. The sodium total in sweat might vary from 200 mg per 32 oz fluid all the way up to 2000+ mg per 32 oz fluid, with the median being around 900 mg per 32 oz fluid. Sweat rates can vary from 16 oz/hr all the way up to 3+ times that.

With those numbers in mind, most athletes can’t get by with the little on-course cups. My guess is that most bonks in road marathons are dehydration bonks more than fueling bonks. My suggestion is to use a race belt where you can store a handheld 500 mL bottle, which you refill at aid stations as needed based on your sweat rate, bringing supplementary electrolytes if you need them beyond what’s in the on-course sports drink. If you’re anywhere near the median, you will need electrolytes to add to your bottles.

The best way to understand your hydration needs is to do sweat testing, which you can do at home. A gross oversimplification for an athlete with median sweat rates would be to suggest 16-24 oz of fluid per hour, with 500-750 mg sodium in that fluid. I personally need 32-38 oz per hour though, with 1000-1500 mg sodium!

In addition to electrolyte totals, determine your fueling plan based on how many calories will be in your hydration mix. I personally love for my hydration to always have some calories when I won’t have access to tons of stops for my favorite gels, like in a road marathon. Calories in drinks go down easily at high intensity, and it’s free jet fuel (and freedom from as many gel wrappers). If you have a high sweat rate on a hot day and you’re hydrating with a drink containing calories, you might need way fewer gels than in the chart above.

Finally, let’s talk about the supplement that’s even more individual: caffeine. Caffeine metabolism is highly genetic, with some athletes thriving on very high amounts, and other athletes having their performance torpedoed by even moderate amounts. My general suggestion to caffeine responders is to have caffeine in your gel at 30 minutes and approximately every hour after. But consult with your genetic testing, your doctor, your preferred clergy member and/or therapist, and your Doordash driver (they know you better than you know yourself).

Marathons are a battle against the bonk, figuring out how you can push harder without burning through your glycogen stores. Modern fueling options are an unfair advantage for all of us. 

The high-carb cheat code is available to everyone. Use it.

Photo Credits: Cody Bare