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The Treeline Endurance Guide to Eating While Running

Using Food As A Tool in Your Best Running Performances

Run fueling blog hero image

It’s what athletes all over the world think about early in the morning, but also into the evening. A staple of long training sessions, but also of recovery days. Something that we all enjoy, but have little interest in while we’re out for a run.

I’m talking about food.

We all know food plays a key role in staying healthy and recovering from hard training, but what might be less clear is how food can help fuel you during your best running performances. The information in this guide will help you better realize the fitness you already have, and will also help you continue to use food as a running performance advantage– in your training, your racing, and all the adventures in between.

Eating Slows You Down- Until It Doesn’t

If you’ve ever tried eating while running, then I don’t need to tell you why it’s one of the least desirable necessities in long distance running. Even if you haven’t tried it, you can probably imagine what it’s like. Your body– including your stomach– are bouncing down the street or the trail as you run. Even a small amount of food or water sits heavy in the stomach, sloshing around with each passing stride.

Yet food is the fuel that powers us forward. When we’re burning calories as quickly as we do when we’re running, shouldn’t some instinct kick in that leaves us not just tolerating, but craving our next gel or oatmeal bar?

Appetites aside, the body’s ability to adjust to the increased output that comes with running, or any endurance exercise, is remarkable. As we settle into a sustainable effort– meaning anything we could sustain for longer than a few minutes– our blood rushes to deliver oxygen from our lungs to the hard working muscles in our legs and core,  which propel us forward. It’s the same reason that our heart rate and breathing rate increase; our body has to work hard to deliver the fuel that’ll keep us going.

When we’re burning calories as quickly as we do when we’re running, shouldn’t some instinct kick in that leaves us not just tolerating, but craving our next gel or oatmeal bar?

The problem is, our blood plays just as instrumental of a role in digestion. Not only does it deliver oxygen to working muscles in the gut, but it also has to “pick up” the calories and nutrients that we absorb through digestion.

When we eat while running, we force our body to split its effort across two tasks simultaneously. One is receiving fuel, and the other is delivering it. The body can learn to do both, but the more strain we put on our working muscles in our legs, the less resources our body has to give to digestion, and the reverse is also true.

Simple graphical representation of how running intensity and ability to eat are related

What we find is that digestion requires resources we could otherwise use to propel ourselves forward. The faster we run, the greater our physical output, and the harder that our heart, lungs, and blood need to work to meet the demands of that output. We decrease the remaining resources that we can devote to digestion when we increase our running effort. As we slow down and settle into a more sustainable running effort, we free up resources that our body can then allocate towards effective digestion. Said another way, if the body has 10 points total to allocate to digestion and working muscles, we can put all those points into digestion, or put all of them into running faster, or split the points in some combination of the two.

It’s this trade-off that has led to the divergence of ultrarunning fueling (more solid foods containing fat and protein) from road marathon fueling (almost exclusively simple sugar gels, blocks, and drinks mixes). We’ll come back to this contrast a little later.

The skill of eating while running is trainable.

Eating while running is a specific fitness. We can be really good at eating, and really good at running fast, but still not be capable of keeping both ships afloat at the same time. The good news is that, like any kind of fitness, the skill of eating while running is trainable.

Carbohydrates Matter Most

Gummy worms- a common simple sugar chosen by long distance runners

Okay, so we know that our body has to split its resources between eating and running. Now how do we put that into practice?

The shortest answer is that if we want to allocate as few resources to digestion as possible, simple carbohydrates (i.e. sugars, glucose, fructose) win out. 


First, simple carbohydrates are small molecules that the digestive system can easily break down into usable units (glucose) and absorb. The faster digestion can occur and the quicker this absorption can take place, the better.

Second, of the three major building blocks that humans rely on (protein, carbohydrates, and fat), carbohydrates are in the shortest supply, yet they are the preferred fuel for our body to turn to for energy production. Protein, on the other hand, is digestively intensive to absorb and even once it’s done, the human body struggles to generate energy from it. Alternatively, fats are very rich in energy, but we all already have an abundance of it in our bodies. As a result, there’s no need to consume supplemental fat. We can use the fat we already have stored in our bodies but we can also burn it in parallel with carbohydrates to produce energy at a much higher total rate.

Unlike fat, we can store very limited quantities of carbohydrates in the body for use in endurance exercise, and we deplete this store of carbohydrates to a problematic level usually within 2 or 3 hours of steady exercise.

Training the Gut

Athlete eats an energy gel to fuel for his best performance

Two valuable takeaways stand out when we look at how digestion and running intensity interact. The first is that we can train to get better at eating while running, and the second is that the length and intensity of our race day efforts play a significant role in the fueling strategies available to us. 

Training the gut requires practice. Eating while running, or “in-run fueling”, is a skill that’s most important for long efforts (more than about two hours). The length of these efforts puts us at risk of depleting our internal stores of carbohydrates. Learning to eat while we run can delay that depletion.

In any good training schedule, the training will mirror the demands of the goal event. The same is true for in-run fueling. Long-and-slow workouts that are meant to get our legs more comfortable with the lengthy nature of the event we’re working towards, also provide the ideal opportunity to train in-run fueling. Above all, it’s a place for experimentation with all the elements you have control over: how to fuel, when to fuel, and what to fuel with, so that on race day you’re dialed into a tested, reliable race-day fueling strategy.

As a starting point, here are the practices we recommend for making in-run fueling an asset to your performance capabilities:

How to fuel 

Experiment with your calorie-carrying systems. It is usually best to keep some plain water available rather than committing all of it to an energy drink. Carry 20% more calories than you think you need. The benefit of having them available if you need them will outweigh any extra effort associated with carrying them.

What to fuel

Races up to 5 hours long should be run on simple carbohydrates alone. Athletes in ultra endurance efforts (5 hours or longer) can start to add fats and proteins to their in-run fueling plan. Research seems to indicate that adding fat into the fueling plan will translate to better performance. Protein will start to benefit performance only in multi-day efforts.

Start with small quantities of fat or protein ingested alongside your staple simple-carb fuels. Drink mixes and gels typically win the day in events that are shorter than 5 hours, are run in extreme heat, or at high altitude (for some individuals).

Below are some common foods used as athletes introduce each macronutrient category into their fueling strategy:

Carbohydrate-rich fuels

  • Energy gels, chews and drinks (Skratch, Honey Stinger, Gu, Clif branded products)

Fat-rich fuels

  • Potato chips

  • Cheese quesadilla

  • Peanuts/ Peanut Butter

  • Pizza

Protein-rich fuels

  • Milk

  • Hamburgers

  • Eggs

Race fueling strategy
The author's fueling plan for Speedgoat Mountain Races

When and where to fuel

Experiment with fuel consumption frequency, eating consistently every 30 to 60 minutes. Top competitors in events over 200 miles still adhere to this fueling schedule most of the time. In flatter races, the best time will be whenever running effort is lower. In hilly or mountainous races, try to eat just before a large climb or while power hiking uphill, as they can be the best opportunities for calorie intake and in a place where the energy will be strongly needed (energy expenditure increases exponentially on steep hills). We also recommend that as a starting point,  in-run fueling should be used on a recurring interval of ~25g every 30 to 45 minutes, starting at the beginning of the run, for any run anticipated to take longer than 90 minutes. Aerobic base impacts the efficiency of carb utilization and may stand to lower your need for fueling in-run calories.

The Rewards of Gut Training
Crossing the finish line fast thanks to a solid fueling plan

Athletes and coaches have often quipped that “long distance racing is an eating contest”. With what we know about how the body responds to food while running, that statement is not so far off.

Learning to eat while you run isn’t an easy task, but it gets easier when you have a strategy. If you commit to training your gut the way you’ve committed to training your legs, we know you’ll be rewarded. With practiced in-run fueling, you stand to realize one or all of the following benefits:

  • Reduced risk of GI distress on race day

  • Reduced risk and/or avoidance of glycogen depletion

  • Improved overall running performance as a result of high fuel intake without GI distress

  • Improved fitness adaptation (e.g. muscular fatigue resistance) as a result of greater output than without exogenous carb supplementation

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