Fueling for the long run

If you are training for a longer race in the spring, you are probably starting to hit some significant long runs on a regular basis at this point. Right around this time in each training season, I start to field a number of questions related to long run fueling. When should I take calories with me? Is there benefit to doing long runs without fuel? How much fuel and how often? This post is a summary of the approach I recommend as a coach to my runners.

Before we talk specifics, here are a few key fueling concepts:

1. The energy system that your body relies upon is dependent upon intensity.
2. Fueling during training and competition should be planned based on time and an intensity not distance and pace.
3. There is a benefit to training your system to efficiently use BOTH energy systems.

Energy System Overview

For simplicity’s sake, let’s focus on the two major energy systems. The vast amount of energy stored in your body is fat, and with proper training you will be relying mostly on this system. When the intensity of your run is relatively low (around 65% of maximum heart rate), you rely almost exclusively on this energy system. As the intensity increases, the ratio of reliance on your other energy system that requires carbohydrate fuel (glucose) also increases. Most importantly, keep in mind that all activity relies somewhat on BOTH fat and carbohydrate fuel, but the ratio changes as the intensity changes. Keep it simple: More easy = more fat fuel. More hard = more carb fuel. The mere presence of carbohydrates in your system doesn’t lead to more carbohydrate use. The intensity is the key.

Focus on Time and Intensity

In all things fueling, get your mind off of thinking in terms of distance. Your energy needs depend on how hard you are running, and for how long you need to maintain that intensity. An elite marathoner should be able to easily run a 3-hour marathon with no carbohydrate fuel, because the intensity of that 3-hour run is relatively low. A weekend warrior, however, may be running a 3-hour marathon at 80% of their maximum heart rate, so they will require carbohydrate replenishment to prevent a bonk. When planning your fuel intake, think in terms of how long you will be running (duration), and how hard you will be running (intensity).

Train Both

A current fad in the endurance community is to push the benefits of training your body to rely on ONLY fat. This is done through a combination of diet (chronic carbohydrate deprivation) and training approach (chronic aerobic intensity). While there is evidence that this approach does increase the body’s reliance on fat as fuel over time, there isn’t evidence that this is beneficial for athletes who want to train or race in a mixed energy system situation. For example, studies have repeatedly shown that diets too low in carbohydrates lead to poorer performance in intense interval workouts and races (particularly those that surpass lactate threshold intensity), likely due to chronic glycogen depletion. In other words, the fat engine does get bigger, but the carb engine no longer exists for the intensities for which it is intended. In my opinion, I want athletes to have access to all their gears. So, we should train both systems to work efficiently, and to work well together. For that reason, some long runs should be very easy and purely aerobic. Others should include some race pace/intensity. Some long runs should be done without fuel, and others should be done with fuel.

So, long run fueling. Here’s a basic rundown of how I think things should be approached, and why:

If you’re running a new distance for the first time in a training cycle, you can fuel at your discretion. This is because you don’t know how your body might respond to the new distance. If you start feeling like you need a boost anytime after an hour, take in 100-300 calories per hour. Because of the distance, you may think you’re running truly easy intensity, but over time it may progress to moderate intensity that is relying on some carbohydrate fuel.
Once you build up your long runs beyond 2 hours with some regularity, you can go with minimal or no fuel for long runs (really as long as you want). The caveat here is this: You must keep these un-fueled long runs sufficiently easy that you are only relying on fat as fuel. That means conversational effort the whole way, or monitoring your intensity with a heart rate monitor and being very diligent to not stray above 65-70% of your max heart rate. I think it’s a good idea to eat breakfast before these runs, so you have some fuel in your body just in case you dip into carb-burning at points.
Anytime you are running long with significant quality (anything more than strides), you should fuel throughout the run to keep carbohydrate reserves topped up. Since your plan for the run is to include a mixture of fat-burning and carb-burning intensity, you want to have access to both fuel sources. Take in 200 calories or so per hour. This also serves the purpose of training your stomach for taking in calories at race intensity.
Lastly, anytime you complete any “race simulator” workouts, follow your planned race intake schedule with the actual fuel you plan to use for the race. This is to ensure that you have managed the logistics and are used to taking in what you’re going to use on race day.

When you’re heading out for your next long run, just ask yourself: How much time am I running, and how hard am I running? Based on the time and intensity, you can make a quick decision about whether or not you need to take fuel.

-Coach Caleb