How the Body Uses Fuel

What’s Included?

  • FIRST: CARBS AS FUEL
  • THEN: FAT AS FUEL
  • LAST: EACH RUNNER IS DIFFERENT

Carbs As Fuel

The carbs you consume are first broken down by the body & either used immediately or stored for energy later in the form of glycogen (basically sugar). Carbs are your body’s preferred form of fuel because they are the EASIEST for your body to create, store, and use again when the time comes.

Glycogen is stored primarily in your liver and muscles – and it’s used whenever we engage in movement or activity. Think of it like a car needing a different amount of gas to get to different locations – the longer or faster that you’re planning to run or exercise, the more carbohydrates you’ll need stored up for easy use.

The body typically uses some amount of carbs to fuel ALL types of movement and exercise, but the DURATION and the TYPE of workout will dictate the rate at which your muscle glycogen reduces. The greater intensity & the longer the duration, the faster your glycogen will run out. For example – if you’re completing sprints or intervals, your body may quickly run through these glycogen stores in the active muscles, even though it’s a shorter workout (think of it like punching the gas pedal and breaks repeatedly while driving vs. driving at a constant easy speed). In addition, during races or high intensity training runs, your body will rely heavily on the carbs you have stored for energy – because it’s the easiest. When these carbs deplete, you may experience a decrease in your performance – often called “hitting the wall.” For this reason, it’s important to ensure that your carb levels are completely topped off before these types of workouts AND ensure that you’ve practiced mid-run fuel so your levels never deplete. However, one important thing to note is that your body typically takes about 16-24 hours to improve your glycogen stores after they’re depleted – so it’s important to practice the TIMING of your fuel strategies as well.


Fat As Fuel

Fat could be called the body’s secondary fuel source. It’s not the most efficient energy source, so the body doesn’t automatically turn to it in the same way that it does for carbohydrates. However, much like with carbs, the body will typically burn SOME amount of fat while engaging in any type of movement or activity – this is because our body moves through different intensities & will switch course accordingly with the type of fuel it uses (making it a more fluid and dynamic process vs. a static one or the other).

Typically, your body relies on fat as fuel when we engage in LOWER intensity workouts and exercise (easy runs, walks, low intensity movement). In addition, the body will rely on fat in the absence of carbs – like if a runner chooses to run fasted in the morning. When your body DOES have to turn to fat in the absence of carbs, depending upon how efficient your body is at using fat for fuel, you may find yourself “hitting the wall” or “bonking” because your body isn’t able to use the fat as fuel fast enough to maintain your energy levels.

You CAN become more efficient at using fat as fuel – so you’re less likely to hit the wall or bonk when your body turns to fat for fuel. A few ways that runners can do this:

  1. Running more: The more you practice running, the more miles you run, and the more frequently you train – the more efficient your body will become. This doesn’t mean you should go out and run as much as you can – it means you should simply aim to get a little more consistent and add a little more mileage gradually.
  2. Fasted running: While fasted running also can come with MANY drawbacks and should be done with caution (and the expertise of a Registered Dietitian) – it will in fact improve your body’s ability to use fat as fuel when done correctly and safely. But again, there are many drawbacks when this is done incorrectly & for extended periods, so PLEASE check with a medical professional before engaging in this strategy.
  3. Keep a balanced diet daily: You can follow the performance plate method for daily fueling – adjusting your carbs, proteins, and veggies/fruits depending upon the work you’re doing. It’s important to remember that while you do need carbs daily, your typical daily plate doesn’t need excess carbs. Rather, your plate should be filled with predominantly fruits and vegetables.

Each Runner is Different

It’s so important to remember that each and every runner is unique and different – and our fuel needs & strategies will vary drastically depending upon our goals, the type, amount, and intensity of our exercise, and our personal health, genetics, preferences, and lifestyle. Figuring out which fuel strategies will work best for YOU can take PRACTICE, LEARNING, ADJUSTING, and TIME. If you feel that nutrition and fuel is holding you back, working with a Registered Dietitian could help you to get faster results & learn more about your personal needs.


Amy Haas
Run with Aim, LLC
UESCA Certified Running Coach
NASM Certified Personal Trainer
NASM Certified Nutrition Coach

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