Fat or Sugar: What Do You Burn When You Run?

On Thursdays we work the most important muscle we own: our heart.  Without it, we most obviously wouldn’t be alive. Just like every other muscle we ask you to exercise, your heart needs exercise too. How do we exercise it? We get it pumping. This is why we do Tabata intervals, sprints, running around/over/through whatever is in our path. We want to keep our heart strong.

Even though getting your heart pumping is a good thing, you can do it both inefficiently and efficiently. To understand this, I’d like to dive into a little more detail of what happens when your heart rate rises above a sitting level. As a preface, I’m sharing knowledge learned from my degree in Kinesiology, and 4 years experience as a metabolic specialist where I’ve done hundreds of exercise assessments using gas exchange (one of the most accurate ways to know what’s going on inside your body).

Your body uses two different fuel sources while you do cardio: fat and sugar. If you’ve ever read a nutrition label, you’ve probably seen that fat has 9 calories per gram, while sugar has 4 calories per gram. This means that inside the same amount of fat lies a little more than double the fuel of sugar. 9 calories versus 4 calories. We obviously burn calories when we exercise. Now, if you were going for a long run, which energy source would you prefer to use based on the above information? Probably fat. It will last longer. And heck, we seem to store a lot of it on our bodies! Why not use it up when we run?

Sadly, when we start exercising, we don’t automatically burn fat at high intensities of exercise. We burn mostly sugar. The higher our heart rate gets, the more intense the exercise is, the shorter we feel we can last, and the more sugar we burn. Also, as our heart rate rises, the percentage of fat we burn drops. The more deconditioned we are, the more quickly fat burn drops. When we’re primarily burning sugar as a fuel source, we start feeling uncomfortable, like we DON’T want to keep going. On the other end, when we burn a higher percentage of fat, we feel comfortable, like we could last for a long time. For example: most individuals burn 70% or more fat while walking (although they don’t burn many overall calories, because walking is not intense).

If you’re one of those people who dread running because it feels so hard, my guess is that it feels hard because you are burning well over 50% of your calories from sugar as you run. Thankfully, it’s possible to change this! We can teach our bodies to burn more fat at a higher intensity. We do this by frequently bringing your heart rate back down to where you burn a high percentage of fat, then pushing it up again. This is also known as “interval work” (sound familiar?).

Practical application: When you do cardio, don’t spend all your time at a high intensity. Recover your heart rate by slowing down for a couple minutes, back to a comfortable place (this may be walking for some of you, or a much slower pace if you’re a seasoned runner). When you’re feeling recovered, increase your intensity. On the weekends, do a recovery workout at a lower intensity than you do during the week. When you’re recovering, it won’t feel like you’re doing much work. Let me assure you, after seeing hundreds of exercise assessments that showed me the exact percentage of fat and sugar burn, your body is doing very valuable work during the recovery time. Imagine if we only drove our car at a high intensity. It would adapt, and it would respond, but it wouldn’t be the most efficient way to drive. Your body is similar. It will adapt to working on mostly sugar at a high intensity, but it’s not the most efficient way to exercise. Vary the intensity of your cardio, spend time recovering, and within a few months, enjoy the difference it can make.

-Trainer Melissa Madgsick