Fasted Cardio: Thumbs Up or Thumbs Down?

The view from my morning LISS cardio on the stepmill. Because I know you care.

The view from my morning LISS cardio on the stepmill. Because I know you care.

Some people will tell you that early morning cardio on an empty stomach is one of the most effective ways to burn fat. Others say it’s catastrophic to muscle mass and will wreak havoc on your metabolism.

So which is it?

The answer, unfortunately, is not quite so simple. Just as there’s no diet or exercise plan that will perfectly fit everyone’s unique physiology and goals, fasted cardio isn’t going to work magic for everyone. There is no silver bullet and nothing is going to beat good, old-fashioned diet and exercise.

With that said — When I’m trying to get lean, I do fasted cardio. Why? Because it works for me. I’m going to attempt to give you the quick and dirty theory behind fasted cardio… so buckle in boys and girls.

Your blood glucose is naturally low in the morning because you’ve spent the last several hours in a fasted state. Blood glucose is your body’s first choice for energy. It’s the sugar floating around in your system waiting to be used (and if it’s not, your body will convert it to glycogen or fat). Glycogen, converted glucose that’s stored in your liver and muscles,  is your body’s second favorite fuel source. It’s easy to access and, like blood glucose, it can provide quick, efficient energy.

Then there’s fat. The red-headed stepchild of the energy source world. While it’s the most energy-dense source of fuel in our bodies, the process of converting stored fat to energy (a process called lipolysis) is cumbersome. For this reason, fat is slow-burning. No matter how much of it you’ve got, you can only burn it at the rate at which your body makes it available. Furthermore, if you’ve got glucose and glycogen available, your body will likely take the path of least resistance and use that instead.

That’s why fat can be so stubborn. You may hate it, but your energy pathways think they and fat are besties. So here’s where fasted cardio can help. Since you’re in a fasted state, fuel choice number 1 (glucose) is out. Now your body has to fuel movement with glycogen or…fat. I already said that glycogen is second in line — but most folks have enough glycogen stored in their bodies to fuel moderate activity for a couple hours (even more if you’re a trained distance athlete). Not only is doing several hours of cardio a day impossible for most people — it’s downright destructive to your muscle mass. You’ll acquire the body of a distance athlete — lean and sinewy. And that’s cool if you’re a marathoner or that happens to be the body type you prefer. However, if you’re going for a harder, more defined physique, you need that lean muscle. So, hours of daily cardio isn’t an option.

So then, how can you bypass the glycogen depletion phase and move right into fat burning when you’re fasted?

That’s where the intensity of your cardio comes in. You’ve gotta keep it low and steady, also known as “low intensity steady state” (LISS) cardio. Because fat is such a pain to convert to glycogen, it’s not as quickly available as glucose or glycogen. Think about it this way — if you went outside right now and ran a 400-meter sprint, your leg muscles would need a large amount of instantaneous energy to fuel that explosive movement. However, if you went for a brisk walk or light jog, you’d burn energy at a much slower rate – a pace much more in line with lipolysis. Studies indicate that keeping your heart rate around 65% of your max (calculated by subtracting your age from 220) can significantly increase the amount of energy your body burns from fat. This is helpful not only for burning fat, but also for sparing muscle. If you do high-intensity exercise in a fasted state, you’re begging your body to burn up muscle mass. It will slurp up your glycogen like a ravenous beast and then go on the prowl for muscle (which can also be broken down into usable energy more easily than fat).

But before you start dragging yourself to the gym for some empty stomach cardio, you’ve got to determine if it’s the best choice for your goals. Fasted, LISS cardio is good if you’re already relatively lean and just want to burn off a little fat. Because it’s low intensity, you won’t get the overall calorie burn that higher intensity exercise will produce. For example, I can burn around 400 calories during an hour of LISS. However, if I bump my intensity up to 80-85% of my maximum heart rate, my calorie burn will be closer to 600. Over the course of five days, that adds up to an extra 1,000 calories — which is pretty significant. So if you’ve got more weight to lose, it’s better to focus on that overall calorie burn and keep your intensity elevated than it is to engage in LISS. Higher intensity cardio also elevates your metabolism post-exercise, a state known as Excess Post-Exercise Oxygen Consumption (EPOC), but that’s a lesson for another day.

It’s important to remember that if you choose to go the higher intensity route, do NOT do it in a fasted state. Not only will you feel like crap, but you’ll also encourage muscle catabolism. And that’s a bad thing. Have a small snack before you do higher intensity cardio to help spare your hard-earned muscle.

So — thumbs up or thumbs down? It depends on where your body is and what your goals are. Fasted or not, I say early morning cardio is one of the best ways to start your day.