Here’s the “theory” behind fasting before a workout:

Working out when your blood sugar is low, e.g. first thing in the morning without breakfast, leaves your body with no choice but to burn fat for energy.

This premise sounds good.  Good enough to become popular among some bodybuilding, fat-loss, and even personal training communities.

But, it’s COMPLETELY FALSE.  In fact, fasting actually works against you.  Here’s why:

1. It’s not based on science.

In general, you should always be skeptical when people make claims that sound good, but haven’t been backed by scientific evidence. You don’t believe in those male enhancement commercials, do you? Fasting before working out is no different.

Repeated studies have shown no difference in body composition or lipid utilization (i.e. fat burn) during a workout between fasted or fed states.

Schoenfeld, et al. studied twenty healthy young female volunteers to ascertain whether fasted or fed states made a difference. Conclusion: they found no difference.

Gillen, et al. studied sixteen overweight women to see if fasted or fed states made a difference. Conclusion: “fed- versus fasted-state training does not alter [the] response.”  No difference.

2. You actually burn more calories throughout the day (i.e. Afterburn Effect) if you’ve eaten before.

Paoli, et al. performed an experiment with eight men to see what effects being fed or fasted had on lipid utilization 12-24 hours after a workout.  They found that the FED group experienced higher VO2 (oxygen utilization) both during the workout and 12-24 hours later, which implies greater excess post-exercise oxygen consumption (EPOC).  In other words, you burn more calories even after the workout ends by eating beforehand.

They concluded that when exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization. On the contrary, it’s actually better to eat before working out.

3. You’ll “hit the wall” very quickly during any moderate to high intensity workout.

Unless your body is in a state of ketosis, which can be very dangerous for your kidneys and liver if you’re not sure what you’re doing, your muscles need a steady supply of glycogen for energy.

Normally, your muscles store enough ATP and glycogen to do about 1-2 minutes of continuous moderate-to-high intensity exercise.

Afterward, your body relies on glycogen reserves in your liver, blood stream, and digesting food to quickly feed your voracious muscles.

When you have low blood sugar or no food being digested, you “hit the wall” pretty quickly during any moderate, HIIT, or other high intensity workout, because your body literally runs out of readily-available energy. This means less calories burned, less cardio, less musculoskeletal training.

4. You risk breaking down muscle protein for energy.

When training in a glycogen-depleted state, studies (here and here) have shown that one may actually break down muscle protein for energy during exercise.  Yes, fat may be broken down too.  But, why risk burning your own muscle for energy at all?

So, What Should You Do?

Simple! Before working out, have something healthy to eat. It can be as easy as a whole fruit and a protein shake. A protein bar with coconut water. Dried fruits and nuts.

Just don’t fast, please!

(photo source: 500px)