Gabriella Rao, RD

In light of hearing from various people about their prior practice of fasted exercise, I wanted to touch on the importance of fueling before going out for a run (or bike ride, lifting session, etc.). 

The dangers of fasted workouts:

Many of us are morning runners, and it can be difficult enough just to roll out of bed in the morning and get out the door, let alone get up early enough to grab something to eat before we head out. However, it is SO important that you grab a carb-based snack before your workout, whether that be even just a banana, a couple of graham cracker sheets, half a bagel, a gel, etc. Fueling with some quick carbs will let your body and brain know that you have broken your overnight fast. Whereas if you go out without eating anything, your body becomes stressed due to its lack of fuel which results in dysregulated levels of cortisol. 

Cortisol is a stress hormone that is necessary in normal amounts. Its levels fluctuate throughout the day under normal, well-fueled and well-rested circumstances. Cortisol normally peaks early in the morning when you wake up, then starts to decline throughout the day. However, fasted workouts can cause cortisol levels to remain elevated throughout the day, since your body is freaking out a bit about the deficit it’s in. Consistently working out without fueling beforehand results in chronically elevated cortisol, which can wreak havoc throughout many systems of the body. Chronically elevated cortisol can result in persistent fatigue, poor sleep, increased anxiety and irritability, brain fog, a suppressed immune system, decreased endurance or muscle gains, increased fat storage, and decreased performance in sport. 

Okay, so how do we avoid the detriments of fasted workouts?

As I mentioned before, we must FUEL! I understand that it can be hard to get something in before early morning workouts, but it’s a necessity. If you’re worried about GI distress, start with something small and over time, you’ll find that you can tolerate a bit more carbohydrate before your workouts. Just like you train your cardiovascular system for endurance efforts, you can also train your GI tract to get used to fueling more. The table below provides some guidelines around fueling before a workout based on how much time you have beforehand. 

Lately, my go-to pre-run snacks have been graham cracker sheets, pop-tarts, or raisin bran cereal. All of these foods are primarily carbohydrate-based, with a bit of protein and fat. I typically don’t have a ton of time to digest before I get out for a morning run, and I find that these foods are easy on my GI system. After my morning workout, I will of course top up with a balanced breakfast of carbs + protein + fat!

Hope this was helpful! Happy fueling!