Whether it’s for a colonoscopy prep, a religious observance, bloodwork, intermittent fasting, or something else entirely, many athletes find themselves needing, or choosing, to train while fasting. At Sonic Endurance, we know endurance athletes are a dedicated bunch (sometimes stubbornly so). The good news? Training while fasting can be done safely and effectively in many situations. The key is knowing when to adjust, when to push pause, and when to talk it through with your coach.
Let’s break it down.
First Things First: Not All Fasts Are the Same
“Fasting” can mean a lot of different things.
- Short fasts (12–24 hours): common before medical procedures like colonoscopies or lab work.
- Daily fasting windows: such as intermittent fasting with a limited eating window.
- Religious fasting: which may include no food, no water, or both for portions of the day (or longer).
- Extended fasts: lasting multiple days.
Each situation changes how your body fuels activity, which means your training plan may need to flex a bit.
When It’s Usually OK to Train
In many cases, low-to-moderate intensity training is perfectly fine during shorter fasts, especially if you’re already accustomed to endurance training.
Examples that usually work well include:
- Easy aerobic runs or rides
- Technique-focused swims
- Mobility work or strength sessions with lighter loads
- Shorter recovery workouts
Your body is actually pretty good at tapping into fat stores for fuel during easier efforts. Many athletes even report feeling surprisingly good during these sessions.
But remember: easy means easy. A fasting workout isn’t the day to chase PRs.
When It’s Smart to Adjust
There are definitely times when modifying your training is the better move.
Consider adjusting if:
- The workout is high intensity (intervals, tempo, long threshold work)
- The fast involves no fluids
- The fast is longer than 24 hours
- You’re feeling lightheaded, unusually fatigued, or mentally foggy
- Weather conditions are hot or humid
In those cases, adjustments might look like:
- Swapping a hard workout for an easy one
- Shortening the duration
- Moving the workout closer to when you can refuel
- Taking a recovery day
A small adjustment today often keeps the larger training plan on track.
Talk to Your Coach (Don’t Guess)
One of the biggest mistakes athletes make is trying to quietly “figure it out” on their own.
Your coach would much rather hear:
“Hey, I’m fasting tomorrow for a medical test” than see a workout marked “Did not complete” or “Felt terrible” after the fact.
Coaches can help:
- Adjust intensity or timing
- Shift key workouts around your fasting window
- Prevent you from digging an unnecessary fatigue hole
- Make sure the bigger training cycle stays intact
Training is a long game. Communication keeps it that way.
Safety Always Comes First
Even experienced athletes should keep an eye on a few key signals when training while fasting.
Watch for:
- Dizziness
- Shakiness
- Unusual fatigue
- Confusion or trouble focusing
- Rapid heart rate at easy effort
If you experience these, stop the workout. There will always be another training day.
Also remember:
- Hydration matters—especially if fluids are allowed during your fast.
- Heat and humidity increase stress on the body.
- Medical procedure fasts often come after restricted eating the day before, which compounds fatigue.
Sometimes the smartest training decision is a walk and a nap.
A Little Perspective
Missing or modifying one workout because of fasting will not derail your season. In fact, sometimes the forced recovery is exactly what your body needed.
Endurance training is built over months and years, not a single Tuesday interval session.
And who knows—you might even discover that an easy fasted workout feels surprisingly smooth.
The Bottom Line
Training while fasting happens. Life, faith, health care, and schedules all intersect with training at some point.
The winning strategy is simple:
- Keep easy days easy
- Adjust when needed
- Communicate with your coach
- Prioritize safety
Most importantly, remember that endurance training should enhance your life—not complicate it.
If you have an upcoming fast and aren’t sure how it fits with your training plan, reach out to your Sonic Endurance coach. We’re here to help you navigate the miles—fed, fasted, and everything in between.
Now go enjoy the process. And when that fast ends… maybe celebrate with a really good
