by Coach Paul Warloski
Early morning training can be a beautiful thing—the roads are quiet, the air is crisp, and you can knock out your workout before the day gets crazy.
But should you eat first?
How long can you ride or run fasted before performance suffers?
Is training empty really making you faster?
The bottom line is that there might be some body composition value in fasted training if you’re just doing an easy endurance workout.
But if you’re doing any kind of intensity, you need some carbohydrates.
What’s Happening in Your Body During an Overnight Fast
Liver Glycogen: The Morning Reality
After 8-12 hours without food, your liver glycogen drops about 20-25%.
This isn’t dramatic depletion—it’s a normal, expected decline.
The research shows that extending your fast from 9.5 to 16 hours doesn’t cause much additional liver glycogen loss, so that overnight window is pretty stable.
Muscle Glygogen: Largely Untouched
Here’s the good news: muscle glycogen stays essentially stable during overnight fasting.
Since muscle glycogen primarily gets depleted through activity and intensity (not passive fasting), your leg muscles are still loaded with fuel from your last meal.
Blood Glucose: Your Body’s Backup System
Fasting blood glucose typically stays in the 70-100 mg/dL range in healthy athletes.
Your liver maintains glucose through glycogen breakdown and glucose production, while the “dawn phenomenon” (early morning hormone surge) can actually push glucose slightly higher.
How Long Can You Maintain Performance Fasted?
The Sweet Spot: 45-75 Minutes
Most endurance athletes can maintain stable performance for 45-75 minutes of morning training after an overnight fast, especially at low-to-moderate intensity (Zone 1-2).
The Intensity Factor
- Zone 1-2: 60-90 minutes of stable performance
- Tempo/Threshold (Zone 3): Performance decline starts around 20-40 minutes
- High-intensity intervals (Zone 4-5): Noticeable decline almost immediately if unfueled
Why It Feels Like a Cliff
Athletes often describe fasted morning sessions as “I felt great for 45 minutes, then the lights went out.”
That’s not psychology, that’s physiology.
Blood glucose stays stable until liver glycogen drops below a threshold, then performance plummets suddenly.
Fasted Training: Worth the Hype?
What the Research Actually Shows
✅ Yes, Fat Oxidation Increases
Multiple studies confirm that training with low carbohydrate availability:
- Increases fat oxidation rates during exercise
- Boosts activity of fat-burning enzymes
- Enhances mitochondrial biogenesis (PGC-1α expression)
- Improves metabolic flexibility
❌ But Performance Doesn’t Improve
Here’s the disconnect: becoming a better fat burner doesn’t necessarily make you faster. Research consistently suggests:
- No improvement in time trial performance
- Reduced ability to hit high power outputs
- Blunted training quality during intervals
As sports nutrition expert Louise Burke puts it: “You become a better fat burner, but a worse carbohydrate user.”
When Fasted Training Actually Has Value
Use It For:
- Zone 1-2 aerobic development rides
- Easy recovery sessions under 90 minutes
- Base building periods
- Body composition goals (when combined with overall energy balance)
Avoid It For:
- Interval sessions of any intensity
- Race preparation
- Performance-focused training blocks
- Back-to-back training days
- High-stress periods (especially masters athletes)
Your Early Morning Fueling Playbook
Easy Aerobic Sessions (Zone 1-2)
Duration: 45-75 minutes
- Fasted training is fine and can be beneficial
- Focus on staying in aerobic zones
- Listen to your body—if you feel flat, fuel next time
Duration: 75-90 minutes
- Consider a small snack if you wake up sluggish
- Bananas, dates, or a small energy bar work well
- The goal is maintenance, not peak performance
Quality Sessions (Tempo/Intervals)
Don’t train fasted for performance work
- Eat 30-60 minutes before starting
- 30-60g carbohydrates works for most athletes
- Better to start slightly overfueled than underfueled
Long Rides (90+ minutes)
Always fuel appropriately
- Start with a good breakfast 1-2 hours before
- Bring fuel for rides over 90 minutes
- Use your normal race-day fueling strategy
The Bottom Line
Use the “Fuel for the Work Required” principle:
- Easy work: can be trained fasted or with minimal fuel
- Quality work: must be well-fueled to maintain performance
- Race preparation: never compromise with fasted training
Fasted training can be a valuable tool for developing aerobic capacity and metabolic flexibility.
But it’s a tool, not a universal strategy.
My recommendation: Start your easy morning rides fasted and see how you feel.
If you’re hitting your power targets and feeling good, you’re doing it right.
If you start feeling flat around the 45-minute mark, grab a small snack next time.
Three Things to Know About Fueling Before Morning Workouts
- You can maintain stable performance for 45-75 minutes of morning training fasted, but high-intensity sessions require fuel to prevent early performance decline.
- Fasted training increases fat oxidation and metabolic flexibility but doesn’t improve race performance – it’s valuable for easy aerobic work, not for intervals or quality sessions.
- Use the “Fuel for the Work Required” principle: train easy days fasted when beneficial, but always fuel properly for tempo, intervals, races, and performance-focused training.
Need more?
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Paul Warloski is a:
- USA Cycling Level 1 Advanced Certified Coach
- RRCA Running Coach
- Training Peaks Level 2 Coach
- RYT-200 Yoga Instructor
- Certified Personal Trainer
- Certified Nutrition Advisor
