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Running on Empty: When Is Fasted Running Actually Worth It?

Running on Empty: When Is Fasted Running Actually Worth It?

Published on: 23 Sept 2025

Author: Phil Knox

Categories: Nutrition & Recovery

Some call it “fasted cardio.” Others call it “madness before 9am.” But whatever your take, running without eating first, also known as fasted running, has become one of those trendy ideas that floats around every now and then, usually accompanied by phrases like “fat burn” and “metabolic efficiency.”

So… is there any actual benefit? Or is it just a polite way of saying I hit snooze and didn’t have time for toast?

What Even Is Fasted Running?

Fasted running means heading out for a run after a decent overnight fast, typically 8 to 12 hours with no food, just water or black coffee. No sneaky banana, no nibble of toast, no last minute jelly baby on the way out the door. It’s you, your stomach, and your glycogen depleted liver having a quiet argument.

When It Might Actually Help

There is some logic to it:

  • Teaches your body to burn fat more efficiently
  • Can improve metabolic flexibility over time
  • Useful for slow, easy runs (think Zone 2)
  • Good for people training for ultras or longer events where fuelling gets tricky

But the key word here is easy. Fasted running is best suited for low intensity sessions where the goal is time on feet, not smashing splits or vomiting on a hill.

When It’s a Terrible Idea

  • Speed sessions
  • Long runs over 90 minutes
  • Back to back hard days
  • If you’re already tired, under fuelled, or mildly cranky

Running fasted in these cases isn’t “smart fat adaptation.” It’s just under eating and expecting your legs to perform a miracle.

Also worth noting: fasted running can increase cortisol levels. That’s fine in moderation, but not ideal if you’re already stressed, sleep deprived, or juggling life like a clown with a coffee addiction.

Does It Burn More Fat?

Technically, yes, during the run itself. But overall fat loss still comes down to total energy balance, not whether you ran on empty. Don’t fall into the trap of thinking fasted means better. It just means different.

Final Word

Fasted running can be a useful tool, but it’s not for everyone, and it’s not magic. If you try it, keep the intensity low, keep the distance modest, and for the love of God, eat something afterwards.

And if you just want to run for the joy of it with a slice of toast in your belly and no existential dread? That’s perfectly valid too.

 

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