Fasted Training Exposed: What the Science Really Says

Estimated read time: 6.2 minutes (about as long as it takes to scroll through every fasted training debate on Reddit and still feel confused). 😉

Fasted Training 101 — What It Actually Does 🧪

Fasted training = working out after 8–12 hours without food (usually early morning).

Here’s what we know:

Fasted sessions increase fat oxidation during exercise by 20–48%.

Translation: your body burns more fat during the workout for fuel.

But...

More Fat Burning ≠ More Fat Loss

You can use more fat as fuel in a fasted session and still gain fat overall. That depends on your daily energy balance.

If you're eating more than you burn, even with fasted workouts, fat loss won’t happen.

The “Train Low” Theory: Cool But Limited

Research shows that low-carb training can boost mitochondrial enzymes and fat-burning.

Cool for cellular adaptation. But performance gains? Meh.

A 2021 meta-analysis of 407 studies found no consistent endurance performance benefit for trained athletes.

You may get better at using fat, but not necessarily better at racing.

Fasted Running — When It Helps, When It Hurts 🏃‍♂️

Fasted running can work, but only in specific situations.

Easy Runs (<90 min): Totally Fine

Your glycogen stores can fuel short, low-intensity runs.

If you feel better running before breakfast or it fits your schedule, go for it.

Just don’t combine fasted runs with underfueling the rest of the day. That’s burnout territory.

Long Runs & Workouts: Fuel or Fail

Fasted long runs reduce:

• Power output

• Time to exhaustion

• Ability to hit prescribed paces

Miss your target pace repeatedly in a training block? That adds up, and costs you key adaptations.

Runner Quick Guide

👉 Fine to fast: 

• Easy runs <90 min

• Low-intensity base building

• Early AM sessions to reduce GI distress

🚫 Fuel required: 

• Long runs >90 min

• Workouts >60 min (tempos, intervals, race pace)

• Double days or high-volume training blocks

Fasted Lifting — Gains Without Breakfast? 💪

Strength Gains: Still Happen

You can lift fasted and build strength or muscle, as long as your overall daily nutrition is solid.

But Quality Might Suffer

Common fasted lifting issues:

• Fatigue or dizziness

• Less total volume

• Higher perceived effort

For strength and hypertrophy, quality drives results. If you consistently push less weight or do fewer reps while fasted, those small deficits might hurt progress.

One study found fed lifters did more reps and that gap grew over time.

Small drop in reps or weights might seem minor. But over months? It compounds.

The Bottom Line for Lifters

If you feel good lifting fasted and hit your numbers, no issue.

But if you want to squeeze out every last ounce of strength or hypertrophy: fuel up.

That pre-lift snack might be a 1% gain that adds up to a big edge.

High Volume = High Recovery Needs ⚠️

If you train a lot, you probably already struggle to eat enough.

Add fasted training and you're digging the recovery hole deeper.

Underfueling consistently can trigger RED-S:

• Hormonal dysfunction

• Bone loss

• Poor recovery

• Increased illness risk

I don’t recommend fasted training for high-volume athletes. Especially during double days or intense training blocks.

That banana before your 6AM session? It’s recovery insurance.

Skip it too often and you're shortchanging your next session and your long-term progress.

When to Train Fasted: A Practical Framework 🎯

What Fasted Training Does ✅

• Increases fat use during workouts

• Works fine for short, easy sessions

• Provides some cellular adaptations (but limited performance payoff)

What It Doesn’t Do ❌

• Lead to more fat loss if calories are equal

• Consistently improve endurance performance

• Benefit long or high-intensity workouts

What It Risks ⚠️

• Lower session quality

• Decreased training volume

• Harder time meeting calorie needs

• Compromised recovery in high-volume training

Should You Train Fasted? Ask Yourself:

1. Am I eating enough daily? If not, skip the fasted workouts.

2. What’s the goal of this session? If it's high quality or long—fuel up.

3. How’s your recovery? Poor sleep, low energy, mood dips? Eat.

How do you feel fasted? Some thrive. Others crash. Use your data.

Practical Summary: Quick Reference Guide

🎯 Key Numbers to Know:

• 1–4g carbs/kg bodyweight 1–4 hours before key sessions

• Fat oxidation during workout < Daily calorie balance

• More volume = more gains (fuel helps you get there)

Jonah’s Take: 

For most athletes and most sessions, training fasted is a personal preference. If it feels good and you’re fueling well, you’re probably fine.

But if you're an elite endurance athlete or chasing peak strength and hypertrophy, timing matters more. Strategic fueling around key sessions could give you a small but meaningful edge. 

As with most things in exercise science: it depends (because of course it does).

💬 One last thing before you go.

After the NFL, I wasn’t sure if obsessing over performance science still mattered—until I started helping this community.

Your training goals? They brought that fire back.

So if you're feeling stuck, second-guessing something, or just want to share how training's going—hit reply. I read every message, and I’m here to help however I can.

🚨 Zone 2 Isn’t Magic. But It’s Not Useless Either

Did you miss my post about when Zone 2 actually matters? You can find it below!

I won’t lie. These posts take me a while to make. If you find it helpful, share it on your story or with a friend. It helps me a ton!

Don’t forget: You + Science = AWESOMENESS 😎

Yours in science,

Jonah

Please email me directly if you’re interested in references for this week!