Fasted Training: 3 Common Myths (and What the Evidence Says)
Training first thing in the morning before breakfast, often called “fasted training”, tends to split opinion. Some athletes build it into their routine, while others wouldn’t consider starting a session without eating first.
So what’s actually true?
What do we mean by fasted training?
In simple terms, it’s training after an overnight fast of around 10–14 hours.
By that point, liver glycogen is reduced, while muscle glycogen is still available (depending on what you ate the day before). That’s why most easy sessions still feel manageable but anything harder quickly becomes a struggle.
The idea behind fasted training is to shift the body towards using more fat as fuel and stimulate certain endurance adaptations.
1. “Training fasted leads to muscle loss”
This is one of the biggest concerns, and it’s often overstated.
Even in a fasted state, your body doesn’t suddenly start breaking down muscle for energy. You still have muscle glycogen available, and that’s what fuels most low- to moderate-intensity training.
Muscle breakdown only becomes a real issue if you push things too far, for example, long duration, higher intensity, or repeatedly training without adequate fuelling and recovery.
In practice, most people will feel fatigue, heavy legs, or a drop in performance well before that point.
The takeaway:
An easy fasted session won’t cost you muscle. Poorly fuelled hard training might.
2. “Fasted training is the best way to lose weight”
This is where things often get misunderstood.
Yes - fasted training increases fat oxidation. You’ll rely more on fat as a fuel source compared to training after eating.
But that doesn’t automatically translate to better fat loss.
Research consistently shows that while you may burn a higher percentage of fat when fasted, total performance - how hard and how long you can train - is better when you’re fuelled. And over time, it’s total energy expenditure that matters most.
The takeaway:
Fasted training can support fat loss, but it’s not superior. Consistency and overall energy balance are key.
3. “Fasted training improves endurance”
This is where a bit of nuance helps.
Fasted training can encourage useful adaptations. It can increase fat utilisation, support mitochondrial development, and improve your ability to switch between fuel sources – often referred to as metabolic flexibility.
But there’s a trade-off.
Sessions tend to feel harder, and you won’t be able to train at the same intensity or volume. That means lower-quality work if you rely on it too often.
And importantly, there’s limited evidence that these adaptations translate into meaningful improvements in race performance.
The takeaway:
It’s a useful tool but not a replacement for well-fuelled, high-quality training.
So, when does fasted training make sense?
Used well, it can fit nicely into a training week.
It works best for easy, steady sessions, that is, low-intensity work where the goal is aerobic development rather than performance. Early mornings, base phases, or lighter training days are where it tends to sit best.
What it’s not suited for is anything that depends on quality, such as intervals, strength sessions, technical work, or race-specific training. Those sessions need fuel if you want to get the most out of them.
What about eating afterwards?
You don’t need to panic about a strict 30-minute “window”, but you also don’t want to delay refuelling unnecessarily.
Eating within an hour or two is a sensible approach, particularly if you’re training regularly. A mix of carbohydrate and protein will help restore glycogen and support recovery.
Final thoughts
Fasted training isn’t a shortcut but it’s not something to avoid either.
Used occasionally, it can support specific adaptations and add variety. Used too often, or in the wrong context, it can limit training quality and recovery.
As with most things in training, the key is simple:
match the fuelling strategy to the goal of the session.
Easy session? Fasted can work well.
Hard session? Fuel it properly.