Fuelling Fasted Training: When It Helps, When It Hurts, and How to Do It Safely

Fasted training has become one of those habits endurance athletes either swear by or avoid completely. Roll out of bed, skip breakfast, ride or run for an hour, then eat. Done properly, it can be a useful tool. Done badly, it leaves you flat, grumpy and slower than if you had simply eaten a bowl of porridge. The difference comes down to intensity, duration and what you do afterwards.

If you are searching for advice on fasted training for endurance, the honest answer is that it is a niche tool, not a default. This guide covers what the research suggests, which sessions suit it, which absolutely do not, and how to fuel around it so you do not wreck your next workout.

What Fasted Training for Endurance Actually Does

After an overnight fast, your liver glycogen is partly depleted. It holds roughly 80–100g of carbohydrate when full, and it is what keeps your blood glucose steady while you sleep. Muscle glycogen, typically 300–500g in a trained athlete, is largely untouched by an overnight fast, so you are not starting from empty. You are starting with a lower liver reserve and no incoming carbohydrate.

The theory behind fasted sessions is simple: train with low carbohydrate availability and your body is nudged to improve fat oxidation and certain cellular adaptations, such as increased mitochondrial signalling. This is the basis of the "train low" approach. Studies do show these signals can be amplified when carbohydrate is scarce.

The catch is that better signalling in the lab has not reliably translated into better race performance. Several trials comparing fasted and fed training over several weeks found little or no difference in time-trial performance. Meanwhile, the cost is real: lower intensity capacity, higher perceived effort and, for some people, more disrupted recovery.

It is also worth remembering that fat oxidation is highly trainable without fasting. Consistent aerobic volume, sensible pacing and enough sleep all improve your ability to burn fat at moderate intensity. You can build that engine while still eating normally, and you will usually feel far better doing it.

Which Sessions Suit Fasted Training

Fasted work is best kept to sessions where fuel demand is low and quality is not the point. Think of it as a small, deliberate dose rather than a lifestyle.

• Easy aerobic rides or runs of 45–75 minutes at conversational pace, well below your first threshold. Fat oxidation is already high here.
• Short recovery spins where the goal is movement, not load.
• Technique or skills sessions with low energy cost.

A sensible limit for most athletes is one, perhaps two, fasted sessions a week during base training. Beyond that, the accumulated under-fuelling starts to outweigh any benefit.

Sessions You Should Never Do Fasted

Anything hard needs carbohydrate. Intervals, threshold work, sweet spot blocks, hill repeats and race-pace long runs all rely heavily on glycogen. Training these fasted means you cannot hit the power or pace that creates the adaptation in the first place, and your perceived effort rockets. You also raise the risk of poor session quality across the following days.

The same goes for anything over about 90 minutes. Once a session runs long, even at easy intensity, you will want carbohydrate coming in. Current guidance for longer endurance work is 30–60g of carbohydrate per hour, rising to 60–90g per hour for events beyond 2.5 hours when using multiple carbohydrate types. Our guide on how many carbs per hour goes into the numbers if you want the detail.

Fasted training is also a poor fit if you are already struggling with low energy availability, disrupted sleep, frequent illness, or a drop in performance you cannot explain. In those cases, eat more, not less.

One practical test: if you cannot hold a full conversation during the session, it is too hard to do fasted. Another is how you feel the next day. If you wake up heavy-legged, hungry and unmotivated after a fasted session, scale it back. Your body is giving you useful feedback, and listening to it is part of being a good athlete rather than a stubborn one.

How to Fuel Around a Fasted Session

The session itself is only half the story. What you do in the hour afterwards decides whether the fasted workout was a smart move or a self-inflicted hole.

Before: Drink water, and a coffee if you tolerate it. Add a pinch of salt if you are a heavy sweater or training in warm conditions. Do not go fasted if you feel light-headed or shaky on waking.

During: Keep a small emergency supply in your pocket. If you start to feel dizzy, cold or weak, eat straight away. Taking 20–25g of fast carbohydrate will usually bring you back within ten minutes. Fasted training is a choice, not a test of toughness.

After: Eat within an hour. Aim for roughly 1g of carbohydrate per kilo of body weight alongside 20–30g of protein. Skipping this step is where most athletes go wrong. A fasted session followed by another long gap without food drives up fatigue and can blunt your next session.

It also helps to think about your whole day. If you train fasted in the morning and then under-eat through the afternoon, you have created a large energy deficit without meaning to. Spread your carbohydrate intake sensibly, and make sure the day's total supports the training load.

What We Use at FTP Endurance Fuel

We do not recommend going fasted for anything demanding, so the first thing we pack for any session is fuel. For easy fasted sessions, we still carry a small pocket stash as a safety net. For everything harder, fuelling starts early and continues steadily.

Our Mixed Fruit Energy Pastilles are easy to portion: each pastille delivers a small, predictable dose of carbohydrate, which makes them handy as a safety net on a fasted run or ride, and just as useful for hitting your hourly target once the session gets longer. Our Tart Cherry Energy Pastilles are the same fuel in a tart cherry flavour; real Montmorency tart cherry is in Power Fluids. If you want to try before committing to a bulk pack, the Taster Pack is a low-risk place to start.

The Bottom Line

Fasted training can be a useful, occasional tool for easy aerobic sessions, but it is not a shortcut to fitness and it is certainly not required for endurance success. Keep it short, keep it easy, limit it to once or twice a week, and refuel properly afterwards. For hard sessions, long days and race preparation, fuel properly and train the way you intend to race.

Ready to build a fuelling routine that works whether you train fed or fasted? Browse our bundles and stock up so you always have something in your pocket.