Should You Train Fasted? What Endurance Athletes Need to Know

Why Fasted Training Is on Every Endurance Athlete’s Radar

Fasted training - typically performed 8 to 12 hours after your last meal, is gaining traction among endurance athletes aiming to optimise fat utilisation, metabolic flexibility, and possibly performance. But is training without food always beneficial?

Whether you're prepping for an Ironman, marathon, or long-distance cycling event, this guide explains the physiology of fasted workouts, the benefits of fat adaptation, and how to use fasted training as a smart, periodised tool rather than a default approach.

Fasted Training Benefits and Limits

Increases Fat Oxidation During Exercise

Fasted sessions reliably increase fat use during aerobic exercise by up to 20–48% compared to fed conditions (Achten & Jeukendrup, 2004). This metabolic shift can enhance fat-burning efficiency, especially in low-to-moderate intensity training zones where aerobic metabolism dominates. Over time, these adaptations may help athletes rely more on fat and spare glycogen in long races.

Does Not Guarantee Greater Fat Loss

Higher fat oxidation during a session doesn’t automatically translate to body fat reduction. Fat loss is governed by total energy balance, not the fuel substrate used during workouts. In a landmark review, Schoenfeld and Aragon (2015) concluded that total caloric deficit, not whether training is fasted or fed, drives fat loss. In other words, you can burn more fat during a workout and still gain fat if you're over-consuming calories overall.

May Enhance Mitochondrial Adaptations

“Training low” exercising with reduced carbohydrate availability, may upregulate molecular pathways involved in mitochondrial biogenesis and lipid metabolism. Philp et al. (2012) demonstrated that low-glycogen training can stimulate greater activation of AMPK and PGC-1α, two critical regulators of endurance adaptation. For athletes in base-building phases, these enhancements could improve aerobic capacity and fuel efficiency.

Doesn’t Consistently Improve Performance

Despite cellular adaptations, a 2021 meta-analysis of 407 studies found no consistent performance improvements in trained athletes from fasted or low-glycogen training (Impey et al., 2021). Athletes may become better at using fat, but if carbohydrate availability is too low, performance metrics such as time to exhaustion and power output can decline, particularly in high-intensity sessions.

What Endurance Athletes Gain from Strategic Fat Adaptation

In ultra-endurance racing, the ability to burn fat efficiently while preserving glycogen can offer a significant advantage. Joel Friel, author of The Triathlete’s Training Bible, emphasises the use of low-carb or fasted workouts during base phases to train the body to become metabolically flexible. The goal is to increase fat utilisation during long, steady-state efforts, reducing reliance on carbohydrate and potentially delaying the onset of fatigue.

Rauch et al. (2021) reinforced this concept in a study published in Frontiers in Physiology, which found that elite endurance athletes oxidised fat at high rates (~0.6–1.0 g/min) even when carbohydrates were consumed before and during exercise. The key factor behind their fat-burning efficiency was not immediate diet, but long-term training adaptations. These findings suggest that structured low-glycogen training, rather than acute dietary manipulation, is what drives meaningful improvements in fat oxidation.

The takeaway is clear: fasted training can improve metabolic efficiency and spare glycogen but only when used intentionally within a structured season plan.

When to Use Fasted Training in Your Season

Fasted training is a tool with specific use cases, not a daily routine. It’s most beneficial when used in alignment with training phases and session objectives.

When It Works Best

• During base phases: Aerobic conditioning efforts at low intensity are ideal for stimulating fat metabolism.
• Short, easy runs or rides (<90 min): These sessions don’t demand large glycogen reserves and are perfect for fasted experimentation.
• Non-critical sessions: Use fasted training when pace, power, or output targets are not the priority.

When to Avoid It

• High-intensity workouts: Intervals, tempo runs, or race-pace sessions require carbohydrate availability to perform at target intensities.
• High-volume training blocks or double days: Energy and recovery demands are high; fasted training can widen the recovery gap.
• Periods of poor sleep, illness, or elevated stress: Fasted training can compound recovery issues, particularly when under fuelling is already present.

Before committing to a fasted session, ask yourself: What is the objective of this workout? If the goal is performance quality, then fuel accordingly.

Strength & Hypertrophy: Fasted Lifting Insights

Gains Still Happen

Fasted strength training is possible and some athletes do maintain or build muscle in a fasted state. As long as total daily calories and protein intake are sufficient, muscle growth can still occur (Schoenfeld et al., 2017). However, fasted lifting comes with important trade-offs, especially when progression and quality matter.

Training Quality May Drop

Research by Areta et al. (2013) showed that protein and carbohydrate consumption before resistance training significantly increased muscle protein synthesis. Practically, athletes training fasted often complete fewer reps or use lower loads per session. While these drops may seem minor, over time they result in less training volume and diminished hypertrophy or strength gains.

Symptoms Can Disrupt Performance

Fasted lifting, especially in the morning, may lead to symptoms of low blood sugar, dizziness, nausea, or reduced focus, especially during compound movements or longer sessions. If these symptoms are frequent, a small carbohydrate-rich snack can significantly improve workout quality and safety.

Recovery Signals May Be Dampened

Muscle repair relies on both mechanical stimulus and timely nutritional support. Delaying protein or carbohydrate intake post-lifting can blunt the anabolic response. For athletes training frequently or under higher loads, consistently training fasted can compromise recovery and adaptation over time.

Small Deficits Add Up

Performing slightly less volume or lifting marginally lighter weights per session may not seem like a concern but over a month or season, these cumulative deficits can slow progression and increase the risk of stagnation or overtraining. The compound effect is real.

The Bottom Line: If you lift well fasted and feel great, there's no need to change. But if you're chasing progress in strength or size, a simple pre-lift snack can make a measurable difference.

Research Summary

• Fat burning increases during fasted exercise, especially at lower intensities, but doesn’t always translate to improved race-day performance.
• Energy balance is the primary driver of fat loss, not whether you train fasted or fed.
• Mitochondrial adaptations may be enhanced through strategic low-glycogen training, particularly in base phases.
• Training quality and recovery depend heavily on carbohydrate availability in high-volume or high-intensity periods.

Carbohydrate Fuelling Guideline: Getting It Right

Recommended intake:
1–4 grams of carbohydrate per kilogram of bodyweight, 1 to 4 hours before a key training session. This flexible range supports different session timings and individual digestive tolerance.

For a 65 kg athlete:

• 65 g carbs (1 g/kg) ~1 hour before training
• Up to 260 g carbs (4 g/kg) ~4 hours before a long brick or race effort

Why Carb Type and Timing Matter

Carbohydrate-rich meals should be:

• Low in fibre (e.g., no raw vegetables or whole grains)
• Low in fat (skip fried foods and nut butters)
• Moderate in protein
• High in fast-digesting carbs (white rice, toast, banana, cereal, sports drinks)

The goal is to optimise glycogen availability without gastrointestinal distress, especially within 1–2 hours of training.

Examples by Timing

• 4 hours prior: Oats with banana and honey, rice with chicken, cereal with milk
• 2 hours prior: Toast with jam, rice bowl, cereal with almond milk
• 1 hour prior: Banana, energy bar, muffin, sports drink (30–60 g carbs)
• <30 minutes: Gel, sports drink, white bread with jam

For Athletes Who Struggle with Solid Food

Use liquid carbs like smoothies or sports drinks for early morning or high-intensity sessions. They digest quickly and still provide necessary fuel.

Practical Framework: Should You Train Fasted?

Ask yourself:

• What’s the session objective? If it's low-intensity base work, fasted may be appropriate. If it’s intense or long, then fuel up.
• How’s your recovery? Poor sleep or fatigue signals a need for better nutrition support.
• Am I under-eating? Fasted sessions add stress to an existing energy deficit.
• Do I perform well fasted? Use your own data, and monitor performance, energy, and mood.

Final Thoughts

Fasted training is neither good nor bad - it’s a strategic option. Used sparingly and purposefully, especially during base training, it may enhance fat adaptation and metabolic flexibility. But overusing fasted sessions can compromise recovery, training quality, and long-term progress.

Periodise your nutrition like your training. Know when to train fasted, and when to fuel for performance.