You wake up early, rush to make your morning workout, and decide to skip breakfast to save time. You feel fine during the session, but does this mean fasted training is a sustainable or effective practice? While some athletes swear by it, experts warn that the reality is more nuanced.


Understanding Fasted Workouts

A fasted state is generally defined as having gone without food for 8 to 12 hours or longer. Historically, proponents suggested that exercising in this state could boost fat oxidation, enhance muscular oxidative capacity, and accelerate weight loss. However, contemporary sports science indicates that the benefits might be outweighed by potential drawbacks for many individuals.


The Impact on Performance and Muscle Growth

Your body relies on three primary energy pathways—carbohydrates, fats, and proteins. High-intensity anaerobic activities, such as HIIT or heavy lifting, require readily available carbohydrates. When you train in a fasted state, these glycogen stores are limited.


According to exercise physiologist Grace Horan,

"Athletes may see decreases in how much work they can do during a session, or experience earlier fatigue. It may reduce training quality for some individuals and it delays the availability of amino acids needed to support muscle protein synthesis."


While some small studies have shown negligible differences in short-term performance, high-endurance capacity often suffers. Specifically, long-duration exercise exceeding two hours can negatively affect your perceived exertion and VO2 max, potentially leading to lower-quality training sessions.


The Risks of Under-Fueling

Consistently skipping pre-workout nutrition can contribute to Relative Energy Deficiency in Sport (RED-S). This condition can interfere with hormonal health, bone density, recovery, and overall performance. For those engaged in serious endurance training like marathons, fueling with adequate carbohydrates and electrolytes is non-negotiable for sustaining performance and muscle mass.


Is Fasted Training Ever Appropriate?

For low-intensity, shorter aerobic sessions, some experts suggest there may be minor metabolic adaptations related to fat oxidation. Additionally, some research indicates that when paired with moderate exercise, intermittent fasting might assist in weight management by maintaining a calorie deficit. However, this is largely driven by total energy balance rather than the timing of the workout itself.


Expert Recommendations for Pre-Workout Fueling

To optimize your results, experts recommend consuming fuel at least one hour before exercise. Consider these options based on your training type:

  • HIIT or Sprint sessions: Quick-digesting carbs like a banana or applesauce.
  • Strength training: A combination of protein and carbs, such as Greek yogurt with fruit.
  • Moderate cardio: Toast with honey or jam.
  • Endurance training (>90 mins): A larger carbohydrate load, such as oatmeal with fruit.

Post-workout recovery is equally critical. Athletes should aim to replenish glycogen stores with carbohydrates and support muscle repair with protein within one to two hours after finishing their session. If you are considering fasted training, it is best to consult with a healthcare professional and monitor your body for signs of fatigue, dizziness, or performance decline.