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Running On Empty? What to Know About Fasted Training

Runners stretching their legs before a race

Fasted training sessions and intermittent fasting are creating quite the buzz with the promise of weight loss and performance benefits. Seemingly everyone is doing it, so what could be the harm?

If you’ve ever jumped out of bed for an early morning workout without eating and didn’t grab a gel or bar, then you’ve done a fasted workout. Some athletes consciously forgo eating since they feel better running on an empty stomach, while others claim pre-fueling isn’t high on their priority list.

Scientific literature defines fasted training as not eating within 10-14 hours before a workout. For most athletes, this applies to their morning workout or to someone who eats breakfast and then goes all day without eating before working out.

Some experts say training on an empty stomach is the best-kept secret, yet others warn against it. So, let’s sift through the chatter and find what works best for you.

A man makes breakfast with a bagel, peanut butter and banana before a race

Is There a Benefit to Running On Empty?

The allure of fasted running hinges on the promise of burning more fat as fuel, weight loss, a leaner physique and enhanced performance.

However, because our glycogen stores are limited, fasted running forces the body to utilize fat. Over time, with adaptation, the body will learn how to burn fat, versus glycogen, providing sustainability during longer aerobic runs.

Primarily relying on fat for fuel versus carbohydrate delays the immediate risk of bonking and helps reduce reliance on supplemental fuel. All this to say, the theory of burning fat over carbs supports weight loss and a leaner physique.

Collectively, the research is clear: Training in a fasted state improves the ability to tap into fat stores sooner and burn a higher percentage of fat during training sessions.

However, beware—the body is smart. In a fasted state, training the body to burn fat will promote intramuscular fat storage, and over time, this plan can backfire.

The Red Flags

Although fat is the primary fuel source in fasted, aerobic workouts, depending on the workout intensity or duration, the body will select the best fuel sources (fats, carbs and protein) for energy production.

In non-fasted endurance training, protein contributes approximately 5 percent of energy. However, in a fasted state, the amount of protein breakdown in muscles is double. Breaking down muscle tissue leads to a decrease in resting metabolic rate, a reduction in strength, poor performance and can lead to injury.

Training in a fasted state to delay or avoid bonking may sound like a good idea, but research warns it’s a major physiological stressor for the body. Athletes who train under fueled experience elevated cortisol levels, deep fatigue, poor recovery, abdominal fat storage and systemic inflammation.

Break the fast by eating just enough to bring cortisol levels down and allow the body to access carbs and free fatty acids so you are physically able to hit top-end efforts in training.

Athletes tend to underestimate caloric needs and sacrifice carbs in our carb-phobic world. Training under-fueled can signal restricted eating and may lead to disordered eating or a full-blown eating disorder.

Relative energy deficiency in sport (RED-S) or low energy availability is rampant in the athletic population from novice to pros, and any time an athlete restricts food to improve body composition or performance, it signals an alarm.

Finally, research suggests the consequence of negative energy availability among female athletes comes at a higher price than male counterparts. Not to suggest fasted training is appropriate for male athletes, but females are negatively affected by their hormonal makeup.

In the female monthly cycle, there is the follicular and luteal phase (low and high hormone, respectively). In the luteal or high hormone phase (days 15-28), both estrogen (anabolic) and progesterone (catabolic) are elevated.

Estrogen promotes fatty acid oxidation and spares glycogen. Therefore, the female athlete is an efficient fat burner since this occurs monthly for 35+ years.

Progesterone dampens the body’s ability to store glycogen, so in the high hormone phase, the body instinctively leans on fat over carbs for fuel. During the high hormone phase, fueling needs change depending on the intention of the session.

A wearing a sports bra stretching her arm

Is It OK to Run In a Fasted State?

If you can’t stomach eating before a run, it’s OK to sometimes go into the session fasted as long as the effort is easy, 60 min or less in duration and you are adequately hydrated.

However, topping off blood sugar after an all-night fast boosts blood glucose and energy, improves mental clarity and mood, allows the body to better access carbs and free fatty acids, and hinders muscle breakdown during the session.

Go with approximately 150 calories made of 20-25 grams of simple carbs, low fat and fiber, with sodium and some protein.

Examples include:

  • applesauce
  • white bread
  • banana
  • non-sweetened instant oatmeal with almond milk watered down to drink
  • rice or potatoes
  • salted sports bar
  • figs
  • dates
  • protein latte
  • 2-3 sports chews

Fuel during your run with sports hydration. This type of fuel also provides an opportunity to test drive your race day nutrition plans.

On long runs, it would be wise to simulate race day with a “pre-race” breakfast within a one- to three-hour window before you head out. Why wait until race day to simulate how you fuel for the race?

Prioritize a post-workout snack within 30 min after high intensity, long and strength-based workouts. Aim for 25-gram protein with some simple carbs, low in fat and fiber. Examples include a whey protein shake, Greek yogurt, protein bar or chocolate milk.

When In Doubt, Go Back to the Basics

When in doubt, always go back to the basics.

Ask yourself, are you eating enough carbs, protein and fat to meet energy demands, maintain health and optimize performance? Is this an eating regimen you can or should keep for life? And, if it’s not sustainable, then what’s the end goal?

Here’s a concession, if you want to include a “fast” in your dietary regimen, consider fasting from right after an early dinner until the pre-run snack or breakfast the following morning. That will deter mindless night snacking, likely void of nutrients and help shed those unwanted pounds.


By Susan Kitchen. Susan is a Sports Certified Registered Dietitian, USA Triathlon Level II Endurance Coach, IRONMAN Certified Coach, published author and founder of Race Smart, a sports nutrition and coaching company.

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