The Pros and Cons of Fasted Workouts

Fasted training and intermittent fasting are creating quite the buzz with the promise of shedding fat and performance benefits. And seemingly, it’s catching fire by the masses, so what could be the harm? If you’ve ever run out the door for an early morning workout without grabbing a quick bite, then you’ve done a fasted workout. It’s okay; it happens to the best of us. Some athletes consciously forgo eating before training, especially early morning runs, since they feel better running on an empty stomach. While others admit 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 those who eat breakfast, then go all day without eating before an evening workout.

Some professionals say exercising on an empty stomach is the magic behind a leaner, meaner fat-burning machine, yet others warn against it. So, let’s sift through the chatter and find what works best for you.

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The benefits of fasted workouts

The allure of fasted workouts hinges on the promise of burning more fat as fuel, weight loss, a leaner physique, and enhanced performance. Sounds appealing, right? 

Because glycogen stores are in limited supply, fasted training forces the body to utilize fat as fuel – which is the main objective. Over time with adaptation, the body becomes very good at burning fat for fuel as opposed to glycogen (stored carbs in the liver and muscles), providing sustainability during longer aerobic workouts. Reliance primarily on fat for fuel versus carbohydrates (carbs) delays the immediate risk of bonking and helps reduce dependence on supplemental fuel. All this to say, the theory of burning fat over carbs supports weight loss and a leaner physique – on paper anyway. 

By and large, 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, a caveat: the body is smart! In a fasted state, training the body to burn fat will also promote intramuscular fat storage, and over time, this plan will backfire.

The case against fasted workouts

Although fat is the primary fuel source in fasted, aerobic workouts (with glucose a close second), the body utilizes a blend of fuel sources (fat, carbs, and protein) for energy production, depending on the workout intensity and duration. In non-fasted endurance training, protein contributes approximately 5% of energy utilized. However, in fasted training, our muscle protein breakdown is double that in a non-fasting state. Repeatedly breaking down muscle tissue for energy leads to decreased resting metabolic rate, reduced strength, poor performance, and ultimately 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 stress hormones such as cortisol. Elevated cortisol levels wreak havoc on the body, causing profound fatigue, poor recovery, abdominal fat storage, glucose intolerance, systemic inflammation, depressed thyroid function, and reducing your ability to relax or sleep. Break your fast by eating just enough to lower cortisol levels and top off blood glucose so the body doesn’t think it’s starving. Plus, having a small snack beforehand allows easy access to carbs and free fatty acids, so you can physically hit top-end efforts in training, enhancing fitness. Bottom line: If you don’t have carbs to pull from, the quality of your workouts will suffer by shortchanging your energy and ability to work hard. 

As for the alleged endurance benefit that fasted training promises, research shows athletes who fueled before and during endurance sessions could perform aerobically for longer than in a fasted state.  

Athletes tend to underestimate the body’s need for fuel and therefore sacrifice carbs in a 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 among the athletic population from novice to pros. Any time an athlete restricts food to improve body composition or performance, it signals an alarm. Be mindful not to be led astray by concepts that involve withholding food/fuel from your body.

What female athletes need to know about fasted workouts

Research suggests the consequence of negative energy availability among female athletes comes at a higher price than their male counterparts. Not to suggest fasted training is appropriate for male athletes, but females have a different hormonal makeup that sets them apart. There is the follicular and luteal phase in the female monthly cycle, low and high hormones, respectively. In the luteal or high hormone phase (day 15-28), both estrogen (anabolic) and progesterone (catabolic) are elevated. Estrogen promotes fatty acid oxidation and spares glycogen. So, as you see, the female athlete is naturally an efficient fat burner since this occurs monthly for decades. In addition, progesterone dampens the body’s ability to store glycogen, so in the high hormone phase, when estrogen and progesterone are elevated, the body instinctively leans on fat over carbs for fuel. During the high hormone phase, fueling should be prioritized depending on the intention of the training session.

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When is it OK to train in a fasted state?

You are not alone if you can’t stomach eating before a run. So, it’s acceptable to go in a workout fasted – some of the time – as long as the effort is easy, 60 min or less in duration, the only workout of the day, and you are well hydrated.

On the other hand, 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. So, consider the intention of the workout before heading out the door without grabbing a quick bite.

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Ideas for a quick snack – approximately 100 calories consisting of 20-25 grams easy to digest carbs, low fat, and fiber, with sodium and a small amount of protein. Examples include applesauce, white toast, banana, rice cake with jam and PB, one waffle, ½ sports bar, figs, dates, 2-3 sports chews, graham crackers, or vanilla wafers.

During high-intensity sessions and those lasting 75 min or more, it’s best to top off blood sugar before the workout. Fuel with a sports hydration beverage and possibly supplemental fuel during the session, depending on the duration and intensity. In addition, fueling provides an opportunity to test drive race day fueling/hydration and train the GI system to digest fuel at race-day efforts. 

On long training sessions, it would be wise to simulate race day with a “pre-race” breakfast within a 1–3-hour window before you head out. Why wait until race day to test drive your pre-race meal when you have many opportunities in training? 

Prioritize a post-workout snack within 45 minutes after high-intensity, long sessions, and strength-based workouts or if you cannot eat a meal after the workout. Aim for approximately 25-grams protein with simple carbs, low in fat and fiber. 

Examples include whey or a vegan protein shake with fruit, Greek yogurt with granola, a protein bar, chocolate milk with a pastry.

The final bite

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, what is the end goal?

Here’s a concession: If you want to include a “fast” in your dietary regimen, consider fasting from after dinner until the pre-workout snack or breakfast the following morning. That will deter mindless night snacking and help shed those unwanted pounds.

Susan Kitchen is a Sports Certified Registered Dietitian, USA Triathlon and Ironman Certified Coach, accomplished endurance athlete, and published author. She is the owner of Race Smart, an endurance coaching and performance nutrition company that works with athletes across the globe as they strive toward optimal health, fitness, and performance.