Fasted training and intermittent fasting are creating a buzz with the promise of fat loss and performance benefits. And apparently, it’s going up in flames by the masses, so what could be the harm? If you’ve ever rushed out the door for an early morning workout without grabbing a quick bite, then you’ve done a fasted workout. It’s fine; It happens to the best of us. Some athletes consciously forgo eating before training, especially when running early in the morning, as they feel better running on an empty stomach. While others admit that pre-fueling is not high on their list of priorities.
The 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 those who eat breakfast and then go all day without eating before an evening workout.
Some pros say that exercising on an empty stomach is the magic behind a leaner, more efficient fat-burning machine, but others warn against it. So let’s break down the talk and find what works best for you.
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The benefits of fasted workouts
The appeal of fasted workouts hinges on the promise of burning more fat for fuel, weight loss, a leaner physique, and better performance. Sounds attractive, right?
Because glycogen stores are in limited supply, fasted training forces the body to use fat for fuel, which is the primary goal. Over time and with adaptation, the body becomes very good at burning fat for fuel instead of glycogen (carbohydrates stored in the liver and muscles), providing sustainability during longer aerobic workouts. Relying primarily on fat for fuel rather than carbohydrates (carbs) delays the immediate risk of bonking and helps reduce reliance on supplemental fuel. All this to say that the theory of burning fat instead of carbohydrates supports weight loss and a leaner physique, at least on paper.
In general, the research is clear; Training on an empty stomach improves the ability to tap into fat stores earlier and burn a higher percentage of fat during training sessions.
A word of warning though: the body is smart! On an empty stomach, training the body to burn fat will also promote intramuscular fat storage, and over time, this plan will backfire.
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The Case Against Fasted Workouts
Although fat is the primary fuel source in fasted aerobic workouts (with glucose a close second), the body uses a combination of fuel sources (fat, carbohydrate, and protein) for energy production, depending on intensity and intensity. the duration of the training. In resistance training without fasting, protein contributes approximately 5% of the energy used. However, in fasted training, our muscle protein breakdown is double that of a non-fasted state. Repeatedly breaking down muscle tissue for energy leads to a decreased resting metabolic rate, reduced strength, poor performance, and ultimately injury.
Training on an empty stomach to delay or avoid hitting may seem like a good idea, but research warns that it’s a major physiological stressor on the body. Athletes who train with low energy experience elevated levels of 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 reduced ability to relax or sleep. Break your fast by eating enough to lower cortisol levels and top up your blood glucose so your body doesn’t think it’s starving. Plus, eating a small snack beforehand allows for easy access to carbohydrates and free fatty acids, so you can physically reach maximum efforts in training and improve fitness. Bottom line: If you don’t have carbs to pull from, the quality of your workouts will suffer by lowering your energy and ability to work hard.
As for the purported endurance benefit that fasted training promises, research shows that athletes who fueled before and during endurance sessions could perform aerobic exercise for longer than fasted.
Athletes tend to underestimate the body’s need for fuel and therefore sacrifice carbohydrates in a carb-phobic world. Low-energy training can indicate restricted eating and can lead to disordered eating or a full-blown eating disorder. Relative energy deficit in sport (RED-S) or low power availability it is widespread among the athletic population, from beginners to professionals. Any time an athlete restricts food to improve body composition or performance, it’s a red flag. Be careful not to get carried away with concepts that involve retaining food/fuel from your body.
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What female athletes need to know about fasted workouts
Research suggests that the consequence of negative energy availability among female athletes comes at a higher price than their male counterparts. I’m not suggesting that fasted training is appropriate for male athletes, but women do 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 hormone high phase (day 15-28), both estrogen (anabolic) and progesterone (catabolic) are elevated. Estrogen promotes fatty acid oxidation and spares glycogen. So, as you can see, the female athlete is naturally an efficient fat burner, as this occurs monthly for decades. Additionally, progesterone reduces the body’s ability to store glycogen, so in the hormone surge, when estrogen and progesterone are elevated, the body instinctively relies on fat instead of carbohydrates for fuel. During the high phase of hormones, feeding should be prioritized depending on the intention of the training session.
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When is it okay to train fasted?
You’re not alone if you can’t eat before you run. Therefore, it is acceptable to perform a fasted workout, part of the time, as long as the effort is easy, 60 minutes or less in duration, the only workout of the day, and you are well hydrated.
On the other hand, topping up your blood sugar after an overnight fast increases blood glucose and energy, improves mental clarity and mood, allows your body to better access carbohydrates and free fatty acids and hinders muscle breakdown during the session. So consider the intent of the workout before you head out the door without grabbing a quick bite.
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Quick Snack Ideas: Approximately 100 calories consisting of 20-25 grams of easy-to-digest carbohydrates, low in fat and fiber, with sodium and a small amount of protein. Examples include applesauce, white toast, banana, rice cake with peanut butter and jelly, a waffle, ½ sports bar, figs, dates, 2-3 sports chews, whole-grain cookies, or vanilla wafers.
During high-intensity sessions and those lasting 75 minutes or more, it’s best to top up your blood sugar before training. fuel with a sports hydration drink and possibly supplemental fuel during the session, depending on the duration and intensity. Additionally, fueling provides an opportunity to test fueling/hydration on race day and train the GI system to digest fuel in race day efforts.
In long training sessions, it would be wise to simulate race day with a “pre-race” breakfast within a 1-3 hour window before heading out. Why wait until race day to take a test drive? your meal before the race when you have many opportunities in training?
Prioritize a post-workout snack within 45 minutes of long high-intensity sessions and strength-based workouts or if you are unable to eat a post-workout meal. Try to consume about 25 grams of protein with simple carbohydrates, 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 cake.
the final bite
When in doubt, always go back to basics. Ask yourself, are you eat enough carbohydrates, protein, and fat meet energy demands, maintain health and optimize performance. Is this an eating regimen that you can or should maintain for life? And, if it is not sustainable, what is the ultimate goal?
Here’s a trade-off: If you want to fit a “fast” into your dietary regimen, consider fasting from after dinner through your pre-workout snack or breakfast the next morning. That will deter mindless late-night snacking and help shed those unwanted pounds.
Susan Kitchen is a Sports Certified Registered Dietitian, USA Ironman and Triathlon Certified Trainer, accomplished endurance athlete, and published author. she is the owner of smart runa performance nutrition and resistance training company that works with athletes around the world in their quest for optimal health, fitness and performance.
