Rapid training and intermittent fasting are creating a buzz with the promise of fat burning and performance benefits. And it seemed to be set on fire by the masses, so what could the damage be? If you’ve ever run out of the door a day to exercise early in the morning without getting a quick bite, you’ve done a quick workout. he is fine; It happens to the best of us. Some athletes forgo pre-workout food, especially early morning runs, because they feel better running on an empty stomach. Others admit that pre-refueling is not at the top of their list of priorities.
The scientific literature defines fasting training as not eating within 10-14 hours prior to exercise. For most athletes, this applies to their morning workout or those who eat breakfast and then go all day without eating before their evening workout.
Some pros say working out on an empty stomach is the magic behind a leaner fat burning machine, while others warn against it. So, let’s scan the chatter and find what works for you.
Related: Is time-limited eating safe for athletes?
Benefits of fasting workouts
The allure of fast workouts hinges on the promise of more fat burning as fuel, weight loss, a leaner physique, and improved performance. It looks attractive, doesn’t it?
Since the stores of glycogen are limited, fast training forces the body to use fat as fuel – this is the main goal. Over time with adaptation, the body becomes very good at burning fat for fuel rather than glycogen (carbs stored in the liver and muscle), providing sustainability during long aerobic workouts. Relying primarily on fats for fuel versus carbohydrates (carbs) delays the immediate risks of overcrowding and helps reduce dependence on additional fuel. All this to say, fat-burning theory on carbs supports weight loss and a leaner physique — on paper anyway.
In general, the research is straightforward. Training in a fasting state improves the ability to utilize fat stores sooner and burn more fat during training sessions.
However, there is a caveat: the body is smart! In the case of fasting, training the body to burn fat will also enhance the storage of fat in the muscles, and over time, this plan will be counterproductive.
The case against fasting workouts
Although fat is the primary fuel source for rapid aerobic workouts (with glucose a close second), the body uses a combination of fuel sources (fats, carbohydrates, and protein) to produce energy, depending on the intensity and duration of the exercise. In non-rapid endurance training, protein contributes about 5% of the energy used. However, in fasting training, our muscle protein breakdown doubles in the non-fasting state. Frequently breaking down muscle tissue for energy leads to a lower resting metabolic rate, reduced strength, poor performance, and eventually injury.
Training in a fasting state to delay or avoid lying might seem like a good idea, but research warns that it is a major physiological stressor for the body. Athletes who train with a lack of fuel experience elevated stress hormones such as cortisol. High cortisol levels wreak havoc on the body, causing profound fatigue, poor recovery, belly fat storage, glucose intolerance, organ inflammation, depressing thyroid function, and diminishing your ability to relax or sleep. Break the fast by eating enough to lower cortisol levels and increase blood glucose so the body doesn’t think it’s starving. Plus, having a small snack beforehand allows easy access to free carbs and fatty acids, so you can put in a lot of effort into your training and boost your fitness. Bottom line: If you don’t have carbs to shed, the quality of your workouts will be affected by your lack of energy and ability to work hard.
As for the purported endurance benefit that fast training promises, research shows that athletes who feed before and during endurance sessions can perform aerobic exercise for longer than a fasting state.
Athletes tend to underestimate the body’s need for fuel, thus sacrificing carbs in a carb-deficient world. Under-fuel training can indicate eating restriction and may lead to an eating disorder or a full-blown eating disorder. Relative energy deficiency in sports (RED-S), or reduced energy availability, is rampant among athletes from beginner to professional. Any time an athlete restricts food to improve body composition or performance, it signals an alarm. Keep in mind not to be fooled by concepts that include withholding food/fuel from your body.
What math you need to know about fasting workouts
Research indicates that negative energy availability among female athletes comes at a higher price than their male counterparts. Not to suggest that speed training is suitable for male athletes, but females have a different hormonal makeup that distinguishes them from others. There is a follicle and yellow phase in the female menstrual cycle, low and high hormones, respectively. In the LH phase (day 15-28), estrogen (anabolic) and progesterone (catabolic) both rise. Estrogen enhances the oxidation of fatty acids and preserves glycogen. So, as you can see, the gym is naturally an effective fat burner because this has been happening monthly for decades. Plus, progesterone suppresses the body’s ability to store glycogen, so in the high hormonal phase, when estrogen and progesterone are high, the body instinctively leans on fats instead of carbohydrates for fuel. During the high hormone phase, refueling should be prioritized depending on the intent of the training session.
Related: Math menstrual cycle tracking
When is it acceptable to exercise in a fasting state?
You’re not alone if you can’t eat before your run. Therefore, it is okay to work out quickly – sometimes – as long as the effort is easy, 60 minutes or less, the only exercise of the day, and you are well hydrated.
On the other hand, consuming blood sugar after an overnight fast boosts blood sugar and energy, improves mental clarity and mood, allows the body to have better access to free carbohydrates and fatty acids, and hinders muscle breakdown during the session. So, consider your intent to exercise before you head out without getting a quick bite.
Related: I’m terrible about eating breakfast. How can I improve this bad habit?
Quick Snack Ideas – Nearly 100 calories consist of 20-25 grams of easily digestible carbohydrates, low in fat, fiber and sodium, and a small amount of protein. Examples include applesauce, white toast, bananas, rice cake with jam and PB, 1 cake, ½ sports bar, figs, dates, 2-3 sports chews, graham crackers, or vanilla chips.
During high-intensity sessions lasting 75 minutes or more, it’s best to lower your blood sugar before exercise. Fuel with a sports hydration drink and possibly additional fuel during the session, depending on duration and intensity. Additionally, refueling provides an opportunity to test drive race day fueling/hydration and train the GI system to digest fuel on race day efforts.
In long training sessions, it would be wise to simulate race day with a ‘pre-race’ breakfast within 1-3 hours before heading out. Why wait until race day to test your pre-race meal when you have so many chances in training?
Prioritize a post-workout snack within 45 minutes after long, intense sessions and strength-based exercises or if you are unable to eat a post-workout meal. Aim for approximately 25 grams of protein, along with simple carbohydrates, low in fat, and fiber.
Examples include whey or vegetable protein shakes with fruits, Greek yogurt with granola, protein bar, and chocolate milk with pretzels.
When in doubt, always go back to the basics. Ask yourself, are you eating enough carbohydrates, protein, and fats to meet energy requirements, maintain health and improve performance. Is this a diet you can or should keep for life? And if it’s not sustainable, what’s the end goal?
Here’s a concession: If you want to include a “quick” in your diet, consider fasting from after dinner until your pre-workout snack or breakfast the next morning. This will deter stray snacking at night and help shed those unwanted kilos.
Susan Kitchen is a Registered and Certified Sports Dietitian, USA Triathlon and Certified Ironman trainer, accomplished endurance athlete, and a published author. she owns Race Smartan endurance training and performance nutrition company that works with athletes around the world as they strive for optimal health, fitness and performance.