Intermittent fasting (IF) is a trending method to lose weight and remain healthy. Intermittent fasting exercise is welcomed, healthy, and has many benefits when you know how to do it. Still, you may need help to safely maintain a workout routine while intermittent fasting. You must understand what happens to your body during IF and exercise and know how it’s safe to follow a workout routine.
Can You Exercise When Intermittent Fasting?
Know what happens to your body during IF to understand whether or not you can exercise. Your body experiences changes during IF, including ketosis, autophagy, and metabolic switching.
Johns Hopkins Medicine explains metabolic switching (15).
Metabolic switching or ketosis is when your body’s metabolism runs out of glycogen or glucose to burn, turning to the fat stores instead. It encourages your body to start burning fat as the primary fuel source.
Dr. Sruthi further explains how autophagy initiates with fasting for hours daily (18). Autophagy allows cells to regenerate and create new ones, even reversing aging. The most popular IF is the time-restricted 16:8 method, where you have a 16-hour fasting period.
Exercises before, during, and after the 8-hour fuel window require tips, facts, and planning.
Is It Safe To Exercise During Intermittent Fasting?
Exercise and IF are safe for most people. However, Johns Hopkins Medicine explains how exercise and IF aren’t safe without a doctor’s guide for the following (15):
- Pregnant or breastfeeding moms,
- People with past eating disorders,
- People with type one diabetes and hypotension (13).
Understand how each fasting exercise appeals to different goals to plan safe routines. Let’s see whether aerobic and strength training is safe for IF.
Fasted Cardio Workout Safety
Cambridge University found that fasted cardio workouts improve your chances of weight loss (7). Meanwhile, they found that fasting cardio is safe for healthy individuals. The Guangxi Normal University published a study explaining the body processes (17).
First, fasted cardio increases insulin sensitivity, which is great for healthy individuals but not type one diabetics. Furthermore, the biochemical changes also revealed how some people’s protein becomes a fuel source instead of glycogen or fat (17).
High-intensity aerobic workouts can burn protein while you try to lose weight. It isn’t safe or suitable for anyone trying to maintain muscle mass. Low-intensity aerobic exercises during fasting windows don’t affect muscle mass but won’t increase it either (17).
In summary, fasted cardio is safe for losing weight, maintaining fitness levels, and speeding up your metabolism as a healthy individual.
Intermittent Fasting Strength Training Safety
Strength training and intermittent fasting can work if you know how to do it safely. The Swinburne University of Technology studied the effects of IF and resistance training (16). Muscle-building isn’t probable during intermittent fasting.
However, following simple safety guidelines preserves your muscles (16). Intermittent fasting can turn protein into a source of fuel. Dr. Carol DerSarkissian explains how muscles need protein to build mass and strength (19).
You must refuel your body with protein before and after workouts to sustain muscle mass during IF. Otherwise, it won’t be safe to exercise during IF. Additionally, Better Me strongly recommends adding carbs for muscle growth, even during IF (6).
In summary, high-intensity, muscle-building, HIIT workouts, CrossFit, and resistance training are safe. However, you must add carbs and protein as fuel sources to protect the muscles.
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How To Exercise Safely During Intermittent Fasting
Safety is the primary concern before you begin exercising while intermittent fasting. Some tips can help you achieve safe workout routines while you enjoy the many health benefits of intermittent fasting (11). Follow these tips to combine exercise safely with IF:
- Chicago’s University of Illinois recommends fueling your body with protein and carbohydrates before and after resistance training (14). Plan workouts 30 minutes after eating carbohydrates and before a protein-solid meal to restore the muscles.
- New York’s Lehman College research shows it matters to start slowly with fasting cardio to see results (5). Their results didn’t show much difference between fasting and non-fasting cardio. They had to get used to fasting first.
- Dr. Klitz also suggests starting exercises during IF slowly because of potential side effects (2). Some include headaches, fatigue, hunger, and lightheadedness. Avoid side effects by allowing your body to change fuel sources before exercising daily.
- Dr. Klitz also explains how dehydration can become an issue, which makes you lose electrolytes (9). Stay well-hydrated during IF, drinking up to 36 ounces of water daily. Use coconut water or calorie-free sports drinks to replenish electrolytes.
- Registered Dietician Nutritionist Miho Hatanaka suggests listening to your body (10). Always speak to your doctor about IF and exercise when you have risks with pregnancy and diabetes. However, your body and symptoms will tell you to stop.
Planning The Best Time To Exercise When Intermittent Fasting
Timing exercise with IF matters for weight loss or muscle mass maintenance. In addition, choosing the right IF routine will help you benefit from timing exercises correctly. The Cleveland Clinic suggests using one of the four popular IF schedules (12):
- Time-restricted eating is the 16:8 or 14:10 method. Choose the 16:8 if breakfast workouts don’t phase you. Alternatively, use the 14:10 method for earlier exercises. You have an eight- or ten-hour eating window to plan your activities (12).
- The 5:2 method allows you to restrict your calories to 500 two days weekly. Nottingham Trent University suggests regular exercises on non-fasting days and ultra-low-intensity workouts on low-calorie days (1).
- Alternate-day fasting is when you fast entirely every second day. Registered Dietician Nutritionist Miho Hatanaka recommends ultra-low-intensity workouts on fasting days (10). Otherwise, your body will burn protein and harm itself.
- Your wellness is a priority. 24-hour fasting is when you go a day without fuel once or twice weekly. It’s difficult to fast and exercise with high-intensity workouts. So, Hatanaka advises sticking to ultra-low-intensity activities during fasting (10).
Benefits Of Intermittent Fasting Exercise In The Morning
The best IF is 16:8 or 14:10 if you want to use any workout. However, the University of Rennes found incredible benefits to morning workouts and IF (8). Those who exercised early in the morning in a fasted state had:
- Better oxygen absorption when using morning workouts with IF,
- Increased aerobic performance during morning exercises with IF,
Furthermore, let’s not forget how Cambridge University confirmed better weight loss during fasted-state exercises (7).
Recommended Exercise While Intermittent Fasting
Consider safety concerns when choosing to exercise during intermittent fasting. Let’s understand which fasting methods and exercises to combine to ensure you have suitable workout routines matched to your fasting schedule.
Intermittent Fasting And Weight Loss Exercises
Registered Dietician Nutritionist Miho Hatanaka suggests aerobic and low-intensity workouts during fasting (10). So, use Cleveland Clinic’s low-intensity exercises during any fasted state on all the IF methods or right before breaking a fast (4):
- Low-intensity pilates,
- Low-intensity rowing,
Intermittent Fasting And Muscle Building Exercises
Hatanaka also recommends anaerobic exercises to build muscle during IF eating windows (10). Follow the tips to eat protein and carbs around your workout. However, Dr. Jabeen Bedum suggests muscle-building exercises, which you can enjoy during eating windows (3):
- Calisthenics (squats),
- High-intensity cardio,
- HIIT workouts,
- Plyometrics (box jumping),
- Strength training,
- Weight lifting.
Is It OK To Exercise While Intermittent Fasting?
Registered Dietician Nutritionist Miho Hatanaka reassures it’s ok to exercise during intermittent fasting (10). However, planning your exercises and meals for maximum effect is essential. Also, you shouldn’t exercise with IF if you’re pregnant or have type one diabetes.
How Does Intermittent Fasting Work With Exercise?
Cambridge University found fasting aerobic exercise and intermittent fasting can help you lose more weight (7). Low-intensity cardio workouts can support the metabolic change to burn fat and glycogen for fuel. Fasting exercise with IF speeds up the weight loss benefits.
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How To Combine Intermittent Fasting And Exercise?
Registered Dietician Nutritionist Miho Hatanaka suggests timing your workouts according to your IF routine (10). Only use low-intensity aerobics during fasting and high-intensity exercises during your fuel windows. That way, you safely time your IF exercises.
When To Exercise During Intermittent Fasting?
Fitness expert Daniel Bubnis recommends timing your exercises before, after, or during eating windows (9). Exercise before fasting to lose more weight. However, train during or after starting your fuel window to maintain muscle mass.
The Bottom Line
Intermittent fasting and workouts are safe when you follow tips, guidelines, and timing suggestions. Plan your workout routine accordingly. Use the correct method, plan which exercises to use on which days, and thrive with weight loss or muscle mass.
This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!
- 5:2 Dieters Risk Undoing Their Good Work By Overeating and Under-Exercising on Non-Fasting Days (n.d., nta.ac.uk)
- 9 Intermittent Fasting Side Effects and Easy Solutions (2022, doctorkilitz.com)
- Anaerobic Exercise: What Are the Health Benefits? (2021, webmd.com)
- Aerobic Exercise Health (n.d., clevelandclinic.org)
- Body Composition Changes Associated With Fasting Versus Non-Fasted Aerobic Exercises (2014, jissn.biomedcentral.com)
- Do You Need Carbs to Build Muscle? (2022, betterme.world)
- Effects of Aerobic Exercise Performed on Fasted vs. Fed State on Fat and Carbohydrate Metabolism in Adults: A Systematic Review and Meta-Analysis (2016, cambridge.org)
- Exercise: Training and Fasting: Current Insights (2020, nih.gov)
- How to Exercise Safely During Intermittent Fasting (2020, healthline.com)
- Intermittent Fasting and Exercise: How to Do It Safely (2020, medicalnewstoday.com)
- Intermittent Fasting: How It Affects Your Body and Health (2023, betterme.world)
- Intermittent Fasting: How It Works and 4 Types Explained (2022, clevelandclinic.org)
- Intermittent Fasting in Cardiovascular Disorders: An Overview (2019, nih.gov)
- Intermittent Fasting Versus Daily Calorie Restriction: Which Diet Regimen Is More Effective for Weight Loss? (2011, onlinelibrary.wiley.com)
- Intermittent Fasting: What Is It, and How Does It Work? (n.d., hopkinsmedicine.org)
- The Effects of Intermittent Fasting Combined With Resistance Training on Lean Body Mass: A Systematic Review on Human Studies (2020, nih.gov)
- The Effects of Six Weeks of Fasted Aerobic Exercise on Body Shape and Blood Biochemical Index in Overweight and Obese Young Adult Males (2023, nih.gov)
- What Happens to Your Body When You Fast for 16 Hours? (2021, medicinenet.com)
- What Protein Does for Your Body (2021, webmd.com)