When it comes to food, intermittent fasting has been suggested to benefit the body in a variety of ways. But when it comes to lifting weights and building muscle, is lifting weights while fasting something you should do?
Lifting weights while fasting is not recommended, especially during heavy lifting sessions or if your goal is to build muscle. For best results, lift weights during the windows of time that you’re eating.
As a nutritionist, I dig into the research and share how and when to incorporate lifting into an intermittent fasting plan to promote performance, endurance, and muscle building.
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A Quick Primer: What Is Intermittent Fasting?
Fasting is defined as the avoidance of food and calorie beverages for a specific period of time.
Intermittent fasting in particular has become a fasting fad being studied for its potential benefits including fat loss, inflammation reduction, and hormone regulation.
Intermittent fasting focuses on the time window that you’re eating. Johns Hopkins Medicine explains the different patterns of intermittent fasting:
Eating between six to eight hours per day, fast for the rest of the 24 hours.
Eating regularly five days a week, for the other two days, limit to one 500-600 calorie meal.
Longer periods without eating, such as 24, 36, 48, and 72-hour fasts.
Some types of intermittent fasting, such as the 6:18 hour pattern (eating for six hours, fasting for 18 per day) has shown promising results in relation to metabolic switches from glucose to ketone-based energy, increased longevity, and decreased incidence of disease such as cancer and obesity.
But this doesn’t necessarily give you free rein to scarf down loads of your favorite fast food within that time frame. For intermittent fasting to benefit health, it’s encouraged to eat mindfully.
Focus on these types of foods:
Daily fresh vegetables and fruits, whole grains (brown rice, whole grain pasta), and healthy fats (nuts, seeds, olive oil).
Weekly fish, poultry, eggs.
Moderate dairy.
Limited red meat and wine.
It can take about two to four weeks for the body to adjust to intermittent fasting in which there may be hunger and crankiness, getting used to the new routine.
It’s important to note that going too long without eating may actually encourage the body to start storing more fat in response to starvation. Your body is designed to help you survive so make sure to eat enough calories in general, and consult a doctor or dietitian before trying the longer fasts (over 24 hours).
Please speak to your physician before trying any new diet or exercise routine. The following groups should steer clear of trying intermittent fasting:
Children under the age of 18.
Pregnant or breastfeeding women.
Diabetes or blood sugar problems.
History of an eating disorder.
Related Article: The Best Pre-Workout For Fasted Cardio
Intermittent Fasting: Cardio vs. Strength Training
The research is inconclusive when it comes to the impact of fasting on exercise including cardio and strength.
Since more extensive research has proven the benefit of adequate nutrition to help build muscle, it’s best to adjust intermittent fasting patterns so that strength-related workouts are done during feeding windows.
CARDIO TRAINING
Some studies show that exercising during a fasting state can increase the breakdown of fats in the body, resulting in weight loss. However, this depends on the type of exercise and how well trained the individual is.
One Journal of Nutrition and Metabolism study found that working out on an empty stomach increased fat breakdown when the participants were doing cardio.
Yet in another study from the Journal of the International Society of Sport Nutrition, found there was no change when comparing the people who worked out fasting versus those who ate before exercise.
When it comes to the effects of fasting on energy metabolism in highly trained athletes, there is conflicting data in terms of the effects on physical performance. Therefore, it’s recommended that endurance athletes avoid high intensity training while fasting.
Key Takeaway: The research is still inconclusive whether extra calorie and fat burn happens when doing easy to moderate levels of fasted cardio. For endurance athletes, it’s best to steer clear of fasting while doing intense exercise such as HIIT.
STRENGTH TRAINING
If your aim is to lift weights in order to slim down and look competition ready, intermittent fasting may help you get lean.
Some evidence supports the concept that fasted exercise may promote fat loss. But this could be at the cost of losing some muscle mass, compromising performance, and eating more after the workout.
Studies show that lifting will use more stored muscle energy (glycogen) more than fat. This means burning through stored fuel which could result in decreased performance and endurance. One study found that skipping breakfast reduced the number or reps participants performed.
Yet another study conducted on 24 resistance trained men, showed a decrease in fat mass when compared to the regular eaters. They were randomly assigned to an intermittent fasting (16:8 hours) diet or normal diet for eight weeks.
One study was conducted on sixteen men in two groups: eight practicing resistance training in the late afternoon while fasting, and eight training in the late evening in an acutely fed state during Ramadan. The participant’s body mass and fat percentage did not change for both groups, suggesting that body mass was not impacted by training during a fasted or fed state.
When it comes to muscle, lifting on an empty stomach won’t necessarily lead to muscle loss, as long as you’re able to eat a combination of protein and carbohydrate (more on macros to come!) to rebuild and restore energy reserves shortly after. The window of time is suggested to vary according to factors such as training level.
Key Takeaway: Some studies show that intermittent fasting may negatively impact performance and muscle mass, while others show no change. What’s most important is to have a post-workout snack or meal shortly after lifting weights.
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Lifting Weights While Fasting: This Is How You Should Do It
SLOW AND LOW
If you’re going to exercise while fasting, go for a slow and lower impact workout. This is particularly important for longer fasts, such as the 24-hour ones, or low calorie days in the 5:2 plan.
For cardio, think about low-intensity steady-state cardio such as a walk, light jog, or bike. This is because easier workouts will help your body use fat. If you’re going to lift weights, aim for lighter ones with more reps rather than heavy weights.
The reason high-intensity interval training (HIIT) isn’t recommended while fasting is because these more intense workouts break down glycogen stores for fuel. As a result, you could be feeling fatigued and sore after the workout. This includes HIIT with weights.
MIND YOUR MACROS
The International Society of Sports Nutrition recommends aiming for a healthy source of protein and carbohydrate after a fasted session, unless your goal is to keep yourself in ketosis, in which case you’ll continue low carb.
To help build muscle, it’s important to eat a combination of protein and carbohydrates either before the workout or directly after. So when it comes to fasting, aim to workout either right before breaking your fast or within the eating period.
Portions can vary depending on your personal body weight and goals, but here are a few examples of meals and snacks containing health sources of protein and carbs:
Eggs with whole grain toast
Oatmeal and milk
Yogurt and fruit
Chicken and rice
Tuna and whole grain pasta
Don’t forget the importance of getting enough calories. We need calories to build muscle and get stronger. So if you’re following an intermittent fasting plan, make sure to include all your meals and snacks.
Looking to gain muscle? Check these tasty tools out: 16 Healthy Bulking Foods For Hard Gainers (With Meal Plan)
MEAL TIMING MATTERS
Meal timing depends on the type of fasting you’re following, whether you can stomach food before a workout, and the intensity of your workouts.
Say for instance you’re following the 16:8 plan and feel fine working out with something in your stomach, In that case, it may suit you best to have a bit of easily digestible pre-workout food.
If you’re exercising at a moderate or high-intensity workout, it’s important to eat close to the workout so your body has some glycogen (stored energy) to use. When it comes to the intense exercise, such as HIIT with weights, exercise closer to when you break your fast.
Slower, steady-state cardio such as LISS, can be done anytime, whether fasting or not. But this is of course while paying close attention to how you feel. If you feel fatigued, weak, or faint while you exercise, eat a meal soon after the workout.
Always consult your physician before starting a new exercise or diet plan, especially if you suffer from a condition such as diabetes. Learn more about low blood sugar and how to treat it: National Health Service.
ONE SCHEDULE DOESN’T FIT ALL
Remember that there’s no one-size-fits-all when it comes to fuel, fasting, and fitness. If you’re new to exercise or intermittent fasting, take your time adjusting to the new way of eating and consider adding workouts slowly.
If you’re following a fast which limits or restricts calories on some days such as the 5:2 diet, don’t push yourself by going above a low-intensity workout. You may also want to consider not exercising on those days, other than daily activities such as walking.
If you’re only available for morning workouts and doing the 16:8 fasting plan, you can adjust the eating window so you can eat right after your workout. If you have availability around lunchtime to lift weights, this would be a great time to plug in strength training since you’ll have fuel before and after the workout. If you’re focusing on lower intensity exercise such as walking or light yoga, you can be more flexible.
Related Article: Strength Training While Hungry: Should You Do It?
Final Thoughts: Intuitive Exercise
Listening to your body and how exercise and eating habits make you feel is the most important thing you can do. For instance, if you find that you feel fatigued during fasted workouts, it may help to shift your eating window and have a bit of food before you get fit. If you’re the type who doesn’t like food in their stomach during a workout, light lifting before eating may be best for you.
What it really comes down to is what pushes you to work hard, but makes you feel your best. Just keep in mind that lifting weights (especially more intense ones) during the window of eating times will provide the best results.
About The Author
Lisa Booth
Lisa is a registered dietitian nutritionist (RDN) with over 15 years of experience in nutrition, fitness, and mental health coaching and education. She studied Foods and Nutrition at San Diego State University and earned a Master of Science in Holistic Nutrition at Hawthorn University.
Having certifications and experience in group exercise, intuitive eating, coaching and psychotherapy, and digestive wellness, she’s enthusiastic about the relationship between the body and mind.
She’s dedicated to helping people understand how to implement healthy habit change, while gaining a deeper understanding of what makes them feel their personal best.
