Combining intermittent fasting and working out may sound extreme, but it’s entirely possible to do it with the right timing and by following safety tips. This article discusses the pros and cons of both approaches and everything else you should know to get the most out of intermittent fasting as someone who’s also passionate about fitness.
Can You Workout During Intermittent Fasting?
The short answer is yes, you can exercise during fasting. In fact, intermittent fasting and exercise provide similar benefits, such as helping you burn body fat, increasing mental clarity and cognitive functioning, and reducing inflammation (* , *, *, *, *). Both can act synergistically, which allows you to get the best possible results.
However, while exercising in a fasted state is generally okay, not everyone feels good doing it. This is especially true if you’re not yet fat-adapted — meaning that your body can derive its energy needs from fat whenever it needs fuel (*). When you’re fat-adapted, you can workout comfortably without craving carbohydrates. Reaching this state requires reducing carbs consistently or adhering to a keto diet.
This is why it’s always important to listen to your body when working out & supporting yourself with the right fuel. If you feel exhausted or depleted during a fasted workout, reduce the intensity of your activity or stop.
Furthermore, those with type 1 and type 2 diabetes should speak with their doctor first. This is because exercising on an empty stomach can cause their blood sugar levels to drop.
When Should You Workout During Intermittent Fasting?
It depends on the time of the day you prefer working out. The key is to schedule your workouts in a way that allows you to nourish yourself after those workouts.
For instance, if you usually exercise at 12:00 pm, then a 16:8 intermittent fast (16 hours of fasting, followed by an 8-hour eating window) should start at 8:00 pm on the previous day and end at 12:00 pm on the following day. This will allow you to fuel yourself with a nutrient-dense meal, which includes quality protein and fats, after the workout.
However, if you’re feeling too weak to workout during a fast, you can always exercise 2 hours after you break your fast with a meal. This is especially true for high-intensity interval training (HIIT) and prolonged workouts, which can cause you to feel sluggish. By eating before an intense workout, you can perform better and benefit from faster recovery (*).
Pros and Cons of Working Out While Fasted
Intermittent fasting and working out have advantages and disadvantages that need to be considered before you get started.
Dr. Sony Sherpa, a holistic physician at Nature’s Rise, an organic wellness company, mentions increased fat-burning as one of the biggest benefits of exercise during fasting. “If your motivation to working out and fasting is weight loss, this may be a great idea. Without a steady supply of glucose, your body burns fat reserves for energy. Increased fat burn leads to increased weight loss,” writes Sherpa.
A systematic review and meta-analysis published by Cambridge University Press found that aerobic exercise in a fasted state — e.g. swimming, cycling, walking, using a cardio machine — at a low to moderate intensity results in a significant increase in fat oxidation (*).
For athletes and fitness enthusiasts, working out during intermittent fasting is a strategy to induce training adaptations. In other words, exercising even when your glycogen stores are low teaches your body to use fat for energy, allowing you to train effectively (*).
“When you work out in a fasted state, it trains the body to tolerate low blood sugar levels by burning the fat and therefore increases our stamina,” said Kristi Smith, a nutritionist and biotechnologist at Honest Brand Reviews.
Another important benefit is improved heart health. “Exercise and intermittent fasting together can lower blood triglycerides, insulin resistance, blood sugar, bad LDL cholesterol, inflammatory indicators, and blood sugar. All of these are risk factors for heart disease,” mentions Jennie Miller, co-founder of MIDSS, an online health and lifestyle publication.
Although a fasted workout helps with fat-burning and weight loss, Smith notes that decreased glycogen levels reduce the intensity of your training sessions. This may explain why people who do HIIT workouts regularly benefit from a ready supply of glycogen (by consuming carbs) so they can power through their sessions. In that case, eating a meal before an intense workout helps.
Miller also mentions the possible loss of muscle mass when exercising fasted. “The body will start to break down muscle tissue for protein for energy.”
While there’s no research to support this, keep in mind to consume enough protein on your fasting and non-fasting days and incorporate resistance training. Protein intake and resistance exercise increase muscle strength and size (*).
The Best Types of Workouts to Do While Fasting
You can perform any type of workout that feels comfortable during a fast, from cardio to strength training.
For those who are new to fasting, low-intensity exercises are a good place to start. Timothy Woods, a renowned professional chef and owner of Carnivore Style, recommends low-intensity steady-state (LISS) cardio. Examples of LISS include walking, steady cycling, and swimming at an easy pace.
It may be best to steer clear of longer and more intense workouts during a fast unless you’ll be breaking that fast shortly. Allow your body to get adjusted first, then gradually increase workout intensity over time as tolerated.
When it comes to lifting weights to boost muscle mass, it’s important to fuel with protein after your workout, when you break the fast.
Intermittent Fasting Workout Schedule
Whether you train during a fast or after you’ve had a meal (during your eating window) is completely up to you. Below, we’ve created a sample schedule you can follow no matter the type of intermittent fasting you’re doing.:
- Monday: Afternoon HIIT (e.g. sprinting and squat jumps) followed by a meal with healthy carbs (avocados, spinach, cauliflower, asparagus).
- Tuesday: Protein-rich breakfast, then weight lifting or afternoon weight lifting followed by a protein-rich snack.
- Wednesday: Steady-state cardio (e.g. walking or biking) followed by a nutritious breakfast.
- Thursday: Full-body strength training followed by a high-protein dinner.
- Friday: Outdoor activity (e.g. recreational sports or equipment-free bodyweight workout in the park) followed by a keto-friendly snack or dinner.
Safety Tips for Working Out While Intermittent Fasting
As you’ve learned, exercise is definitely possible during any type of intermittent fasting. We would also recommend following these tips to help you feel as comfortable as possible and minimize risks.
1. Consume electrolyte drinks.
Drinking electrolytes during a fast prevents an electrolyte imbalance, which can happen when you’re not eating anything and you’re also exercising. Additionally, glycogen depletion due to not eating any carbs will cause your kidneys to excrete water and electrolytes.
Thankfully, there are zero-calorie electrolyte drinks that do not break a fast. It replenishes your sodium, potassium, chloride, and magnesium stores.
2. Do low-intensity or short-duration workouts.
If you’re new to intermittent fasting, it may be a good idea to stick to low-intensity workouts. Walking, cycling, swimming, and yoga are opportunities to stay active even when you’re low on energy. But even though you’re so hyped up, reducing your caloric expenditure by keeping your workouts mild will prevent you from feeling so exhausted afterward.
A low-intensity workout can wear you out if you do it long enough — for instance, cycling for one to two hours. Whatever your usual workout duration is, cut it in half while fasting.
3. Stay hydrated.
Don’t forget to drink water throughout your fasting window. It’s a simple way to stay hydrated. Hydration is essential for controlling body temperature (thermoregulation) and preventing tiredness (*).
Aside from plain water, you can try adding a slice of lemon to a large water pitcher to create lemon water for a refreshing drink! This has almost no calories so it shouldn’t break a fast.
4. Exercise at the beginning or toward the end of your fast.
Instead of working out in the middle of a fast, do it after your pre-fast meal. That way, your body has the energy to push harder during the workout.
But if your schedule doesn’t allow it, alternatively you can exercise as you approach the end of the fasting window. Then make sure to replenish with a healthy meal that has protein, fat, and complex carbs.
5. Listen to your body’s signals.
There is no one size fits all intermittent fasting and exercise plan. What’s optimal for you may not be the same for others. For example, you have no issues combining a 16-hour fast and a full-body strength training workout at the beginning of the fast.
That said, always listen to your body. Pay attention to your energy levels and symptoms of low blood sugar, such as dizziness, nausea, blurred vision, sweating, and tingling. You should discuss with your doctor if you have a medical condition and want to try fasting.
Below are common questions and answers on the topic of exercise during fasting:
Can you build muscle while intermittent fasting?
Yes, it’s entirely possible to gain muscle mass on an intermittent fasting plan that includes eating enough protein (about 1 gram of protein per pound of weight or depending on how active you are) and resistance training.
That said, fasting in combination with proper nutrition and exercise promotes muscle growth — and not fasting alone.
Who should not combine intermittent fasting and working out?
Pregnant or breastfeeding women should avoid combining intermittent fasting and exercise. On the other hand, if you are diagnosed with diabetes, consult your doctor first. Your healthcare provider may need to advise you to adjust your medication and monitor you closely to maintain good blood sugar control.
How long should I workout on intermittent fasting?
It depends on the person. While the usual advice is to engage in at least 30 minutes of physical activity each day, you may feel the need to keep your workout short or less intense during fasting. Listen to your body and do what feels good for you.
The Bottom Line
Fasting and working out make a good combination that can improve weight loss, increase training adaptations, and boost cardiovascular health. Ultimately, the type of exercise and duration will vary for every individual — although easy workouts (like low-intensity cardio) are best for those who are just getting started.
Remember to stay hydrated and take electrolytes. Exercise during your eating window, after your pre-fast meal, or right before you break a fast to ensure that your body has nutrients to support that workout and help with recovery.