Working Out on an Empty Stomach - Are Fasted Workouts Worth It?

There are several pros and cons to working out on an empty stomach. Here’s what to know before you begin fasting before exercise.

To start, it’s worth mentioning that fasting before a workout can present challenges, regardless of the potential benefits. The right choice for your body ultimately comes down to your performance goals and how you feel during and between workouts. Here’s a rundown of the pros and cons of working out on an empty stomach to help you decide what feels best for you.

What are the Potential Benefits of Fasted Workouts?

Research has shown that fasted workouts can result in a few key benefits many athletes seek.

1.    Burning more fat during workouts and improving endurance.

This helps reduce body fat percentage, which may be beneficial if your goal is to lean down (more on this below).

2.    Utilizing the ‘train low, compete high’ strategy.

Training in a fasted state forces your body to adapt to using more fat for fuel at all intensity levels. This switch may help your carbohydrate stores last longer during a non-fasted endurance event.

3.    Promoting a happier belly.

Many of my clients prefer to exercise first thing in the morning and eating very early can be unappealing. Some people simply have no appetite when they wake up to train before dawn, and it’s not pleasant to force yourself to eat when you don’t want to.

Eating Before a Workout & GI Distress

Even if you eat a nutritious, easy-to-digest food pre-workout, like a banana, dates, leftover sweet potato, or oatmeal, you need to allow some time for digestion before you get your heart rate up. That’s because exercise diverts blood flow away from the gut and towards your muscles, heart, and lungs.

So, if you eat too close to the start of the workout, you can end up with a brick-sitting-in your-stomach feeling, or an unhappy belly, from the undigested food in your stomach.

This can result in discomfort and poor performance. For this reason, fasted workouts may feel better digestively speaking for people who exercise very early in the morning.

What Are the Possible Cons of Fasted Workouts?

There are several reasons why working out on an empty stomach can hinder your overall goals. Here are some of the potential downsides.

1. Poor performance.

Some athletes adapt better than others, but I have worked with many athletes who simply cannot complete training sessions fasted or cannot train as intensely as they would like to.

2. Lightheadedness or weakness.

Low blood sugar or under-eating can lead to dizziness and weakness during exercise, which can cause an accident or injury. If you decide to exercise fasted, pay close attention to how you feel. If you feel shaky, spacey, or not quite like yourself, reassess your strategy.

3. Muscle breakdown during the workout.

Consuming adequate protein and calories during your eating window can offset muscle breakdown during fasted workouts. But fasted exercise combined with an imbalanced diet can lead to muscle loss, up injury risk, and negatively affect immune function, mood, and sleep.

4. Possible hydration issues.

Sugar helps absorb electrolytes, including sodium, which is lost in sweat. Sweating heavily and drinking only plain water during a workout (because you’re fasting) can lead to an electrolyte imbalance, which can be quite serious.    

What the Research Shows About Fasted Workouts

One study that compared fasted vs. non-fasted exercise among 16 women over six weeks found no difference in fat loss between both groups.

Another study compared fasted exercise and fed exercise with and without a post-exercise meal. The results showed that eliminating the post-exercise meal for those who fasted decreased their total daily calorie intake. However, it also decreased the total number of calories they burned per day, and increased hunger.

This is a valuable look into the complexity of metabolism and how each body adjusts to a reduced energy intake. Burning fewer calories and being hungrier isn’t a recipe for successful, sustainable fat loss or performance. If you ultimately prefer fasted workouts, don’t skip your post-workout recovery meal.

How to Test the Fasted vs. Non-Fasted Phenomenon

If you’re interested in experimenting with working out on an empty stomach, try training both fasted and fueled (ideally with easy-to-digest nutrient rich carbs). Then keep track of how your body responds.

If you’re committed to trying fasted training keep in mind that it can take a few weeks for your body to adapt. During that time and afterwards, prioritize the quality and balance of what you eat during your eating window to optimize exercise recovery and overall nutrition.    

What Time of Day is Best for Fasted Workouts?

If you do choose to experiment with fasted workouts, they tend to work best after an overnight fast and when you can eat a healthful, balanced recovery meal shortly after your workout ends. This is crucial for muscle repair and to meet your overall nutrient needs. 

My Professional Performance Dietitian Opinion on Working Out in a Fasted State

Fasted workouts are trendy, but that doesn’t mean they’re right for you. And while fasted workouts may result in burning more fat, you can still successfully reduce your body fat percentage without fasted training (I’ve helped many athletes do this and it’s research-backed).

Bottom line: I advise my athlete clients to pay close attention to how they feel while they train and throughout the day. If they struggle during exercise, need to use caffeine or other stimulants to get through a workout, or feel run down between workouts, fasted training may not be a good fit.

Looking for more guidance with your own fueling? Schedule an individual nutrition coaching session today.