What’s The Deal With Fasted Workouts?

Should you workout on an empty stomach? An RD weighs in on the science behind fasted workouts and how they affect performance & weight loss.

Working out on an empty stomach (aka fasted workout) is a heavily debated topic in health magazines and on social media lately. Those that workout fasted swear by it, while most Registered Dietitians don’t recommend it.

You will often hear me say that food is fuel and chat about what to eat before a workout. But after receiving many questions about working out in a fasted state, I decided to take a look at the pros and cons of doing so. 

As always, I consulted the scientific research so that I can provide you with proven evidence for or against fasting before a workout. 

Before digging into all of this, know that everyone is different and you need to find the right fueling practices that work for you. Just because your friend fasts before they strength train doesn’t mean it will work for you before running. 

It’s very difficult to give nutrition advice to EVERY individual because people are so different in their goals, activity levels and lifestyle. But if you want to try fasted exercise, here’s what you should know. 

Why people workout on an empty stomach

You may be thinking, “Why would I even want to workout on an empty stomach?” Well, there are a few reasons that people try working out without any “gas in the tank”.

1. They want to burn fat. 

It’s well known that the body uses stored carbohydrates, known as glycogen, and dietary carbs as the primary fuel source for exercise. But what happens if you starve the body of carbs?

If you don’t eat before a workout, your body will tap into the glycogen stores to burn them as fuel. But glycogen stores only last for a short period of time. Rather than collapsing on the spot, the body turns to fat stores to break them down for energy.

That sounds awesome, right?! Well… breaking down fat is harder on the body and takes more energy than using carbs. There is some merit to this, as some research suggests that working out in a fasted state may reduce body fat percentage.

That said, there is also research to show that working out in a fasted state will decrease overall performance, but more on that later.  

2. They think it will help them lose weight.  

Since fasted workouts use fat as the primary fuel source for energy, it’s natural to assume that they will help with weight loss. There is some evidence to suggest that fasting at the right times may result in weight loss. 

But a fasted workout alone is not enough to help you lose weight. You need a mixture of a well-balanced diet, a calorie deficit and exercise in order to lose weight. [Related: 6 Healthy Tips For Runners To Lose Weight]

Some research even suggests that fasting before a workout can make you hungrier later in the day, which can lead to overeating. 

3. They want to prevent indigestion.

We’ve all been there– either you eat the wrong thing before a workout or maybe you eat too big of a meal. Either way, it’s a common complaint among athletes that they experience stomach issues if they eat before a workout. 

Because of this, some people choose to workout on an empty stomach. If there’s nothing in your stomach, nothing will jostle around and make you feel sick. [Related: How To Avoid Stomach Issues While Running].

How fasted workouts affect performance

First, it’s important to always have a goal in mind when making nutritional choices. For many of my readers, their nutrition goal goes hand-in-hand with their fitness goals. You want to use food to enhance your performance and increase fitness.

That said, others want to lose weight or try different styles of eating to figure out what works for them. I highly encourage you to think of the reason why you’re making a certain eating choice, like fasting before a workout. 

If you’re working out on an empty stomach to increase your overall performance– run faster, lift heavier, grow stronger and more agile– you may want to think twice. Research on fasting before exercise shows mixed results on performance. 

For example, some studies reported that not eating before exercise decreased performance, while others showed no effect. I was not able to find any studies that showed an increase in performance when fasting before a workout. 

There are a number of reasons for such differences, including the differing ways the studies were designed, the length of the fast, and the varying athletic levels of study participants. In general, most of the studies use sedentary people, which doesn’t always correlate to trained athletes.

The study authors recommend that “athletes train at relatively low intensities when fasting to ensure that they recover adequately to optimize performances in competitive events.” 

If you want eat something light before a workout, try one of these options: 10 Healthy Foods To Eat Before An Early Morning

What if I’m working out for less than 60 minutes?

If you’re working out for less than 60 minutes, you may think that fasting before a workout is not a big deal. After all, you can power through a workout for 60 minutes without any gas in the tank.

If you want to workout fasted and you feel fine doing so, then go for it! Just keep in mind that although you feel good, you may feel even better if you were to eat a little something.

Sometimes, eating enough the night before may increase glycogen stores so that you might have enough fuel for a morning workout. That means eating a large dinner with plenty of carbs, protein and fats, which will sustain you in the morning.

This all comes down to trial and error. If you feel like working out on an empty stomach works better for you, then that might be the case! But if you don’t eat before a workout, make sure you eat some recovery foods immediately after. This will prevent you from overeating later in the day. [Related: 10+ Best Recovery Foods For Vegetarian Athletes].

How to try a fasted workout

Have realistic expectations. It takes time to adjust to working out while hungry. It’s an uncomfortable feeling that might negatively affect your performance. It won’t automatically fast track your fitness goals.

In addition, meeting with a Registered Dietitian can help you learn how to make sure you don’t eat too little or too much. 

If you have a medical condition, definitely speak with your doctor before making any diet changes. If you’re generally healthy, talk with an RD to make sure you still eat a well-balanced diet.


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I’m Natalie Rizzo, an NYC-based Registered Dietitian.

My mission is to help everyday athletes fuel their fitness with plants.

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