Fasting. Exercise. Results?

Most people follow some sort of “wake up, eat, and workout” routine. Why not? It works and most people swear that you need food before you workout.

But, do you really? Can you exercise while fasting? Is it okay to workout while fasting? And, if you can what are the guardrails to ensure you do it properly? Are there any drawbacks? Benefits? Is it for you?

There are a whole lot of ways to skin a cat. Or, as other people say, many ways to peel an onion for the vegans reading this.

Let’s discuss.

First, What the Heck is Fasting?

Well, let’s talk about what fasting isn’t. It’s not about starving yourself into a skeletal state. Nor is it related to eating disorders, although people with eating disorders may do it.

In the simplest terms, fasting is simply giving your food processor (aka stomach) some time-off. No food, no working overtime to digest. Just some much-needed R&R for your dear gut.

And guess what? People are riding the fasting train for a couple of compelling reasons:

  1. Health Overhaul: Some argue that fasting gives their bodies a chance to reboot and repair. Think of it as your smartphone periodically needing a reset/reboot.
  2. Weight Vanisher: A few use fasting as a clever trick to help them shrink their waistline. The calories in vs calories out equation still works.
  3. Food Tamer: Fasting is often a tool to reign in those wild eating habits. When your body runs low on nutrients it will focus more on what it needs rather than what you want.

Fasted Exercise?

Having an empty stomach puts you on a different level. Being hangry is real. Your energy levels can drop. You might be uncomfortable.

But, here’s a proposal that might throw you for a loop. Add in exercise while fasting. Instead of getting up, forcing yourself to eat something, and then heading off to the gym, imagine just getting up and going.

Technically, fasted exercise can be any exercise you do if you haven’t eaten in a few hours but to most fasted exercise is when you do any exercise without eating for 12+ hours beforehand.

Nutrition and Energy

Understanding two key aspects can turn your idea on whether it is okay to workout while fasting from timidity into confidence:

  • Energy: Your body sources energy from a few different places in order to power itself. First, it uses the energy already stored in muscles and organs. Then, it looks for food to digest. If there is no food to be found it moves onto stored reserves in the form of fat deposits. Then it will move into cannibalizing tissues. Judging by this list, there is plenty of energy to go around, at least at first.
  • Nutrition 101: Your daily diet will dictate how successfully you can do fasted exercise. A diet full of twinkies and soda will not allow you to perform very well on an empty stomach, but a diet full of whole foods like vegetables, lean meats, and complex carbs will.

The Whys, Whats, and Hows of Fasting

Fasting, as you know, is essentially putting a “closed for lunch” sign on your stomach for a while. There are a few different ways to fast, and if you choose to do partake, which route you choose is completely your decision.

  • Intermittent Fasting (IF): You have periods of fasting mixed with periods of eating. IF says “eat-stop-eat-stop.” There are different fasting periods ranging from 12 hours to 18. IF is NOT a traditional fast as you do not go more than 24 hours without eating.
  • Water Fasting: IF too mainstream for you? There is the more hardcore water fasting option. Instead of having eating windows with real food, your meals are replaced with water for a certain number of days.

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Fasting Trends

You are probably here because of the popularity of IF. Fasting has exploded onto the fitness these past few years. Why would someone want to do this, you may ponder?

  • Body Autonomy: Nothing beats the feeling of controlling yourself. People who can control themselves are powerful, and what’s harder to control than your diet? Self-control is a huge part of the overall fitness world, so this makes sense.
  • Health Factor: Some people find fasting as an effective way to let their bodies hit the reset button. There is some good research that shows certain length fasts can help reduce toxins from your body and help your body repair itself.
  • Lean Mass: If your calories in are lower than your calories out you lose weight, and one of the first places you lose it is in your fat stores. I won’t deny that aesthetics are a huge part of why people work out.

The Unseen Magic Wrought by Fasting

Some pretty awesome things happen during a fast that you may not be aware of.

  • Switching Fuel: When food’s not coming in, your body learns to switch gears, switching to stored fuel in your body. This makes your body more adaptable while also helping you to lose extra pounds that you do not want.
  • Cell Repair: When your body doesn’t need to focus on digesting food it can focus on other things that it doesn’t always get to apply as much energy to. During a fast your cells buckle up and start cleaning shop. Toxins, old cells, things that don’t need to be there.

Fasting and Exercise: The Basics

Do fasting and exercise go together? The answer, like so many other things in fitness, is that it depends. That might be a terrible vague answer, but it’s true. Some people should not do fasting of any kind. Others should not exercise while fasting.

A Dynamic Duo or a Dangerous Dance?

Here’s the thing: Fasting and exercise are a bit like peanut butter and jelly. They might seem like an odd pair, but when done right, they can work together in harmony. However, when done incorrectly or by the wrong person it can go terribly wrong.

What Happens When You Fast and Exercise?

When you exercise, your body burns fuel (aka energy) to keep you moving. Normally, the food you eat is digested and stored in your cells. When that energy runs out, your body will look for more energy sources to pull from. But, when you’re fasting there’s no food in your stomach. So where does your body get its energy from?

Energy Sources during Fasting

Whether you’re a pro or just dipping your toes into the water, understanding where your body gets its energy from is key. There are three energy amigos we can pull energy from at any given time: glycogen, body fat, and muscle protein.

  1. Glycogen: Glycogen is like your body’s personal piggy bank, storing extra energy for those “just in case” moments. When you fast, your body starts by breaking down glycogen into little energy nuggets called glucose that keep you fueled up.
  2. Body Fat: You know that stubborn fat that just won’t go away? As you fast, your body happily munches away at those fat stores.
  3. Muscle Protein: Last but not least is muscle protein. Our body prefers to save this as a last resort (or skip it altogether, if it can). But when the fasting clock keeps ticking, it eventually breaks down some muscle protein for energy.

Benefits of Fasted Exercise

Our western minds have trouble comprehending that sometimes being hungry is a good thing. We are so well fed that it pains us to think of being hungry as anything but a bad thing that needs to be remedied. Why wouldn’t we? We see pictures of starving people around the world and correspond that as what happens when you are hungry. There is a difference between actual starvation and hunger.

It might not seem like doing anything on an empty stomach is good for you. In some cases it isn’t, but for most of us fasted exercise can be an absolute game changer.

There are a few really good things scientists have found when studying the effects of fasting.

Reduction in Stored Fat

Fasting is already a lack of calories. A lack of calories is how anyone loses weight. Because you aren’t taking in any calories your body needs to find additional energy stores. It looks for them in your fat deposits and burns them down to power itself.

Improved Insulin

Insulin, that hormone that breaks down food into glucose, gets even better at its job when you fast and work out. Improved Insulin Sensitivity means you are better able to control blood sugar spikes.


An increased metabolism happens, at least temporarily, due to fasting. Since you aren’t taking in any calories and your body has to search for other sources, your body is more efficient. It will break down nutrients (carbs, fats, proteins) faster when it finds them.

As an added benefit, brain-loving chemicals, like endorphins, get released in abundance, helping you stay happy and focused.

There are a ton of benefits to fasting, that when stacked with exercise are even more impressive.

(Possibly) A Bumpy Ride

Where there are rewards there are also drawbacks. While it can be effective, fasted exercise isn’t for everyone. There are some things to consider when deciding if you want to go this route.

Slowing Down

Fully fed you might run a 7 minute mile or bench two plates. At the beginning of a water fast, or while doing IF you might even keep that speed and strength. However, as you progress into a fast and hit more than 18-24 hours you WILL slow down. It will happen. You won’t have the same energy levels. That’s normal, and you will continue to slow down as you progress in, despite the waves of energy that you feel at various periods. Do not push too hard.

Muscle Breakdown

While fasting, remember when your body turns to other sources of energy? Short term IF (18 hours or less) will have you using fat stores more likely than not. Depending on who you are you might even have enough stores to keep going without burning muscle for a few days. However, after days of not eating your body will start cannibalizing itself to a degree and that will continue as you go on. Probably explains why no bodybuilders fast for more than a few hours at a time and marathon runners rarely, if ever participate in the lifestyle.

Low Blood Sugar

Depending on the length of the fast and who you are, working out while fasting might drop your blood sugar a bit too much, sending it crashing down. What happens then? Dizziness, shakiness, and crankiness. Of course, low blood sugar is expected because you aren’t eating anything so you only have internal resources floating around.

Fasting & Nutrition

Fasting and nutrition? Isn’t the point of fasting to not take in any nutrition? To an extent, yes. Of course you will want to prep ahead of time and there are some considerations while fasting that you will want to take in.

Pre-Fast Prep: The Nutrition Route

Before diving into fasting, it’s important to fuel up with the right nutrients and prepare your body for what is coming. Pre-fast nutrition is like stacking up your energy stores with top-notch supplies. Favor whole, unprocessed foods with high fiber content like veggies, fruits, whole grains, and legumes. These slow-digesting foods will keep you satiated longer while you’re fasting. Power up the protein with lean meats, fish, eggs, or plant-based protein sources like tofu and beans and do not forget the healthy fats! Avocado, nuts and seeds, and olive oil are all good choices.

How about hydration? Drink plenty of water before your fast begins. Staying hydrated is key to good health and can help curb the hunger pangs.

IF Eating Window

If you choose the IF route you’re allowed to eat for a specific time called the “eating window.” This can last anywhere from a single meal to 12 hours. How long it is is dependent on your own schedule. During the window pack in as many nutrient-dense foods as you can while sticking to your usual calorie intake. Don’t go overboard. Eat normally. You want to remain in a calorie deficit, not eat twice the amount of food in a shorter period of time.

Grab these goodies and have them on hand:

  • Fruits and veggies: Fresh produce of all sorts.
  • Protein: Chicken, fish, and beans are great choices.
  • Carbs: Whole grains, rice, and sweet potatoes.
  • Healthy fats: Avocados, almonds, olive oil, butter.

Must-Have Essentials: Electrolytes

Electrolytes can do wonders for you during a fast. Sodium, magnesium, and potassium are not only recommended, they should be required. Why? Because they can help keep you hydrated. Dehydration during a fast is exceptionally bad so just avoid it.

Longterm Fasting Guidelines

When doing a multi-day or multi-week fast keep these things in mind:

  • Drink lots of water and other no calorie beverages.
  • Stay away from caffeine for the most part. You can enjoy black coffee, but stay away from all the sugars, sugar replacements, creamers, and other stuff that go into it. Same for tea
  • Water fasts are considered any extended period of time where you consume 500 calories per day or less. The purists will tell you to stay away from any calories. Most people will simply stay under 500 calories. How you approach this knowledge is up to you.

Breaking the Fast Properly

nstead of diving into a feast, your body will thank you for a gentle wakeup call. Start by drinking some water with a pinch of salt for electrolytes. Then, focus on smaller, lighter meals. Foods like fresh fruits, raw or cooked vegetables, and broths are excellent choices to ease your digestive system back to normal functioning.

Proteins and fats? Yes, but just a little later. Start with smaller portions and gradually increase them over several days.

Do not rush. You’ll be sorry. Google it if you want to know, but it can be painful.

Nutrition is the key that unlocks your full potential, even when you’re fasting and exercising.

Exercise Considerations while Fasting

How do you know what kind of exercise to do while on a fast? Turns out there is no right or wrong answer, at least at first. A few hours in and your energy levels are fine. You can probably tackle your normal workouts the entire time while you are on IF.

Water fasts are different. You might feel great at first, but that won’t last. Don’t run a marathon, try to tackle a Spartan, or hit a powerlifting competition while on a water fast. You can probably get away with some lighter lifts or good walks the first few days, but that will get progressively harder as you go further into a fast.

Don’t forget about the energy balance, dizziness, and lethargy mentioned above. Do not try to PR any lifts or runs. Keep that for after you break the fast.

Low vs. High-Intensity Workouts

When fasting and exercising, what workout should you go for?

  • Low-intensity: A leisurely walk or a gentle yoga session can be perfect for fasters. Generally safe for most people.
  • High-intensity: Fast-paced exercises like running or lifting heavy weights can be tough on an empty stomach. But with your doc’s thumbs-up, and if you are feeling up for it, go for it while on IF. You should probably avoid this entirely while on water fast.

Who Should and Who Should Not Attempt Fasted Exercise

Honestly, most healthy adults can probably try it out. Obviously, I have to legally tell you to go ask your doctor to double check. Chances are if you’re in good health they won’t say no. That is, unless you go to one who is beholden to pharma or other special interests.

Just because you can doesn’t mean you must. Listen to your body. If it says, “Hey, ease up on the exercise, buddy,” then take a break.

So, Who Shouldn’t?

It’s probably easier to talk about who shouldn’t try this under any circumstance. For some folks, fasting and exercising is bad such as:

  • People with diabetes. The low blood sugar won’t put you on a good course.
  • Pregnant ladies and breastfeeding moms. Take care of your baby.
  • Folks with eating disorder: Your safety comes first!

If you do not have any underlying issues and your doctor says you can, go for it if you’d like to try it out.

Bottom Line: Is Exercise and Fasting Okay?

For some people, sure. Let’s go over some key points.

  • Energy Balance: Our body uses glycogen, then fat, and finally, protein for energy when we’re fasting. It uses them quicker while fasting and exercising.
  • Exercises: How you pick out your own exercises is dependent on what kind of fast you are doing, who you are, and a lot of other variables.
  • Nutrition: How you start and end a fast is important. What you consume during a fast is also important.
  • Dial-a-Doc: Always chat with a healthcare professional before you begin.

Fast & Fitness: A Conclusion

Is it okay to work out while fasting? Yes, it can be! But remember, not everyone’s journey has to look alike. Healthy adults? You’re cleared for take-off if your doc says you are. Diabetes, pregnant or breastfeeding, or coping with an eating disorder? You should probably stay away.

The most important takeaway? Listen to your body. It’s the best whistle-blower.

Remember, the finish line isn’t just about being fit, but also about being happy and healthy.

Let Us Help You Out

I want to help you achieve better physical and mental health through exercise. Check out the plans I offer to my clients and see if you could benefit from working with me. If you have questions, you can always contact me through the Contact Us page

Personal trainers, like myself, can help guide you on your pathway towards reaching your fitness goals, whether that is getting bigger, stronger, faster, more lean, or just generally feeling better.

If you choose to join one of my programs I’ll get you setup with a periodized workout plan, supplement information, and advice on nutrition to help you reach your goals. I also do advisory services that guide you along and answer questions you have if you don’t want a fully customized service.

The only thing you need is some motivation and a willingness to change some old habits.

Get into contact with me to find out what, if any, membership is right for you.

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