Fact-checked by Dr. Anthony Gustin, DC, MS.
Written by Emily Ziedman
The all too common question of “what should I eat before a workout?” has shifted to “should I eat before a workout?”
Fasted training, intermittent fasting, and getting into ketosis are replacing the long-popular pre-workout shakes and bars.
And while it may sound like a fitness industry buzzword, fasted training has some pretty compelling scientific backing.
What Is Fasted Training?
Fasted training is exactly what it sounds like — exercising in a fasted state. That usually means training after not having eaten for several hours or working out in the morning when your last meal was dinner the night before.
So, how could working out on an empty stomach possibly be good for you? Will your body start to break down muscle to create more energy?
Will you bonk? What about people with hormone imbalances or adrenal issues?
Fasted State vs. Feeling Hungry: What’s the Difference?
Believe it or not, being in a fasted state vs. a fed state has less to do with what’s happening in your stomach, and everything to do with what’s happening in your blood. Or more specifically, what’s happening with your blood sugar and insulin.
It’s important to understand that skipping a meal, feeling hungry, or having an “empty” stomach may be associated with a fasted state, but don’t necessarily mean you in a truly fasted state.
You can eat a low-protein, low-fat meal and feel hungry again in a couple of hours, but your body is still working on metabolizing that meal. You’re in a truly fasted state when your body has completed the process of breaking down, absorbing, and assimilating the nutrients from your last meal.
How Do I Know if I’m in a Fasted State?
So how do you know if you’re in a fasted state? When you’re digesting food, or your body is absorbing and assimilating nutrients, you’re in a fed state. Yes, even if you’re hungry.
The presence of fuel — whether it’s in the form of glucose from carbohydrates or fatty acids and ketones from a keto diet — in your blood stimulates insulin.
Insulin is a hormone that helps shuttle that fuel out of your blood and into your cells where it can be used for energy, stored for later use, or excreted.
Depending on the size of your last meal, the entire digestive process can take between 3 to 6 hours.
Once this process is complete, insulin levels drop and your body switches from using blood glucose or fatty acids as its primary source of fuel to using stored energy as fuel.
4 Main Benefits of Fasted Training
Now that you know what a fasted state is and how to get into it, let’s talk about some of the benefits of fasted exercise.
#1: More Fat Burning
The primary purpose of training in a fasted state is to be able to tap into stored energy in your tissues, aka stored body fat.
When there’s no glucose in your bloodstream, your body has no choice but to turn to your fat stores and liberate fat to be used as fuel.
Studies on fasted training show that you’ll not only burn more fat during fasted training, but you’ll also increase the amount of fat that’s liberated from your cells[*].
Important note: the research shows that the type of fat you’re burning in a fasted state is primarily intramuscular triglycerides or IMTG[*]. That means you’re burning the fat stored in your muscle tissue, not necessarily that extra flab around your middle.
What does this mean for overall fat loss? It’s not totally clear.
But there is a strategy to fasted training that will not only enhance fat burning, but also protect your muscles: you can use fasted training to get into ketosis faster.
#2: Get into Ketosis Faster
Fasted training is an efficient way to use up your muscle glycogen stores, which is the key to getting into ketosis.
When insulin is doing its job of shuttling glucose out of your blood and into your cells, it stores that glucose as glycogen in your muscles. You can think of your glycogen as the low hanging fruit of energy stores in your body.
It’s pretty easy to break down, and it can enter the bloodstream with fewer steps then fat or protein. That’s why your body loves going after glycogen stores for energy before it shifts over to fat stores.
Both fasting and training use up glycogen in your body, which quickens the process of switching over to burning fat for fuel.
#3: Increased VO2 Max
When you’re doing cardio or aerobic exercise, your endurance is only as good as your body’s ability to bring oxygen to your cells.
Fasted cardio can help increase this process of oxygen delivery, which is measured by something called VO2 Max[*].
Your VO2 Max is the maximum amount of oxygen your body consumes during an aerobic workout when you’re working your hardest.
This means that when you increase your VO2 Max, you’re increasing your ability to take in oxygen and deliver it to your muscles so you can work harder during an aerobic workout.
This is great news for endurance athletes or weekend warriors. Maybe eating all those protein bars before a race isn’t the best way to increase performance.
#4: Increased Human Growth Hormone
Fasting prior to training naturally increases a protein called human growth hormone (HGH)[*].
HGH, which is released by your pituitary gland, boosts muscle growth, as well as the growth of your bones and cartilage. This means bigger, stronger muscles and protection against age-related bone and muscle degeneration.
HGH tends to rise during adolescence and puberty, and slowly declines as you age.
Potential Pitfalls of Fasted Training
So fasted training is looking pretty good right now, right? Well before you start skipping meals before a strenuous workout, there are a couple of downsides.
Not Being Able to Train as Hard
If you’re used to eating before your training sessions, then your body is more accustomed to a steady flow of fuel during your workouts.
When you begin training fasted, you may notice a dip in energy faster than if you had eaten a pre-workout meal[*].
This happens because that readily available glucose is no longer hanging around in your bloodstream waiting for you to burn it up.
Some athletes call this phenomenon “bonking,” which happens when your glycogen stores are depleted and the steady flow of fuel to your muscle cells comes to a halt.
Even if fasted training increases your VO2 Max, oxygen is only part of the formula- you still need fuel to be burn.
If you’re used to hours-long, high-intensity workouts, fasted training might not be for you.
Potential Muscle Breakdown
While fasted training does signal your body to start breaking down fat stores, your muscles aren’t completely out of the woods[*]. Yes, it’s possible for your body to break down muscle tissue in its hunt for fuel.
One simple way around this is to refuel your protein stores after your workout. In one study, muscle breakdown after a fasted cardio workout didn’t begin until an hour and half post-workout[*].
A protein-rich meal within about an hour post-exercise will ensure your muscles have the fuel they need to maintain and recover.
But while there may be some muscle breakdown during fasted training, this doesn’t seem to be the case for fasting in general.
How To Maximize The Benefits of Fasted Training
HIIT Protects Muscle and Burns More Fat
If you really want to get the most out of fasted training, high-intensity interval training (HIIT) workouts are the way to go.
HIIT workouts are also extremely time efficient. A typical workout will last anywhere from 10-30 minutes, with a huge calorie burn that keeps your metabolism humming for hours.
Know your limits
This is as true for endurance athletes as it is for resistance training. You’ll most likely have less energy and endurance when training in a fasted state, so be sure to listen to your body and make sure your form is not suffering.
It’s much better to do a shorter workout with good form, rather than push yourself past your limits and let your form slide.
As your body gets used to working out in a fasted state, it’s likely you’ll be able to access your fat stores more easily, but knowing your limits is essential to preventing injury.
Take Supportive Supplements
Fasted training won’t work unless you’re, well … fasted. So, pre-workout snacks and supplements are out of the picture.
However, there’s still plenty you can do to support fasted training to maximize strength, endurance, and recovery.
- Exogenous Ketone Base Exogenous ketones may be the one exception to the rule of “no supplements pre-workout.” Whether you’re already in ketosis or you’re working towards it, exogenous ketones can boost your workout and help prevent that drop in energy that you may experience when transitioning to purely fasted training. Exogenous ketones will supply your body with energy to fuel your workout without triggering an insulin response.
- Whey Protein Post-Workout Whey is an excellent source of branched-chain amino acids (BCAA), which are essential for muscle building and exercise recovery. Fasted training can trigger some muscle breakdown, so replenishing your muscles with BCAAs is a great way to avoid this[*]. Whey is also a powerhouse supplement with benefits ranging from liver health, immunity, and weight loss to name a few. Be sure to get your post-workout protein in within an hour after your workout to optimize it’s buffering effects on muscle loss[*].
Who Shouldn’t Try Fasted Training?
The Bottom Line of Fasted Training
All in all, it looks like fasted training is a great way to push your workout routine to the next level.
With a boost in HGH and some post-workout protein, you can get all the benefits of fasted training with none of the pitfalls.
Worried about hitting a wall? Just take some exogenous ketones to keep you going strong through your workout.
And the good news is, with that increased VO2 max your endurance should only get better over time. But if you’re looking to get the most bang for your buck, make sure you’re doing high-intensity interval training for maximum fat burning and muscle preservation. Happy workout!