The recent popularization of intermittent fasting in the fitness world has led to a new trend of working out on an empty stomach, or as fitness lovers call it, fasted workouts. This new fitness trend has caused a lot of lifters and avid gym-goers to ditch their breakfast for an early morning workout due to the fat-burning benefits it claims to have. But for some, working out on an empty stomach in the early a.m. sounds like a nightmare, and a little too risky.
Fortunately, there are pros and cons of working out in a fasted, as there are for in a fed state. We're going to break them down for you so you can decide whether you should trade in your breakfast for an a.m sweat session!
Exercise on an empty stomach
First, let's talk about what training in a fasted state means. Being in this state means that your stomach is completely empty, but how long do you have to fast for that to happen? It can be anywhere between 8-12 hours, but those practicing intermittent fasting will already be in that state since it requires a minimum fasting period of 14-16 hours. It also depends on how big your last meal was and how fast your digestive system works because being in order to reap the benefits of being in this state your body must be done breaking down food so insulin levels are low and glycogen stores are empty.
So, what are the benefits of working out on an empty stomach?
There are two significant effects being in a fasted state produces that can help with your training efforts. First, it helps increase the growth hormone which provides the body with wonderful benefits, including promoting fat loss. Aside from helping burn fat, it helps make new muscle tissues and reduces your risk of bone fractures by boosting the growth in your muscles, bone, and cartilage. So, basically, when your stomach is empty there is no glucose in your bloodstream and since there is an increase in growth hormone production your body is effectively able to tap into your fat stores to use fat as a fuel source. The downside is that the increased growth hormone production ends when the fast does! However, fasting regularly will help keep this fat-burning and muscle-building hormone at its peak.
The second significant effect of working out on an empty stomach is that it can help increase your VO2 Max, which refers to the maximum amount of oxygen your body consumes during high-intensity cardio/aerobic exercise. To put it simply, a fasted cardio session can increase your ability to take in oxygen and deliver it to your muscles quickly so that your muscles are able to work harder! A study in the Journal of Science and Medicine in Sports found this claim to be true! The study examined healthy, but untrained men and women who were instructed to fast overnight and partake in endurance training the next morning. They found that the fasted group had a significant increase in their VO2 Max compared to those who trained in a fed state. 
These are the two main reasons this fitness trend of working out on an empty stomach has exploded! But before you start ditching your breakfast for an early morning workout session, let's discuss the potential downsides of fasted workouts...
Potential decrease in exercise performance: Working out on an empty stomach can lead to a decrease in energy, therefore, affecting your training performance. However, this might only happen if you're barely starting out with fasted workouts. If you previously ate a pre-workout snack or meal then your body is accustomed to having an available energy source that provides steady fuel to crush your high-intensity training at an optimal level. So, taking away that fuel source will be an adjustment for your body. You might notice a dip in your energy the first few days or weeks, but eventually, your body will adjust to fasted training.
Potential risk of muscle breakdown: If you're trying to build muscle, there's a slight risk of muscle breakdown when working out fasted. Although your body should tap into stored fat to use as fuel, it can also tap into another source, your muscle tissues. Instead of tapping into the adipose tissue for fat, your body might use the protein that makes up your muscles instead which is not ideal for anyone trying to build serious muscle mass.
The research/evidence is mixed: As with all research, there are other factors that can contribute to mixed evidence. Fasted training is fairly new in the fitness world so a lot of the studies that have been done in the past few years have mixed results. Some studies resulted in positive effects of fasted training, while others saw no significant difference between training on an empty and full stomach. Although the evidence is not conclusive, a lot of people feel that fasted workouts work better for them.
Luckily, the first two downfalls mentioned can be avoided if you know how to approach fasted training safely and in a way that doesn't negatively affect your results.
Tips for working out on an empty stomach
For those trying to achieve weight loss or any other fitness goals, consider giving fasted workouts a try! There's no harm in trying it as long as you know how to approach it. So, here are a few tips to help you maximize your fasted training efforts:
Drink plenty of water beforehand, and have black coffee for an energy boost: Drinking water should be a given, but it’s something that is especially important for those working out on an empty stomach. If your energy is super low in the mornings, consider having a cup of black coffee or green tea! It won't break your fast, and these drinks are actually known for stimulating the metabolism and improving insulin sensitivity, so win-win!
Have a healthy, balanced post-workout meal: An immediate post-exercise protein shake might not be necessary for someone who had a meal or two before their workout, but it is important for those who workout fasted! After your fasted workout has a meal that includes plenty of carbohydrates and protein to help replenish glycogen stores, minimize the risk of muscle protein breakdown, and promote muscle growth and repair. If you need some post-workout meal/snack ideas, consider using foods that promote better muscle recovery.
Choose a lower intensity workout: If you're just starting out consider starting with low to moderate intensity cardio like jogging, walking, avoid high-intensity workouts like HIIT, or heavy weight lifting. There are many lifters who successfully train while fasting, but it’s something you should work your way up to. If you see a significant decrease in performance then consider working out in a fed state or just stick with fasted cardio and strength training in the evenings!
Consider intermittent fasting regularly: Remember the increased GH production ends when the fast does, but fasting regularly can help ensure that your GH levels are at their peak. So, if working out on an empty stomach is a good fit for you then consider intermittent fasting! It'll help with your GH production and your body will also adjust to working out this way thus eventually becoming effective at tapping into your fat stores instead of your muscles or glycogen.
Exercise on a full stomach
Despite its downfalls and the mixed studies, working out on an empty stomach has worked and demonstrated success for a lot of fitness lovers! But despite all its benefits, some can't fathom the idea of working out without their pre-workout energy bar or sports drinks. Fortunately, working out fast isn't the only way to lose fat...
Studies show that fueling up with carbs and other nutrients post-workout can lead to a lower calorie intake throughout the day!  As we all know that the key to reducing body fat percentage and body weight is a calorie deficit, so your fat loss and weight loss efforts won't be doomed just because you don't want to work out on an empty stomach! In fact, there's another study that was published in the Journal of Functional Morphology and Kinesiology that found that although there was an increase in fat burn when exercising fasted, it did not result in greater fat loss overall than those exercising after breakfast!
Aside from what the studies say, working out in a fed state vs a fasted state reduces your risk of hitting a wall. Sudden fatigue and loss of energy happen when glycogen stores are depleted which happens after intense exercise and fasting. But it's not because your muscles need the energy, it's your brain. Liver glycogen is the primary source of energy for the brain and it requires a steady supply of blood glucose to function at optimal levels. The lack of steady supply to the brain can increase the risk of fatigue and hitting a wall when training in a fasted state. This may not always happen, but it can. This is why it's best to do high-intensity workouts like heavy weight lifting after eating, to ensure that your performance is at its best.
Exercising on a full stomach can get you the same results as working out fasted, but there are two downsides that you should be aware of...
It can be uncomfortable: A hearty big meal filled with carbohydrates and protein might sound like a good idea before a workout, but it can actually make your workout uncomfortable. Exercising on a full stomach can cause bloating, cramps in the stomach, hiccups, or nausea.
The food you ate might not have been turned into energy yet: Despite contrary belief, your hearty pre-workout snack or meal probably won't give you the huge boost of energy you're hoping for! To explain, we have to briefly discuss how your stomach digests food. After eating the food sits in your stomach as it prepares it for digestion by mixing acids and enzymes that break the food down. Once that's done it gets pushed into your intestines, and this is when food gets extracted and turned into energy.
So to fully benefit from your pre-workout snack your stomach has to be nearly empty to avoid discomfort and in order for it to be used as a sustainable energy source.
Tips for working out on a full stomach
To avoid the two potential downsides of eating before working out, here are some tips to help make your workouts comfortable and effective...
Avoid having a huge meal: Keep your morning breakfast small if you plan to workout after! A fatty meal of eggs, hash browns, and pancakes is only going to lead to an uncomfortable workout session and will leave you feeling sluggish as opposed to giving you the energy to push harder.
Eat one to two hours before: Don't chomp down on your snack or meal and then hit the treadmill or weights. Give your stomach a little bit of time to settle and digest! This is also why you should choose a lighter and easy-to-digest meal because you won't have to wait hours for your stomach to be nearly empty. So, choose a small nutritious snack to eat at least one hour before your workout.
Avoid high-fiber and high-fat foods: Choose the foods you eat wisely... Even something healthy like beans or cruciferous vegetables can impact your workout! So, choose something light that has mostly protein and carbs, like a banana with peanut butter or whey isolate protein shake with fruit.
To summarize, working out an empty stomach vs on a full stomach has its pros and cons, but as you can see, the downsides can easily be avoided by following the tips given! It really comes down to a matter of personal preference, so create a workout regime that is effective, but most importantly, comfortable for you! At the end of the day, the best workout regimen is one that you can stick to!