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Fasted Cardio: Should You Try It for Fat Loss?

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Fasted cardio - Dr. Axe

Is it okay to workout on an empty stomach? Ultimately, it depends on your body composition, goals, overall health and preferences.

Working out while fasting, also called “fasted cardio,” does have some advantages — such as potentially leading to more fat loss and preventing indigestion while exercising. On the other hand, it’s not for everybody, since it may make some feel weaker and lethargic while working out.

If you’re curious about how meal timing can affect your workout performance and results, read on to find out about the benefits and drawbacks of working out while in a fasted state.

What Is Fasted Cardio?

Fasted cardio describes doing aerobic or endurance exercise on an empty stomach, without eating anything beforehand. This is called being in a “fasted state,” which is considered to be 4 to 6+ hours after your last meal or snack.

In order for you to truly be in a fasted state, in which your glycogen levels are low, some experts say you’d have to go more than 9–10 hours without eating anything. Fat and carbohydrates are the most important fuel sources for skeletal muscle ATP synthesis, so when carbohydrates are mostly not available due to fasting, fat is utilized instead.

Most people do fasted cardio workouts in the morning, before eating anything for breakfast. This may mean that someone has been fasting for 8 to 16 hours or more overnight, depending on their schedule and when they stop eating at night. For many people, the morning is the easiest time to workout without any “fuel” in their system because they’ve just gotten up and have already fasted through the night.

Related: A Beginner’s Guide to Intermittent Fasting

Health Benefits

What are the benefits of fasted cardio? Based on available research, here’s what we know about the potential perks of working out without any food in your system:

1. May Boost Fat Burning and Weight Loss

Does fasted cardio actually work for weight loss? There’s some evidence from studies suggesting that it can. Researchers involved in a 2016 meta analysis concluded that “aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state.”

When in a fasted state, your body doesn’t have any glucose/glycogen available to be used as a quick source of energy, so it utilizes stored energy instead.

This means that your body pulls from energy stored in your muscles and from stored body fat (via fat lipolysis and fat oxidation) in order to keep you fueled. Lipolysis is the metabolic pathway through which lipid triglycerides are broken down into fatty acids and glycerol to be utilized during fasting or intense exercise. The result is that you may boost your “fat burning” potential, although it’s likely not to have a dramatic effect in most cases.

Another way in which fasted cardio may support fat loss is by spiking post-exercise calorie-burning. Essentially after your fasted workout is done, your body uses up extra calories to help you recover, which raises your metabolic rate a bit for about 24 hours.

That being said, not every study has found this benefit to be true. A 2020 article published in the Open Access Journal of Sports Medicine states that “Our review of the literature suggests that there is little evidence to support the notion of endurance training and fasting-mediated increases in fat oxidation, and we recommend that endurance athletes should avoid high intensity training while fasting.”

The researchers add that …

Fasting decreases body weight, lean body and fat content in both trained and untrained individuals … However, there are conflicting data regarding the effects of fasting on glucose metabolism in highly trained athletes … Differences in experimental design, severity of calorie restriction, duration, and participant characteristics could, at least in part, explain such discordant finding.

2. Can Decrease Nausea

If you struggle with feeling nauseous while exercising, is it good to workout on an empty stomach in the morning? It can be, assuming that a full or partially full stomach is to blame for your indigestion. If eating before doing cardio makes you feel uncomfortable, you can try either having nothing beforehand and perhaps some water or coffee.

If you find that the sensations of “lightness” in your stomach feels better while exercising, then fasted cardio might be a good fit for you.

Everyone is a bit different when it comes to their eating preferences around workouts; some like a small pre-workout snack before exercising, some prefer a bigger meal a couple hours before a workout, and others like to consume nothing at all. Feel free to experiment and see what works best for you.

Drawbacks

1. Might Make You Feel Fatigued More Easily

There’s a chance that fasted cardio might cut your workout short if you leads you to feel tired and unmotivated more easily. Again, this comes down to the individual.

Overall, the effects of fasting on physical performance remains unclear, with some studies reporting decreased performance, some showing increased endurance, and others reporting no significant correlation or effects.

If you’re waking up early for a workout after a good night sleep, then you might have plenty of energy even without a meal. However, if you work out later in the morning on an empty stomach, after you’ve already been up for several hours, fatigue might be an issue. So it seems to depend on your unique schedule, body type and other factors.

2. Could Lead to Weakness and Less Power Output

Some find that fasted cardio causes side effects like dizziness, low blood sugar and lightheadedness. You may find that you aren’t able to push yourself as hard when fasted due to feeling weak, in which case you’ll be scarifying physical performance.

For example, one study concluded that “Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance.” Another meta analysis uncovered findings indicating that pre-exercise feeding enhanced prolonged, but not shorter duration aerobic exercise performance.

If you tend to experience lethargy when “running on fumes” during a workout, consuming a snack plus water to keep you hydrated before working out can be a better solution than skipping food altogether.

3. May Contribute to Muscle Breakdown

Although we’ve mostly been talking about fasted aerobic workouts, it’s still important to point out that fasted exercise may negatively impact muscle growth and strength.

Some studies have found evidence that fasted workouts cause muscle tissue to be broken down for energy, thereby making it more difficult to put on muscle and to build strength and endurance.

So if you’re into bodybuilding, cross training and lifting weights, there’s potential for fasted aerobic workouts to hinder your results. It’s not a deal breaker, but you’ll want to be mindful about how often and how intensely you do fasted cardio.

Should You Try It?

So what’s the bottom line, is fasted cardio better than fed cardio?

As you can probably tell by now, one approach isn’t necessarily better than the other. While fasted cardio can offer some benefits for amplifying fat loss, if weight loss is your primary goal, your total energy balance and calorie intake will be the ultimate determinant of whether or not you lose weight.

Keep in mind that you may experience an increased appetite after exercising in a fasted state, which can cause you to overeat afterwards if you’re not careful. If eating a very calorie-dense meal following a fasted cardio workout causes you to have a positive energy/calorie balance at the end of the day, this isn’t going to help you to reach your weight loss goals.

Here are some things to consider if you do want to give fasted cardio a try:

How long should you do fasted cardio?

Experts tell us that it’s probably best to stick to moderate intensity cardio for up to an hour when fasted. However, if you personally have energy to do even more without feeling negative side effects, a longer or high-intensity workout can also be appropriate.

Your best bet is to probably start out by trying 20–30 minutes of fasted exercise, and then increasing intensity and duration if you feel good doing so. Listen to your body and avoid pushing too hard to the point where you feel lightheaded or famished.

What should you eat after fasted cardio?

You’ll want to fill up on a combination of protein and complex carbohydrates, which help you refuel and support muscle recovery. Fiber and healthy fats are also important components of a healthy post-exercise meal because they’ll help control your appetite and prevent you from overeating.

Some examples of good recovery meals post exercise can include: a protein smoothie with fruit, hemp seeds and coconut milk; a salad with protein and avocado; quinoa with hard boiled eggs and veggies; an open face sandwich with meat/fish/eggs and a side salad dressed with olive oil.

Can you have coffee before fasted cardio?

Most people find that coffee in the morning before working out gives them a welcomed boost of energy that helps them push through a workout. As long as you also drink water to prevent dehydration, and don’t feel jittery or nauseous from having coffee alone, then this seems to be a fine option.

Should you combine intermittent fasting and fasted exercise in the morning?

This can be a good strategy if you’re already accustomed to practicing intermittent fasting (IF). People who incorporate IF into their routines likely have more opportunities to do fasted workouts since they are skipping meals anyway, most often breakfast in the morning. You’ll want to listen to your body and look out for weakness and other warning signs, but as long as you feel good combining these two approaches then there’s no reason not to.

Final Thoughts

  • What is fasted cardio? It’s another way to describe working out on an empty stomach. Most often people do this in the morning after they’ve fasted overnight for 8+ hours.
  • Potential benefits of fasted cardio include increased fat burning, help with weight loss, and less indigestion. However, drawbacks can include increased fatigue and weakness while exercising, and a bigger appetite once the workout is over.
  • Does it work for weight loss? It can, but weight loss really depends on someone’s overall day and energy/calorie balance. A fasted workout can slightly boost calorie and fat burning, but in order for weight loss to happen, someone still needs to consume less calories than their body is using each day.
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