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Why You Should Work Out While Fasting

Updated: Aug 9, 2021

One of Dr. Johnson’s favorite ways to start her morning is with a quick cardio session. Not only will it leave her energized for the day ahead of her, but working out in the morning is a painless way to get in a fasted workout.

Working out while fasting has been linked to numerous health effects, including weight loss, higher fat burning, and increased endurance.

weight loss, workout, exercise, fasting

Fasted workouts 101:

A fasted workout is pretty self-explanatory- it’s simply working out while you’re also in a period of fasting. But how long do you need to go before entering this fasted state? It’s usually defined as 8-12 hours without food. However, depending on how efficient your digestive system is at processing food, it could take as little as 3-6 hours.

When you’re in a state of fasting, your body has stopped breaking down food and insulin levels have dropped. There is also no available glycogen in your blood for your body to use as fuel. This forces your body to find another source of energy- usually fat. Lowered insulin levels will also allow body fat to be easily broken down and oxidized (burned) as free fatty acids.

In theory, this is why a fasted workout is more beneficial than a regular workout. As you push your body to find new sources of energy, your body eats away at stored fat. For decades, this technique has been used by professional athletes and bodybuilders as a quick way to cut fat.

Benefits of fasted workouts:

While Dr. Johnson enjoys a fasted workout and recommends it to many of her patients, the research behind fasted workouts is limited. However, what research has been performed has gleaned positive results such as the ones below:

Increased endurance: Many studies have found that fasted cardio, if done over an extended period of time, can lead to an increase in VO2 max (a measure of endurance capacity). One study in New Zealand found that after four weeks of fasted cycling, both male and female participants experienced significant increases in VO2 max. Moreover, another study found that fasting participants saw great improvements in endurance while exercising when compared to non-fasting participants.

Greater fat-burning potential: As mentioned above, one of the clearest advantages of working out while fasting is its potential for fat burning. Without carbohydrates to use for energy, the body must rely on other sources, such as fat. Several studies have demonstrated that working out while fasting burned up to 20% more fat. Another study found that fasted cardio resulted in a 6.2% greater loss of body fat percentage.

Increased growth hormone: Human growth hormone, or HGH, plays an essential role in your metabolism, regulation of body fluids, body composition, and muscle and bone growth. HGH is often accredited for slowing the aging process. One study found that after a 24 hour fast, women and men experienced a 1,300-2,000% increase in HGH production.

Improved insulin sensitivity: Fasting will activate a protein called AMPK. When activated, AMPK will decrease insulin levels so that you can burn fat. When you’re in a state of fasting, your insulin will be low, further enabling the burning of fat. When insulin levels are high, your body thinks you are well fed and actively resists burning fat and stores it instead. The best part of working out while fasting is that the more you do it, the less insulin your body will release, resulting in more fat loss in the long term.

Weight loss: Many studies have found positive results when looking at fasted workouts for weight loss. One study demonstrated that working out while fasting burned 20% more calories than a regular workout.

Easy on the GI system: For some people with a slower digestive system, if they work out after eating, they may be left feeling nauseous or cramping. Some researchers say you should wait between 30 minutes to 2 hours after eating before attempting a workout. Fasting before a workout will alleviate these problems and save you time, as you will have nothing currently digesting in your GI system as you exercise.

When to do a fasted workout:

The most important thing to keep in mind when working out while fasting is what kind of workout you choose to do. Experts agree that you should only do light, low-intensity cardio workouts while fasting. You should avoid any type of high-intensity interval training, boot camp, or CrossFit classes. And you should especially not weight lift or do strength training while fasting.

Workouts that get your heart rate very high require carbohydrates for quick energy. If you’re working out while fasted, you won’t have any to offer. This might leave you exhausted, sore, weak, and potentially lightheaded or nauseous. Further, doing intense workouts while fasted has been found to cause a breakdown of protein in muscle tissue- causing muscle loss.

How to workout while fasting safely:

Before jumping into a fasted workout, you should tread carefully at first to ensure your safety. Further, if you want to burn body fat and retain your fitness level, you should follow these tips below!

  • Stay hydrated → When you’re fasting, water is your best friend. Not only will it keep you hydrated all day long, but it is also one of the few things you can consume while fasting. Some experts recommend drinking more water while fasting.

  • Boost your electrolytes → While fasting, and especially while working out fasted, you should make sure to keep your electrolytes up. This can easily be done by adding some sea salt to your water!

  • Keep the duration and intensity low when doing long fasts → If you’re doing a fast 24 hours or more, you should stick to walking, yoga, pilates, and other low-intensity workouts.

  • Eat a meal after your workout → You should follow your fasted workout with a healthy, protein-heavy meal!

What to eat after a fasted workout?

So, what exactly should you be eating after completing a fasted workout? The International Society of Sports Nutrition recommends following a fasted workout with a combination of protein and carbohydrates within an hour after completing the exercise. Following this routine will help to promote muscle growth.

Some examples of this combo include:

  • Eggs and nuts

  • Oatmeal and milk

  • Vegetables and avocado

  • Yogurt and fruit

  • Salmon and sweet potato

  • Chicken and rice

One important thing to note about fasted workouts is that you’ll likely to hungrier than normal after their completion. So drink a full glass of water before eating a meal to ensure you don’t accidentally overeat.

If you’re interested in integrating a fasted workout into your health routine, contact our office at or call 276-235-3205 to schedule your complimentary 15-minute discovery call.

***For people with certain medical conditions, working out while fasting may not be the best choice. You should make sure to check with your doctor before beginning this new fitness regime. Moreover, if working out while fasting leaves you light-headed or nauseous, it may not be right for you.***

The Johnson Center for Health services patients in-person in our Blacksburg and Virginia Beach locations. We also offer telemedicine for residents of Virginia and North Carolina!