Intermittent Fasting And Working Out – What You Need To Know

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By Thomas Youngerman

Reviewed by Juliana Tamayo, MS, RDN - Last Updated

Intermittent Fasting And Working Out

People fast for health, religious, or cultural reasons. People exercise to build muscle, increase endurance, manage their weight, and for general health. Both intermittent fasting and exercise are effective methods to help manage weight and body composition. But can you combine them? Should you work out when practicing intermittent fasting (IF)? What do you need to know about intermittent fasting and working out?

This article will answer that question as we explore the various types of time-restricted feeding (TRF), present the pros and cons of exercise when fasting, and discuss the best types of exercise in a fasted state. The short answer is a resounding, “It depends!” To learn the various factors affecting exercise performance and how to safely and effectively exercise when fasting, read on.

Intermittent Fasting And Working Out Together 

Intermittent Fasting

If you’re reading this article, we assume you have some familiarity with intermittent fasting and calorie restriction (CR) diets. Both have garnered significant attention on social media and health and fitness publications as weight management methods and for their potential to increase longevity and delay the onset of age-related disorders.  

This article focuses on intermittent fasting; time-restricted feeding patterns that mimic the effect of calorie restriction. There are various forms of IF that people practice, each featuring fasting and a scheduled time for meals. Research indicates a fast must last a minimum of twelve hours before you enter a fasted state. 

Intermittent Fasting

The 16/8 Method

Sixteen hours of fasting, followed by eight hours for your meals. This is perhaps the easiest and most sustainable TRF pattern – and one frequently used by lifters and athletes. 

The 12-Hour Method

This approach features the shortest fasting period necessary to trigger the benefits of fasting. 

The Fast 5 Method

With the Fast 5, you abstain for nineteen hours and eat as much as necessary in the remaining five hours in the day.

Other more aggressive patterns include the One Meal a Day Method, you fast for twenty-three hours of the day, and the Alternate Day strategy, where you fast for twenty-four consecutive hours. People also cite the 5:2 Method as a TRF fast. In the 5:2, you eat as you normally would for five days and practice two days per week of a restricted-calorie intake, in essence, more of a CR diet than fast.

Intermittent Fasting And Working Out: Pros And Cons

There are several important considerations when addressing exercise in a fasted state:

  1. What are your primary goals and objectives? Fat loss, strength and muscle gains, delaying age-related conditions, and longevity? 
  2. What type of exercise routine are you doing while fasting? Light cardio, endurance training, resistance training, circuit workouts? 
  3. What pattern of IF are you following? You’ll need to schedule your workouts strategically, based on fasting versus fueled windows. 

Your reason for working out while training and the type of routine you intend to do will inform the discussion on your program’s advisability and effectiveness. 

Girl Working Out

Intermittent Fasting And Working Out For Fat Loss

The benefit of exercise while fasting is that your stored glycogen levels are depleted, so you burn more fat to fuel your workout. Bodybuilders and other strength athletes endorse “fasted cardio” as a strategy to help get ripped. We concur you will burn more fat in a fasted state, provided the cardio session is low-moderate intensity.

Intermittent Fasting And Working Out For Muscle Gains

If your main objective is muscle or strength gains, it’s not optimal to exercise in a fasted state. It is difficult to build muscle in a calorie deficient state. Conversely, if fat loss is your goal, you may still want to incorporate some resistance training to maintain your lean muscle while fasting. You may not gain lean mass, but you can protect what you already have. 

Intermittent Fasting And Working Out As An Anti-Aging Strategy

We’ve previously mentioned that the physiological effects of intermittent fasting mimic that of a calorie restriction diet. Long term laboratory studies have well documented the anti-aging and life extension potential of CR and intermittent fasting. Exercise is yet another component of an anti-aging strategy. 

Combining exercise and IF effects changes in hormones and cell function. Glucose and insulin levels drop, Human growth hormone (hGH) levels increase, and autophagy (a process that helps destroy unwanted or damaged cells to regenerate newer and healthier ones) increases – all keys to staying youthful and lean.

When To Work Out While Intermittent Fasting

When to work out? It depends; you have three options for scheduling your workouts while intermittent fasting, before, during, or after your fueling window. Before your eating window is ideal for optimizing fat-burning. It’s better to do higher intensity activities during the fueling window and allows you to capitalize on post-workout nutrition. After the window is for those who like to exercise after fueling, the protein you ingested during the window will help support your workout and muscle recovery. 

We recommend you schedule your more aggressive, more intense lifting sessions during the fueling window so that your body has some glycogen stores to tap into to fuel your workout, and a potential post-workout protein drink for regeneration. 

If practicing the Alternate Day IF Method, we recommend a lower intensity workout during your fasting period. And regardless of your type of workout or eating pattern, be sure to stay hydrated and maintain your electrolyte levels.

Working Out While Fasting

Repair And Recovery From A Fasted Workout

When fasting beyond twelve hours, your body’s liver glycogen stores are depleted, fatty acids are mobilized, and ketones are produced. Your body begins to use the energy reserves stored in fat for fuel. This facilitates the fat-burning process, but what about your muscles? Recovery after an intense fasted workout is dependent on fluid and nutrient intake to optimize rehydration and protein synthesis for muscle repair. 

We suggest a post-workout protein meal that contains all the essential amino acids. Ketones help spare muscles from oxidative stress and increase the circulation of branched-chain amino acids. Leucine, one of the BCAAs, plays an essential role in muscle protein synthesis. The combination of muscle protein synthesis and resistance training will help you maintain lean muscle tissue even during a fast. Just be sure to eat the right meal after your workout. 

Benefits Of Intermittent Fasting And Working Out

The benefits of intermittent fasting and calorie restriction are well documented. Likewise, we intuitively know that regular exercise is healthy. There is evidence that the synergistic effect of combining the two may actually increase the benefits of both. This is obvious in weight loss; however, slowing the aging process and disease avoidance are also amplified due to changes in metabolism, hormones, and cell function.

Main Benefits

  • Insulin levels decline
  • HGH – human growth hormone levels increase
  • Exercise in fast state aid in the release of testosterone
  • Autophagy is increased

Additional Benefits

  • Fasted training improves endurance and increases muscle glycogen stores
  • Weight training while fasting reduces the likelihood of regaining lost weight as muscles increase metabolism
  • Training in a fasted state helps your body become more efficient at burning fat for energy 

Other Documented Benefits Of IF And CR

  • Reduced risk of type 2 diabetes
  • Improved cardiovascular health
  • Reduces oxidative stress and chronic inflammation 
  • May help prevent Alzheimer’s Disease
  • May reduce the risk of cancer – proven effective in animal laboratory studies

Working Out While Fasting

Clinical Studies – Working Out When Fasting

The NCBI (National Center for Biotechnology Information), in an overview of clinical trials performed to validate the effectiveness of exercising while fasting, states, “It is now increasingly recognized that physical training and fasting have beneficial effects on body composition and health. Exercising during a fasting state increases lipolysis in adipose tissue while also stimulating peripheral fat oxidation, resulting in increased fat utilization and weight loss.”

So far, so good. Clinical confirm that exercising in a fast state helps us to burn fat and lose weight. Further studies attempting to establish if intermittent fasting and exercise improve physical performance were not as positive – several studies found decreased performance, and others found no significant effects.

Other well-constructed studies validate the insulin sensitivity, endurance gains, and muscle glycogen benefits cited in the benefits above. 

Is Intermittent Fasting And Working Out Safe?

The most frequently cited adverse effects of working out while fasting relates to muscle retention and performance. Experts warn that you may start burning muscle to fuel your workouts as well as the stored fat you are attempting to eliminate. Others tell us that in the absence of carbs and glucose, you may “hit the wall,” decreasing your performance. 

Other cautions are lowered blood pressure when combining IF with exercise causing “lightheadedness” or low blood sugar, resulting in fainting. For some individuals, fasting and exercising may have more potential risks:

  • People with diabetes
  • Those with low blood pressure
  • Those with eating disorders
  • Pregnant women or women who are breastfeeding

As with any new diet or exercise regimen, we highly recommend you check with your doctor before combining exercise and IF.  

Supplements To Support Workouts When Intermittent Fasting

A potential side effect of intermittent fasting is the potential to create nutritional deficiencies. When you add exercise, it becomes even more essential to provide your body with the right nutrients. Below are several dietary supplements you may want to add to your regimen when fasting:

  • Multivitamin/Mineral 
  • Omega 3 Fatty Acids
  • Vitamin D3 
  • B Complex – for energy and to enhance the absorption of minerals

Several additional supplements that will help support your exercise-related needs:

  • BCAAs (Branched-chain amino acids)
  • L-Glutamine
  • Exogenous ketones
  • MCT Oils
  • Caffeine

If exercising during a fueling window, you may want to include a pre-workout supplement. Some choices worth looking at include Total War Pre Workout, BlackWolf Pre Workout, and MRI Black Powder. We also recommend a post-workout protein with all the essential amino acids, such as Clear Muscle. Regardless of your exercise schedule, stay hydrated, and be sure to replenish your electrolytes.

Supplements for fasting

Our Take On Intermittent Fasting And Working Out 

The typical Western pattern diet is heavy on red meat, refined sugars, processed foods, and saturated fats. The diet is low on protein intake (15%) and higher in fats (35%) and carbohydrates 50%). Combined with a sedentary lifestyle, the Western diet can lead to weight gain, a higher risk of obesity, type 2 diabetes, premature aging, and the onset of age-related illnesses. 

In this article, we’ve set out the many benefits of intermittent fasting and calorie restriction – clinically documented benefits. Regular exercise has its own set of advantages, and now we know that when you practice both IF and exercise, the benefits are compounded. 

In our opinion, it is not only okay to combine them; we believe you should combine working out when fasting to maximize their synergistic benefits.   

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Thomas Youngerman

Thomas Youngerman is an entrepreneur and author in the health and wellness space with extensive experience in the supplement industry. He has owned multiple sports nutrition stores and served as the Category Manager and Director of Business Development for a regional chain of nutrition stores. Thomas created a successful line of men’s healthy aging supplements that was distributed in GNC, The Vitamin Shoppe, and Kroger, and later sold to a West Coast corporation. Thomas was previously a certified nutrition coach. He has a strong understanding of nutrition, supplement formulations, DSHEA, cGMP, and FDA regulations.