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August 27, 2018

Smart Exercise During Intermittent Fasting

Exercise During Intermittent Fasting

exercise during intermittent fasting
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Exercise during intermittent fasting – how do you do it? Before we go into the options for exercise when following an intermittent fasting program, let’s first discuss what intermittent fasting is.

Intermittent fasting is when you only eat during a rather short window of time during the day, or eat very limited calories some days of the week. The 8 hour eating plan we discussed already is an example of intermittent fasting. The Every Other Day Diet is another form of intermittent fasting where you alternate eating a normal amount of food one day with typically 500 calories a day the next.

The Rationale Behind Intermittent Fasting

One of the main reasons people follow an intermittent fasting program is because of the positive metabolic and hormonal changes fasting induces that lead to improved health and fat loss.

Intermittent fasting more closely mimics the way our paleo ancestors ate. They did not have 24/7 access to food like we do today. Our ancestors would have gorged for a day or two on animal that was killed and then they would go stretches without much to eat. Plus, they were simply more active throughout the day than we are today. Going through stretches were you are not eating actually makes the body work more efficiently.

Exercise in Fasting or Fed State

When you are intermittent fasting you have two choices when it comes to exercise. You can exercise during the time you are actually fasting. Or, you can exercise during your feeding window. This would be called exercising in a fed state.

There are pros and cons to exercising during the fasting phase as well as exercising during a fed state. Also, how you exercise depends on your exercise goals. Are you exercising to lose fat, exercising to gain muscle, or exercising to improve cardio fitness?

Exercise During Intermittent Fasting

Exercise during intermittent fasting will facilitate fat loss when you exercise in the fasted state. So if fat loss is your goal then try to exercise in a fasted state.

Since you have already been fasting so your insulin levels will be low. Exercising when insulin levels are low enables your body to tap into fat stores to provide calories for your workout. In other words, you are more likely to burn fat during your workouts when in a fasting state.

Plus, you set the tone to increase your metabolism for the rest of the day by exercising in a fasting state through hormonal changes enabling you to burn more fat during the remainder of the day.

But, when exercising during intermittent fasting it is likely you will not be able to exercise at the same intensity compared to if you were to exercise during the feeding window because your body is not metabolically prepared to handle too much of a strenuous workout.  When you exercise in a fasted state you are walking a fine line between burning fat and burning muscle. You do not want to burn muscle which you are a risk of doing if trying to max out on your lifts or doing excessive cardio.  So ……

It is recommended that you not do any prolong cardio or aerobics in a fasting state (more than 30 minutes) as is doing so leads to a catabolic state (catabolism) where you will break down muscle and burn that as fuel.

Regardless of your reasons for exercise you never want to break down muscle. Muscle is your friend.  Ultra-endurance athletes really need to be careful of breaking down muscle. Making sure you get plenty of protein during your feeding window will minimize muscle loss.

Also, avoid lifting at your maximum in a fasted state as you are not going to get the same benefit if you do maximal lifts in a fed state and you risk teetering from fat burning to muscle burning. If weight loss is your goal then focus on full body exercises.

These include body weight exercises and exercises with dumbbells where you are moving most of your body simultaneously – arms, legs, and core muscle groups. Doing such exercises will trigger larger hormonal responses in your body that preserve muscle and burn fat.

Exercise in Fed State

If you exercise during your feeding phase of the day you can still lose weight but your exercise will need to be more intense and/or of longer duration to achieve the fat loss results as exercising during the fasting period. You might have to workout an additional 30 minutes to get the same weight loss effects as you would if you exercised in a fasted state.

But, that should not be a problem because your body will have enough energy stored in the blood stream and muscles to handle a more intense or longer duration exercise session.

If your goal is to gain muscle and strength then exercise during the fed state as you will be able to really push the intensity of your workouts and your muscles will have the nutrients to handle maximum loads. Using free weights and weight machines will likely give you better strength results than body weight exercises.

At the end of the day, you have to consider what is the most convenient time in the day to exercise regardless if you are fasting or in a fed state. Personally, I do both –  I will exercise in a fasting state and fed state.

During the week I workout as soon as I am done seeing my last patient when I am in a fed state and on the weekends I exercise early in the morning when I am in a fasted state. I typically focus on strength training and high intensity interval training. I also do 3 days of cardiovascular exercise for 45-60 minutes


Exercise in Fasted State

  • Best for weight loss (fat loss).
  • Avoid prolong cardio and maximum lifts.
  • Focus on body weight exercises.

Exercise in Fed State

  • Better for maximum strength gains.
  • Can burn fat but will need to workout longer (30 additional minutes) compared to exercise in fasted state.

Exercise during intermittent fasting can be done and may aid in weight loss.



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Dr. Joe Jacko

Dr. Joe is board certified in internal medicine and sports medicine with additional training in hormone replacement therapy and regenerative medicine. He has trained or practiced at leading institutions including the Hughston Clinic, Cooper Clinic, Steadman-Hawkins Clinic of the Carolinas, and Cenegenics. He currently practices in Columbus, Ohio at Grandview Primary Care. Read more about Dr. Joe Jacko

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