Is exercise while fasting mutually exclusive?

Fasting as in:

  • one to three days on just water, and
  • five days a month on a reduced calorie diet (fasting-mimicking diet)
  • NOT a one meal, eight hour period of no eating

Exercise as in:

  • 10 minutes of cardio with > 70% of max heart rate
  • 15 minutes of calisthenics (prison workout)
  • 5 minute of stretching

Are there any studies on how much or how intense exercise is safe while on a fast? I'm looking for exercise guidelines while being on both a total fast (water only) and a fasting-mimicking diet

Fasting-mimicking diet:

day one of the diet, they would eat 1,090 calories: 10 percent protein, 56 percent fat and 34 percent carbohydrates. For days two through five, 725 calories: 9 percent protein, 44 percent fat, 47 percent carbohydrates.

EDIT: there are some concerns raised about my particular biometrics, so I've posted it in my profile.

The reason I'm asking this question is not because I want to rapidly lose weight, as I already believe I'm at a healthy weight. In a nut shell, I've seen articles indicating that fasting can have a positive effect on the body (detox, etc.. ). The articles did not touch on activity while fasting, I was wondering if there were any studies that did.

  • If you have undiagnosed McArdle disease, it could kill your kidneys. Commented Jul 2, 2015 at 21:16
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    @IronPillow - Again, you are fear mongering. Please stop. Anyone can have a number of undiagnosed diseases. Your citing of rare conditions to discredit valid answers is unwelcome. Commented Jul 3, 2015 at 4:48
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    "Anyone can have a number of undiagnosed diseases." Absolutely true. And bad outcomes occur when no one stops to think about the possibilities. We screen high school athletes for hypertrophic cardiomyopathy, which is rare. The military screens every recruit for sickle trait, to avoid the small number of sudden deaths it causes during exertion. This site takes cavalier liberties with people's health that it shouldn't. I'm sorry if that is unwelcome news, but suppressing alternate viewpoints is hardly the mark of an open community. Commented Jul 3, 2015 at 6:38
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    @IronPillow I think this conversation should go on meta. FYI, the help center has this already stated: If your question is requesting personal medical advice, it is off-topic and instead should be directed to your personal physician. (emphasis added by me). If you feel strongly that the disclaimer is not adequate, please open a question on meta instead of hijacking this question.
    – anon
    Commented Jul 4, 2015 at 4:12

2 Answers 2


Provided that you are healthy, not suffering from a cold, disease state or other suppression of the immune system, moderate exercise during fasting periods should not impact your health. For submaximal efforts, you may notice earlier fatigue, and there will be definite performance impacts the closer you get to maximal efforts.

Caveat: You are still introducing an artificial state (fasted), which may have health complications. I would schedule a consult with a physician and explain your plans, and have them give you a checkup with that in mind before starting this.

However, there will be some impact on the effective level of the exercise, and how well you are able to perform the exercise during the later parts of the fasting period. These changes include depression in max VO2 (Although this value is more of a performance predictor, not really a measurement metric), higher levels of free fatty acids (FFA's) in the blood, as well as a somewhat suppressed gluconeogenesis as well as a concurrent rise in fat based oxidation for energy.

What this means, is that your body will (mostly) use up all muscle and hepatic (liver) glycogen storage, and will turn to fax oxidation (ketone bodies) for fuel. There is not that much difference in the lactate/pyruvate levels during this time, suggesting that the switch is adequate to fuel performing muscles. There are other short term changes in several hormone and substrate (glycogen, pyruvate, lactate, etc) levels, but these are returned to normal after resuming normal dietary intake.

While not an ideal state to pursue, intermittent periods of short fasting ( < 12 hours) or even up to a few days shouldn't have any health impacts, although you may notice some performance impairment.

This study I was able to obtain, only the first page is freely available. This took 5 obese subjects, and in a clinical setting (hospital), underwent a 3-5 week fast, with only water and vitamin supplements for health. They showed the changes described above in hormone and substrates, with a drop in VO2 max levels at the 2 and 4 week exercise marks. There was also substantial weight loss, but that is to be expected. They showed no other adverse health effects during the prolonged fast, but to emphasize, they had adequate water intake and daily vitamin and potassium supplementation.

Another paper available through Dartmouth Sports looked at 12 hour fasting for Ramadan and the effect on athletic performance, and found that even in the limited fasting state, there is some performance impact. One passage stands out in the paper:

An extensive review of the older literature on the effects of fasting on endurance performance was published by Aragón- Vargas.(21) The conclusion of this review was that a short period (24 h to 4 days) of fasting in humans resulted in a decreased capacity to perform endurance exercise. In spite of a rather consistent effect of fasting, however, there was no clear evi- dence as to the mechanism responsible for the earlier onset of fatigue.

That is corroborated by other sections in the paper, one detailing performance impacts on cycling at 100% VO2 max level after 24 hours fast, and impacts on high intensity events such as the 100m and 800m runs. Interestingly, one section shows no difference in muscle glycogen stores in the absence of exercise during fasting.

There may be some effect of a reduction in the muscle glycogen store on the maximal rates of muscle glycogenol- ysis with a consequent loss of exercise performance dur- ing high-intensity exercise, but a few days of fasting in the absence of exercise has little effect on muscle glycogen content.(18)

They do postulate that because of the metabolic acidosis that starts rising with prolonged fast is part of the reason for early fatigue in exercise, which is interesting.

The second paper also has 57 cited studies relating to exercise in a fasted state.

  • awesome answer, i'll read over the referenced articles when i get home from work.
    – anon
    Commented Jun 30, 2015 at 16:48
  • @ton.yeung - Some (if not many) may be behind paywalls. The Ramadan one is fully available, and you can use Google Scholar to see if the others will come up. Also see what I recommended for physician consult first.
    – JohnP
    Commented Jun 30, 2015 at 16:57

If you weigh 400 pounds, the answer is: No.

If you are an Olympic distance swimmer, the answer is: Go ahead.

If you are in between, the answer is: See your doctor.

In other words, no one can give you a safe answer without knowing a lot more about you. It is unwise to ask questions like this on the internet, and even unwiser to heed the answers.

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    This is a health site, and this is not an unanswerable question. Commented Jun 30, 2015 at 4:25
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    Sure, it is answerable if you want to be irresponsible. A responsible answer, however, requires too many caveats, or, without them, too many assumptions. OP has signs of being desperate to lose weight (i.e. combining fasting plus exercise). This suggests s/he is overweight and probably sedentary, which raises the probability of complications, no matter what answer is given. What else hasn't OP told us? There is no hint as to age, nor prior medical history -- certain medications could make this dangerous indeed. OP's question has many moving parts, any of which could lead to harm. Commented Jun 30, 2015 at 5:35
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    "Are there any studies on how much or how intense exercise is safe while on a fast"? I'm not looking for individualized care. As for my physical health, see edit above.
    – anon
    Commented Jun 30, 2015 at 14:20

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