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How Often Can You Do A 3-Day Fast?

Robyn Openshaw - Dec 30, 2019 - This Post May Contain Affiliate Links

Photo of woman holding up a chocolate protein shake from "How Often Can You Do A 3-Day Modified Fast?" by Green Smoothie Girl

One of the top questions people ask after completing their first modified fast is, “How often can I do this?”

And no wonder! Modified fasting is simple, doable (especially compared to water fasting!), and packs so many health benefits, including cell repair and detoxification,1 weight loss,2 better immune function,3 and even a longer life.4

In this post, I’ll answer your top questions about how often you can fast, and why:

In this article:

How Often Can I Do a 3-Day Modified Fast?

How often you choose to complete a modified fast should really depend on your health goals. While most of us think of three square meals a day as “normal,” it can be helpful to remember that food scarcity was the norm for most of human history.

Our bodies are hardwired to take advantage of missed meals on a very regular basis!

You can safely fast (without lowering your metabolism,5 more on that in a bit) as often as once per week if you have a significant amount of weight to lose,6 are working to improve insulin resistance or lower your A1C,7 or as part of an autoimmune treatment protocol.8

Many people with generally good health choose to fast once per month, or once per quarter to regularly reap the protective health benefits of modified fasting!

Will Regular Modified Fasting for Three Days Cause Muscle Wasting?

A lot of people worry that fasting for any length of time will make their body start using up muscle for fuel. But think about that logic for a second: why would mother nature cannibalize your muscles during times of food scarcity? In the wild, you’d need those muscles more than ever, to search for food, run from predators, etc.

During a fast (unless your body fat percentages are extremely low— we’re talking less than 4 percent),10 your body will use fat stores for energy first. That’s why they exist! You’d have to fast for a very long time, with zero calorie intake, to start using muscle for energy. In one large-scale study,11 where participants fasted 35 out of 70 days, they lost an average of 6% body mass and 11% fat mass. Translation: your muscles are safe!

[Related: Modified Fasting 101: The Complete Beginner’s Guide]

Will Modified Fasting Slow Down My Basal Metabolic Rate If I Do It Too Often?

As fasting expert Dr. Jason Fung says,12

“If periodic starvation caused our metabolism to decrease, then we would have less energy to hunt or gather food. With less energy, we would be less likely to get food. So, another day passes, and we become even weaker, making us even less likely to get food — a vicious and unsurvivable cycle.”

Dr. Jason Fung

The root of this fear may come from the fact that our basal metabolic rate DOES fluctuate based on calorie increases and restrictions. If you usually eat 2,000 calories per day and restrict to 1,500 calories per day, your basal metabolic rate will decrease to match your caloric intake. However, as Fung adds,

“Decreasing food intake is matched by decreased energy expenditure. However, as food intake goes to zero, the body switches energy inputs from food to stored food (fat).”

Dr. Jason Fung

In other words, fasting doesn’t trigger your basal metabolic rate to slow down. It triggers a switch from using consumed energy for fuel to using stored energy for fuel.

[Related: A 3-Day Modified Fasting Eating Plan With Menu And Recipes]

What Effect Will Regular Modified Fasting Have on My Hormone Levels and Metabolic Pathways?

What we eat and how often we eat has a big impact on our hormones and metabolic pathways. Which begs the question: how does regular fasting impact them?


Insulin is the hormone that allows our cells to use energy. Insulin is also responsible for storing energy as fat. A poor diet high in sugar and highly-processed carbohydrates results in major problems in insulin production— either way too much or way too little.

Fasting is the most effective way to improve insulin sensitivity and help the body heal its pathways for producing insulin.13 During a fast (in individuals who do not have diabetes), blood sugar levels stay steady, while fasting insulin levels and insulin sensitivity improves.

Growth Hormone

During a fast, your body will produce significantly more growth hormone than usual. Growth hormone helps maintain bone density and muscle mass (even more evidence that shows your body won’t cannibalize its muscles!). It also helps the body effectively use fat stores for energy during a fast.

For the most part, people see a steady decrease in growth hormone as they get older. But when you fast? It turns out that fasting doubles the amount of growth hormone released!14


Fasting increases adrenaline production slightly (by about 3 percent),15 which is actually your metabolism revving up instead of slowing down, like it would in response to many other dieting strategies.

Scientists speculate that this is so our bodies have the energy to search for food and stay alert to predators during times of scarcity. This mild increase in adrenaline is one of the reasons you don’t want to stay in a fasted state ALL the time. Your adrenal glands need to rest.
[Related: 9 Tips For Sailing Through A 3-Day Modified Fast]

TLDR: How Often Can You Do a Three-Day Modified Fast?

For most people, fasting once per month or once per quarter is just the ticket for preventative healthcare that helps improve lifespan, lower inflammation, improve insulin sensitivity, regenerate healthy cells, and flush waste and old or mutated cells.

If your health goals include significant weight loss or you are looking to support insulin function or autoimmune conditions, you can fast (with the support of your functional medical practitioner) as often as once per week without fear of muscle wasting, slowed metabolism, or problems with hormone levels.

Read Next: Why I Developed The Flash Fast: Modified Fasting Made Easy and Fun

Photograph of Robyn Openshaw, founder of Green Smoothie GirlRobyn Openshaw, MSW, is the bestselling author of The Green Smoothies Diet, 12 Steps to Whole Foods, and 2017’s #1 Amazon Bestseller and USA Today Bestseller, Vibe. Learn more about how to make the journey painless, from the nutrient-scarce Standard American Diet, to a whole-foods diet, in her free video masterclass 12 Steps to Whole Foods.

Disclosure: This post may contain affiliate links that help support the GSG mission without costing you extra. I recommend only companies and products that I use myself.

Photo of woman holding up a chocolate protein shake with post's title text overlay from "How Often Can You Do A 3-Day Modified Fast?" by Green Smoothie Girl


  1. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254–268. doi:10.1002/oby.22065
  2. Ganesan K, Habboush Y, Sultan S. Intermittent Fasting: The Choice for a Healthier Lifestyle. Cureus. 2018;10(7):e2947. Published 2018 Jul 9. doi:10.7759/cureus.2947
  3. Mohapatra, Sipra, et. al. “Short-term starvation and realimentation helps stave off Edwardsiella tarda infection in red sea bream (Pagrus major).” Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology. Volume 206, April 2017, pages 42-53.
  4. Heilbronn L.K., de J.L., Frisard M.I., DeLany J.P., Larson-Meyer D.E., Rood J., Nguyen T., Martin C.K., Volaufova J., Most M.M., et al. Effect of 6-month calorie restriction on biomarkers of longevity, metabolic adaptation, and oxidative stress in overweight individuals: A randomized controlled trial. JAMA. 2006;295:1539–1548. doi: 10.1001/jama.295.13.1539.
  5. Fung, Jason Dr. How to Fix Your Broken Metabolism by Doing the Exact Opposite. Intensive Dietary Management.
  6. Byrne NM, Sainsbury A, King NA, Hills AP, Wood RE. Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. Int J Obes (Lond). 2018;42(2):129–138. doi:10.1038/ijo.2017.206
  7. Furmli S, Elmasry R, Ramos M, Fung J. Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin. BMJ Case Rep. 2018;2018:bcr2017221854. Published 2018 Oct 9. doi:10.1136/bcr-2017-221854
  8. Choi IY, Piccio L, Childress P, et al. A Diet Mimicking Fasting Promotes Regeneration and Reduces Autoimmunity and Multiple Sclerosis Symptoms. Cell Rep. 2016;15(10):2136–2146. doi:10.1016/j.celrep.2016.05.009
  9. de Groot S, Pijl H, van der Hoeven JJM, Kroep JR. Effects of short-term fasting on cancer treatment. J Exp Clin Cancer Res. 2019;38(1):209. Published 2019 May 22. doi:10.1186/s13046-019-1189-9
  10. Fung, Jason. The Obesity Code, page 242. Vancouver : Greystone Books, 2016.
  11. Bhutani S, Klempel MC, Berger RA, Varady KA. Improvements in coronary heart disease risk indicators by alternate-day fasting involve adipose tissue modulations. Obesity (Silver Spring) 2010;18:2152–2159.
  12. Fung, Jason. The Obesity Code, page 244. Vancouver : Greystone Books, 2016.
  13. Cheng, Chia-Wei et al. Fasting-Mimicking Diet Promotes Ngn3-Driven β-Cell Regeneration to Reverse Diabetes. Cell. 02/2017.
  14. Ho KY, Veldhuis JD, Johnson ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968–975. doi:10.1172/JCI113450
  15. Drenick EJ, Swendseid ME, Blahd WH, Tuttle SG. Prolonged Starvation as Treatment for Severe Obesity. JAMA. 1964;187(2):100–105.



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One thought on “How Often Can You Do A 3-Day Fast?”

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  1. Kris says:

    Good succinct summary and VERY nice work!

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