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3-Day Modified Fasting: The Basic Rules

Robyn Openshaw - May 11, 2020 - This Post May Contain Affiliate Links

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If you’ve heard the buzz about modified fasting (the easiest way to get the amazing health benefits of fasting1), your first question is probably something along the lines of, “Okay, what are the basic rules?”

We hear you! And the good news is, the basic rules of a 3-day modified fast are pretty darn simple (you may also want to check out our answers to FAQs about modified fasting, and tips for preparing to fast!

But without further ado, let’s get to the basics!

In this article:

1. Stay Under 800 Calories

The magic number is 800. Why? Because when you keep your total calories under 800,1 your body goes into recycle mode: a process called autophagy.2 During times of food scarcity (like while you sleep, fast, or during times of illness), your body takes the opportunity to “clean house” by destroying and consuming damaged or mutated cells to make room for healthy cells.

Autophagy works a little bit like a faucet, always trickling and turning up the flow anytime food scarcity is detected. Keeping your total daily intake between 600-800 calories keeps the faucet open wide, like it would be if you were doing a water-only fast!

2. Drink At Least 2 Quarts Of Water

We all know that water is important! After all, even a 1% water deficiency3 can cause all kinds of problems, including impaired kidney function and an overly acidic stomach. Hydration is especially important while fasting, since your organs and cells will be working extra hard to flush toxins and waste (including heavy metals that have been tucked away in your fat stores!)

Drink at least two quarts of fresh water each day of your modified fast (along with any other liquids allowed during a fast!) And don’t wait until you feel thirsty! (By the time you feel thirsty, you’re already well on your way to dehydration!). Sip water regularly throughout the day. You should be making plenty of trips to the bathroom!

picture of water being poured into a glass cup
"Drink at least two quarts of fresh water each day of your modified fast."

3. Eat 5 Mini Meals Of Clean, Nutritious Food

Each day of your modified fast, you’ll eat five mini-meals. Remember, the grand total of your meals should stay under 800 calories, so each one should be between 100-200 calories.

Since you’ll be eating a minimal amount of calories, with the goal of detoxing and encouraging autophagy, the quality of these meals is important! I recommend around 50 grams of protein each day, as well as plenty of fresh, raw or steamed vegetables, a small amount of fruit, and healthy omegas from chia and flaxseed.

4. Maintain A 12-Hour Eating Window

During a fast, when you eat is just as important as what you eat! Each day of your modified fast, you’ll need to choose a 12-hour eating window to consume all of your calories (for example, from 6 am to 6 pm).

Consuming all of your calories within this window will give your body a full 12 hours of advanced autophagy each day of your fast. Remember, autophagy is like a faucet!4 By lowering your caloric intake beneath that magic 800 number, you’re opening the faucet of autophagy to a steady stream. And by maintaining a 12-hour eating window, you’re turning up the faucet to high!

Remember, you should continue to hydrate even while you’re not consuming calories.

5. Skip the Sugar, Caffeine, Alcohol, and Artificial Sweeteners

I know this sounds like bad news, but trust me: Your results are worth it! Completely avoid alcohol,5 coffee6 (even decaf, it’s dehydrating and acidic), caffeinated drinks,6 sugar,7 and artificial sweeteners8 during your modified fast.

All of these substances place a heavy burden on your internal organs and digestive system. Not to mention that they create a more acidic environment in your body (leading to more inflammation), add toxins to your body (you’re trying to flush those!) and trigger cravings.

Summary: The Basic Rules of a 3-Day Modified Fast

Modified fasting is the easiest way to gain all the benefits of fasting — without going completely hungry. Just eat five nutritious mini-meals that total less than 800 calories, consume those meals within a 12-hour window, drink at least two quarts of water per day, and stay away from alcohol, caffeine, sugar, and artificial sweeteners during your fast. That’s it!

Interested in learning more good stuff about modified fasting? Check out these tips for sailing through a 3-day modified fast! You can also see my videos showing how and why I fast on YouTube.

Read Next: What (And When) Can You Drink While Doing A Modified Fast? Tips, Recipes, and Best Practices

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.


  1. Longo, V. D., & Mattson, M. P. (2014). Fasting: molecular mechanisms and clinical applications. Cell metabolism, 19(2), 181–192. doi:10.1016/j.cmet.2013.12.008.
  2. Mizushima N, Levine B, Cuervo AM, Klionsky DJ. Autophagy fights disease through cellular self-digestion. Nature. 2008;451(7182):1069–1075. doi:10.1038/nature06639
  3. Riebl SK, Davy BM. The Hydration Equation: Update on Water Balance and Cognitive Performance. ACSMs Health Fit J. 2013;17(6):21–28. doi:10.1249/FIT.0b013e3182a9570f
  4. 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
  5. Shield KD, Parry C, Rehm J. Chronic diseases and conditions related to alcohol use. Alcohol Res. 2013;35(2):155–173.
  6. Cornelis MC. The Impact of Caffeine and Coffee on Human Health. Nutrients. 2019;11(2):416. Published 2019 Feb 16. doi:10.3390/nu11020416
  7. Rippe JM, Angelopoulos TJ. Relationship between Added Sugars Consumption and Chronic Disease Risk Factors: Current Understanding. Nutrients. 2016;8(11):697. Published 2016 Nov 4. doi:10.3390/nu8110697
  8. Sharma A, Amarnath S, Thulasimani M, Ramaswamy S. Artificial sweeteners as a sugar substitute: Are they really safe?. Indian J Pharmacol. 2016;48(3):237–240. doi:10.4103/0253-7613.182888

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.

Posted in: Fasting, Healthy Weight, Preventive Care

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