QUOTE from Dr Fung (emphasis mine):
FASTING IS DEFINED as the voluntary act of withholding food for a specific period of time. Non-caloric drinks such as water and tea are permitted. An absolute fast refers to the withholding of both food and drink. This may be done for religious purposes, such as during Ramadan in the Muslim tradition, but is not generally recommended for health purposes because of the accompanying dehydration.
Fasting has no standard duration. Fasts can range from twelve hours to three months or more. You can fast once a week or once a month or once a year. Intermittent fasting involves fasting for shorter periods of time on a regular basis. Shorter fasts are generally done more frequently. Some people prefer a daily sixteen-hour fast, which means that they eat all their meals within an eight-hour window. Longer fasts are typically twenty-four to thirty-six hours, done two to three times per week. Prolonged fasting may range from one week to one month.
During a twenty-four-hour fast, you fast from dinner (or lunch or breakfast) the first day until dinner (or lunch or breakfast) the next day. Practically, this means missing breakfast, lunch and snacks on the fasting day and only eating a single meal (dinner). Essentially, you skip two meals as you fast from 7:00 p.m. to 7:00 p.m. the next day.
During a thirty-six-hour fast, you fast from dinner on the first day until breakfast two days later. This means missing breakfast, lunch, dinner and snacks for one entire day. You would be skipping three meals as you fast from 7:00 p.m. the first day to 7:00 a.m. two days later. (See appendix A for sample meal plans and fasting protocols.)
Longer fasting periods produce lower insulin levels, greater weight loss and greater blood sugar reduction in diabetics. In the Intensive Dietary Management Clinic, we will typically use a twenty-four-hour or thirty-six-hour fast two to three times per week. For severe diabetes, patients may fast for one to two weeks, but only under close medical supervision. You may take a general multivitamin if you’re concerned about micronutrient deficiency. 
NOTE – What does it say?
The choice to fast is a voluntary decision on my part. Consequently, it will require either will power or routine to make this decision. Therefore the most effective way to fast for me would be to establish a routine of fasting.
FASTING IS DEFINED as the voluntary act of withholding food for a specific period of time.
Options include the 16 hour fast, which means eating within a defined period of the day. This could be 8 hours, from 9 to 5, with a late breakfast at 9 am and an early supper at 4 pm. Or it could be a 20 hour fast with eating lunch and supper within 4 hours, 1 pm and 4 pm. Or simply “one meal a day” (OMAD).
A 24-hour fast would last from supper one night to supper the following night. This is barely enough to activate the body’s fat-burning routines and processes. This is the pattern of Krista Varady’s Every Other Day Diet, in which you eat whatever you wish one day, and a limited supper on the next day which is no more than 25% of your daily caloric need. (11 calories per pound divided by 4.)
A 36-hour fast would be through one entire day and then to breakfast the following morning. In other words, after supper Sunday night, fast all of Monday and then eat Tuesday breakfast. A 40-hour fast starting Sunday evening would break on Tuesday at lunch and a 48-hour fast would break on Tuesday at supper. This length of time would be sufficient for the body’s fat-burning process to be activated.
we will typically use a 24 hour or 36 hour fast, two to three times per week.
At Dr. Fung’s Intensive Dietary Management Clinic, 24 and 36 hour fasts, 2 to 3 times a week, are frequently used. They are the basic building blocks of an intermittent fasting routine.
STRATEGY: Follow the basic IDM strategy of 2-3 fasts per week, 24 or 36 hours in duration.
1. What gets your attention?
2. What does it say about Reality? (Any misperceptions to correct?)
3. What does it say that I need to obey? (What should be added to my strategy?)
4. How can I incorporate what is true into my Disciplines? (Specific methods?)
5. What questions do I have for the experts? What might be the answers?
SOURCE – Footnotes:
 Quote from APPENDIX B, FASTING: A PRACTICAL GUIDE, The Obesity Code: Unlocking the Secrets of Weight Loss by Dr. Jason Fung; Greystone Books (March 3, 2016). Page 252.
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Key: Fung-10-30.01 Last Revision: 07/21/2020