Jesus began his instructions on fasting with “when” you fast, not “if” you fast (Matthew 6:16). The implication is that fasting was an integral part of every Christian’s life. Certainly by the time of the Didache (circa a.d. 100), Christians were fasting “on the fourth day and the Preparation” (i.e. on Wednesdays and Fridays). This wasn’t a special rule for particularly pious Christians; it was the norm for all Christians.
Fasting therefore came with religious connotations. This situation persisted until the late nineteenth century, when two physicians ushered in a new mindset. First, Dr. Henry S. Tanner completed a 40-day fast, proving that such a long fast was possible for ordinary human beings and not just for Jesus. Second, Dr. Edward H. Dewey popularized fasting for health with books such as The No-Breakfast Plan and the Fasting-Cure.
Based on my experience with fasting for up to 7 days, here are my top ten hints and tips for fasting, especially for extended fasts of more than 24 hours.
Fasting can be uncomfortable at times. You’ll be better equipped to deal with difficulties if you have a clear idea of your motivation. Be prepared to remind yourself why you’re fasting. That’s how you’ll pull yourself through the difficult spells. Think of the benefits you want to obtain.
If you’re fasting for weight loss, weighing yourself daily provides additional motivation. It’s so satisfying to watch the pounds come off. Don’t panic if you hit a plateau for a day or two. It’s the long-term results that count. If the plateau really bothers you, then insert the occasional longer fast into your fasting routine.
A couple of other ways of dealing with hunger when fasting for weight loss:
The fasts in the Bible were for a fixed length of time: Daniel’s fasts of 10 days and 21 days, Jesus’ fast of 40 days, and so on.
In the nineteenth century, Dr. Edward H. Dewey invented the concept of a “complete” fast. He observed that ravenous hunger disappears after a few days. There’s then a long period where the patient is not particularly hungry. A real hunger then returns. It’s at that point that you break the fast. He called this cycle a “complete” fast. You can’t determine in advance how many days your fast will last.
You need to decide whether you’re going for a fixed-length fast or a variable-length fast. (You don’t need to take it as far as a “complete” fast.) Obviously a medical emergency will cause you to break your fast early in either case
People fasting for weight loss often begin with fasts of 16 hours or 18 hours. These are the intermittent fasts that correspond to what Dr. Dewey called the “no-breakfast plan.” Dieters sometimes work their way up toward one-meal-a-day (OMAD) or even one meal every two days (called “rolling 48s”). After a while, they may want to try longer fasts of 3 days, 5 days, and so on.
People fasting for spiritual reasons, or as an experimental cure for chronic illnesses, tend to dive right in and go for 10, 20, 30, or 40 days. These longer fasts are often done just once in a lifetime, whereas intermittent or short-term fasts are repeated many times over.
Old-school water fasts were just that: fasting on water only.
During the twentieth century, people discovered the value of supplementing electrolytes. These are the elements — sodium, potassium, magnesium, etc. — that keep your body’s electrical signaling functioning properly. When dissolved in water, electrolytes split into positively and negatively charged ions that control various cellular processes.
A certain amount of weakness is normal during a fast. However, without electrolyte supplementation, symptoms such as fatigue, headaches, nausea, blood pressure changes, muscle cramps, and low energy can become more common. To avoid these difficulties, people nowadays often supplement electrolytes on fasts over 72 hours long.
You can buy pre-packaged electrolyte mixes that you add to water to make a drink, or you can just buy individual ingredients in a grocery store or pharmacy. The packages are more convenient, while the do-it-yourself option is cheaper.
Personally I mix a teaspoon of salt (sodium chloride), a teaspoon of No Salt (potassium chloride), and a scoop of Jamieson magnesium powder (magnesium bisglycinate and magnesium citrate) in a pitcher of water. Then I sip that slowly throughout the day.
For your reference, the United States Food and Drug Administration recommends 2,300 milligrams of sodium per day; 4,700 milligrams of potassium; and 420 milligrams of magnesium for men, 320 milligrams for women.
With no shopping, prepping, cooking, eating, and clean-up, you’ll have a lot of time on your hands. You may also sleep less.
Plan what you’ll do to stave off boredom. You don’t want to spend your fast thinking about food and how you’re not getting any/
Edward Purinton’s suggestion forms his rule 7 out of the 20 in his book The Philosophy of Fasting: “Plan work congenial but not compulsory.”
Only you can determine what fits that description for you. But do your planning in advance of your fast, not after you’ve already started.
You want to have a supportive environment, if possible. Surround yourself by positive people who are doing something to improve their lives.
At the very least, see if you can avoid the company of unsupportive people. You don’t want people around you saying: “You haven’t had a cheeseburger for at least three hours! You’ll collapse from exhaustion!”
Purinton suggests that it’s better to be surrounded by strangers than to have unsupportive people around you.
The main symptoms of fasting are hunger and weakness.
Many people experience headaches and fatigue not because of the fasting, but because of withdrawal from addictions.
Caffeine is a well-known addiction that produces headaches if you suddenly stop consuming it. Consider quitting in advance of your fast.
Carbohydrates are a less well-known addiction. A fast forces your body to do metabolic switching. If you’re not fat-adapted, you can mistake the “keto flu” for symptoms of your fast. Consider going on a ketogenic diet for a few days or weeks before you fast.
The general rule is to start eating slowly after a prolonged fast. You can estimate the time you’ll need to get back to normal after a fast as one-half the length of the fast itself. For example, if you fast for 7 days, plan to spend 3.5 days getting back to normal eating.
Most people start with foods that are easy to digest, such as liquid foods. After a long fast, that might mean broths and soups. For others, it might mean fruit or salads. Gradually add more solid food.
Cramps and refeeding syndrome can be prevented by getting enough electrolytes.
You may or may not want to plan how you’ll eat after the fast.
One purpose of Edward Purinton’s “conquest fast” is to get back in touch with your instincts. That might mean that you actively avoid planning in advance how you’ll eat after the fast. You don’t know yet.
On the other hand, if you’re fasting for weight loss, going back to a normal way of eating will likely mean you’ll regain all the weight you worked so hard to lose. In this case, you’ll definitely need a plan for how to eat after the fast is over.
Do any planning you need to do before you start the fast. You don’t want an excuse to think about food during the fast itself.
FAQ #9.1. “Can I eat/drink X while I’m fasting?” It’s up to you. If you’re fasting for religious reasons, you’ll have to consult your religion’s governing body. Otherwise, you can set your own rules. There’s no “World Council on Fasting” laying down rules on what you may or may not consume during a fast! Just as a matter of practicality, if you’re fasting for weight loss, a few calories here and there won’t make a substantial difference to your progress. Black coffee or black tea have minimal calories. On the other hand, if you definitely want to call what you’re doing a “water fast,” that means just water, possibly with electrolytes dissolved in it. The rules for a non-specific “fast” are up to you.
FAQ #9.2. “When does autophagy kick in?” Much information on the internet comes from unreliable sources and/or animal studies. While autophagy is held to be a good thing, and while insulin and protein may downregulate it, the exact timing in humans cannot be measured. Dr. Guido Kroemer, cellular biology researcher and professor of medicine at Paris Cité University, puts it like this: “We don’t know.” YouTube video, “Minimum fasting length required for autophagy”
FAQ #9.3. “How much weight will I lose?” Part of your weight loss will be temporary and part permanent. Your temporary weight loss will be made up of glycogen, water, and poop. You’ll regain that when you start eating again. You can estimate your permanent fat loss as one pound (0.454 kg) for every 3,500 calories of food not eaten.
FAQ #9.4. “How much exercise can I do?” Some people treat their fast like a complete retreat and rest for the entire duration. Others do light to moderate exercise. I’ve never heard of anyone successfully doing intense exercise while fasting.
FAQ #9.5. “How long should I wait before doing another fast?” The minimum would be zero. Some people refeed for a few hours then go straight into the next fast. For 48-hour and 72-hour fasts respectively, this is called “rolling 48s,” and “rolling 72s.” The rule of thumb for extended fasts is that you should eat normally for three times the length of the original fast. For example, after a one-week fast, eat normally for three weeks before fasting again.
FAQ #9.6. “Is there a scientific paper that covers my exact situation and my exact fasting plan?” Probably not. Pharmaceutical companies aren’t lining up to pay for clinical trials of fasting. In any case, imagine how difficult it would be to recruit volunteers who might be randomly assigned to fast for 30 or 40 days. If you want to search for what little research exists, you can look on PubMed.
If you want to be more than a beginner, read Fung’s book on fasting. Michalsen and Longo offer alternative views, biased toward plant-based diets.
The old public-domain classics (Dewey, Carrington, Macfadden, Purinton, and Sinclair) contain some practical tips of value, but their medical knowledge is outdated.
Hall, Idleman, and Prince offer Christian views on fasting for spiritual purposes.
Dave Asprey. Fast This Way: Burn Fat, Heal Inflammation, and Eat Like the High-Performing Human You Were Meant to Be. New York: Harper Wave, 2021.
Hereward Carrington. Vitality, Fasting and Nutrition: A Physiological Study of the Curative Power of Fasting, together with a New Theory of the Relation of Food to Human Vitality. New York: Rebman Company, 1908.
Edward Hooker Dewey. The True Science of Living: The New Gospel of Health. Norwich, Conn.: Henry Bill, 1895.
———. The No-Breakfast Plan and the Fasting-Cure. Meadville, Pa.: Edward Hooker Dewey, 1900.
Jason Fung and Jimmy Moore. The Complete Guide to Fasting: Heal Your Body through Intermittent, Alternate-Day, and Extended Fasting. Victory Belt Publishing, 2016.
Franklin Hall. Atomic Power with God thru Fasting and Prayer. Originally published 1946. Revised and enlarged second edition 1950. Reprint, Mansfield, Conn: MartinoPublishing, 2016.
David R. Hawkins. “Weight Loss.” In Healing and Recovery, pp. 329–361. Sedona, Ariz.: Veritas Publishing, 2009.
Shane Idleman. 40 Days to Reset Your Life.
Valter Longo. The Longevity Diet: Slow Aging, Fight Disease, Optimize Weight. New York: Penguin Random House, 2019.
Bernarr Macfadden. Fasting for Health: A Complete Guide on How, When and Why to Use the Fasting Cure. New York: Macfadden Publications, 1923.
Andreas Michalsen. The Fasting Fix: Eat Smarter, Fast Better, Live Longer. Trans. Laura Wagner. Originally published in German as Mit Ernährung Heilen, 2019. New York: Penguin Viking, 2020.
Derek Prince. Shaping History through Prayer and Fasting. Original edition, Old Tappan, N.J.: F.H. Revell, 1973. New edition, New Kensington, Pa.: Whitaker House, 2018.
Edward Earle Purinton. The Philosophy of Fasting: A Message for Sufferers and Sinners. New York: Benedict Lust, 1906.
Upton Sinclair. The Fasting Cure. New York: Mitchell Kennerley, 1911.