Research on fasting is fascinating. There are so many health benefits to fasting that it’s worth trying for yourself. With multiple ways to do it including several types of intermittent fasting and the more prolonged extended fasting, you’re likely to find a version that works for you.
If you decide to try it, be prepared for some backlash. Unless they’ve read the research, a lot of people see it as a form of an eating disorder or start to worry about you if you skip a meal.
This thinking is in part because we’ve been brainwashed by the packaged food industry to believe that breakfast is the most important meal of the day. And that eating every few hours to keep one’s metabolism going is necessary. Plus, if you’re still a sugar burner, a few hours without a meal can make your blood sugar crash and make you downright hangry. It makes it hard to imagine going for very long without a meal.
Fortunately, in functional medicine circles and particularly the keto community, extended fasting for autophagy, insulin regulation, weight loss and health is generally accepted as normal and beneficial.
Once you become fat adapted by following a ketogenic diet, fasting becomes much easier since your body is trained to easily access body fat. Whether you choose to experience the benefits of fasting as a sugar burner or a fat burner, the health advantages still apply. It’s just that fasting is so much easier when you’re following a keto diet.
If you’re wondering how to get started with the keto diet, be sure to check out our comprehensive guide to keto.
Science-Backed Benefits of Fasting
A reminder here that I am not a doctor or licensed to give nutrition advice. I’m just a very good researcher that’s been interested in my own health and wellness for over 25 years. Please consult a health professional if you have concerns about fasting or your diet.
There is also evidence that fasting improves lipid levels 4 (if we believe this is important…more on that here), helps osteoarthritis5, diabetes prevention,6 decreases body fat7 and promotes weight loss8. While some of the research has been on mice and monkeys, there are also plenty of human studies and mounds of antedoctal evidence.
Clearly, the health benefits of fasting are numerous. Skip down to the additional benefits of fasting here, or read on about the types of fasting below.
Types of Fasting
There are two types, intermittent fasting and extended fasting, also known as prolonged fasting. Intermittent fasting is basically any amount of time under 24 hours or based on doing fasting 24 hours at a time, various times of the week. You may also like to think of intermittent fasting in terms of having an eating window, which seems like a more positive way to view it.
The most popular types of intermittent fasting include 16/8, 5:2, eat stop eat, one meal a day (OMAD), alternate day and convenient. You can get some of the health benefits of fasting with fasts as little as 12 hours at a time. When the benefits of fasting start to kick in depends on when glycogen stores are depleted. If you’re keto or very active at the beginning of a fast, the benefits kick in sooner.
Intermittent Fasting Types
14/10 16/8 or 18/6 or 20/4 Fasting
When you see people talk about intermittent fasting in terms of two numbers, they are referring to the fasting window and eating window. The first number indicates the number of hours of fasting, and the second number represents the number of hours that are the eating window.
One of the most popular is 16/8 since it is easily achieved by having dinner by 8:00 pm then simply skipping breakfast the next day. If you had dinner by eight, you could eat 16 hours later at noon which fits well with most people’s work schedules. This schedule gives your body 16 hours in which to avoid the release of insulin caused by food, thus promoting blood sugar regulation.
Two meals a day improves glucose tolerance and promotes autophagy,9 further discussed below.
Rather than a number of hours fasting, 5:2 refers to the number of days involved. This protocol involves eating normally for five days of the week and then restricting calories to 25 percent of normal on two non-consecutive days. Or you could fast completely for 24 hours on those chosen days.
Eat Stop Eat Fasting
The eat stop eat fasting method is pretty simple. Eat normally for most days of the week, but then fast for 24 hours once or twice a week. Dinner to dinner may seem the easiest to most, but you could choose any 24 hours to fast.
Alternate Day Fasting (ADF)
Alternate day fasting (ADF) has you eating every other day of the week. There are multiple studies on PubMed on ADF with varying results but most show that it’s effective for weight loss10 and affects some other biomarkers as well.
One Meal A Day (OMAD) Fasting
With the OMAD method of fasting, you eat once a day. This could be literally one giant meal in which you eat all of your calories for the day, or it may be divided into smaller meals within a two to three-hour time window.
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Extended Fasting aka Prolonged Fasting
Extended fasting is typically thought of as any fast over 24 hours. There are some additional health benefits that begin to take effect with fasts of this length. If you choose to participate in extended fasting, it’s a good idea to consult your health professional first. People who are underweight or have very low body fat should be cautious about fasting for too long.
What Is Allowed on an Extended Fast?
Strictly speaking, an extended fast consists of water only and should most definitely include electrolyte supplementation. Any nutrients taken in during this time even in small quantities can disrupt some of the fasting benefits. However, most agree that black coffee and tea can be consumed and may even offer benefits to the fast.
For longer fasts, I supplement with magnesium. I actually take this every night since it helps with sleep. I keep it simple for sodium and potassium. Morton’s Lite Salt has both and I simply dissolve it in water. Just don’t do too much at a time. About a half teaspoon in eight ounces of water doesn’t taste too strong. Or you could get an electrolyte supplement in pill form such as keto electrolytes. There are also powdered electrolytes, but I haven’t been able to find one without sweeteners, which should be avoided. I prefer the lite salt since it’s inexpensive and easy to find in any grocery store.
I’ve also been doing a ketone, broth and electrolyte assisted fast called the Reboot. The kits go on sale at the beginning of each month for a few days. You can do the fast with community support through the Facebook group, or use the kit to do your own assisted fast anytime you like. My experience with this 60 hour fast is that it’s much easier to do than a water and coffee only fast. However, you are consuming a small amount of protein which could lessen autophagy slightly.
There are two schools of thought to the use of caffeine during a fast. The first is that fasting should be a rest and repair period for the body. Since caffeine stimulates the adrenals you may want to avoid it while fasting. The other thought is that caffeine can be of benefit while fasting by acting as an appetite suppressant, increasing ketones11 and inducing autophagy12.
The additional benefits of extended fasting will be discussed below.
What Type of Fasting is the Best?
The best type of fasting is the one that works best for each individual. Start with simply skipping a meal for a few days in a row and see how it works for you. Assuming all goes well, start to experiment with some of the other types and see what works best for your body, your goals and your hormones.
It’s normal to experience some hunger of course, but it typically goes in waves and passes quickly especially if you stay well hydrated. It’s also important to keep an eye on your electrolytes while fasting by supplementing if necessary. Typically electrolytes only become an issue on fasts longer than 24 hours.
Women can be more sensitive to fasting than men since their bodies are designed to carry children. If a woman’s body feels it’s not safe to bear children (lack of food and nutrition) it may begin to alter hormones. Hormone levels can get thrown off by excessive fasting so be sure to start out slow if you’re a woman. Leanne Vogel is a great resource for keto and fasting as it relates specifically to balanced hormones. The hunger hormones leptin and ghrelin can also be affected in anyone with fasting.
Benefits of Fasting by Length of Time
Benefits of Intermittent Fasting: Blood Sugar and Insulin Regulation
Blood sugar and insulin decreases can be seen in fasts as little as 12 hours and improve over time. Regular intermittent fasting of 20 hours was shown to reduce insulin levels and increase insulin sensitivity13.
Insulin drops up to 70 percent in the first 24 hours and at 54 hours, insulin is at its lowest level and are more insulin sensitive14.
Benefits of Intermittent Fasting: Weight Loss
Intermittent fasting of varying types produces weight loss over time15.
Benefits of Fasting 18-24 Hours: Ketosis
When you follow a ketogenic way of eating, you’re in ketosis most or all of the time. When not following a ketogenic diet, fasting can promote ketosis as soon as glycogen stores have been depleted which depends on a number of factors such as the person’s diet and how active they are.
According to a 2018 study published in Obesity, ketones may start being produced after 18 hours of fasting16. According to the same study, more significant ketones are generated a couple of hours later resulting in “heavy ketosis.”
Benefits of Fasting 24 Hours or More: Autophagy
Autophagy is one of the coolest things about fasting. In 2016, Yoshinori Ohsumi won the Nobel Prize for discovering the miraculous process of autophagy. Autophagy comes from the Greek words auto and phaegin which mean self and to eat. In other words, autophagy is the process of self-eating. He discovered autophagosomes accumulate in the absence of food.
So what does this mean to you? When you’re fasting for autophagy, the body is literally eating itself by feeding of deteriorating and damaged cells. As you can imagine, this has huge implications for your health! It’s an opportunity to get rid of dangerous cells that can cause disease. Disrupted autophagy has been noted in Parkinson’s, type 2 diabetes and other disorders.
Fasting for 24 hours our more to induces neuronal autophagy which may protect against Alzheimer’s17.
Autophagy may also explain why some studies show a small loss of lean mass during fasting. It’s likely at least some of that loss can be attributed to autophagy of damaged cells rather than the body feeding off of the protein in muscles. If you think about it, losing muscle during times of famine would not make evolutionary sense. Clinical evidence backs up this assertion18 and shows that no lean mass was lost with fasting. A more recent study showed a slight loss of lean mass, but less with fasting than in a calorie restricted group19.
It’s important to know that autophagy can be slowed or stopped by consuming any amount of protein or carbohydrate. Fat has less of an effect, but for maximum autophagy, it’s best to avoid consuming anything with calories. Artificial sweeteners should probably be avoided as well to get the full benefits of fasting as they can cause insulin responses in the body despite being noncaloric.
Benefits of Fasting 48 Hours or More: Human Growth Hormone (HGH)
After 48 hours of fasting, human growth hormone (HGH) is five times the level when you started20. Since this hormone declines significantly with aging, it could play a role in rejuvenation in older people.
Human growth hormone generally declines every year after 30. By age 60, your level of HGH is half of what it was at age 2521. One way fasting promotes HGH is through the hunger hormone ghrelin. It promotes the secretion of HGH22.
With HGH at such a high level during fasting, it may be a good time to work on building muscle mass if that is a goal and you feel well enough to do so. Personally, my experience is that I can exercise without much trouble while fasting. The key here, as always is to listen to your body and stop anything that makes you feel unwell.
Benefits of Fasting Varied Time Frame: Weight Loss
While there is a theoretical limit for fat mobilization23 that has been discussed quite often in the keto community, that might not be the whole story. New research is showing that keto-adapted athletes24 may be able to more easily access body fat and burn it in accordance with their activity. A 2016 study by Volek found that fat oxidation in low carb subjects was over twice as much as that in the high carb group25.
Whether or not we know if this fat mobilization increase holds true for those that aren’t athletes is irrelevant. With no intake of food, people lose weight, and there is research cited above that explores many different types of fasting. Most if not all of them result in weight loss.
Benefits of Fasting 72 Hours or More: Immune Regeneration
After 72 hours, key regulators of cellular protection and tissue regeneration kick in. Pathways that give cells the ability to resist toxins are activated. Prolonged fasting may protect cells from chemotoxicity by reducing circulating insulin-like growth factor-1 (IGF-1). 26 This has implication for both the immune system and possibly on the ability for patients to better withstand cancer treatments.
Benefits of Fasting 4 to 5 Days: Longevity and Multiple Health Effects
Dr. Longo has been doing a lot of research on the effects of prolonged fasting and is one of the fast mimicking diet proponents. The fast mimicking diet does include some food, but is mostly fat and calorie restrited to get the benefits of fasting while still being able to consum some food. However, you can get the same results by simply fasting. Fasting for five days has been shown to produce multiple positive effects.
The fasting-like diet reduced body weight and body fat, lowered blood pressure, and decreased the hormone IGF-1, which has been implicated in aging and disease. A post hoc analysis replicated these results and also showed that fasting decreased BMI, glucose, triglycerides, cholesterol, and C-reactive protein (a marker for inflammation). These effects were generally larger in the subjects who were at greater risk of disease at the start of the study. A larger study is needed to replicate these results, but they raise the possibility that fasting may be a practical road to a healthy metabolic system.27
Other research by Longo shows that fasts of five days are the sweet spot for shifting stem cells from dormant to a state of self-renewal. This length of fast also prevents immune system damage, which again, has huge implications for chemotherapy.
Benefits of Fasting: Cancer
There is starting to be more research on the role of fasting in cancer. However, it’s tough to get research funded for a modality that doesn’t result in an expensive drug to be sold.
The obvious benefit here is autophagy, which promotes higher levels of tumor-suppressing genes28. AS described above, there is also an effect on the immune system and the ability to withstand toxins which could help prevent some of the side effects of chemotherapy. Breast cancer survivors have a better outcome when practicing intermittent fasting29.
Results of a Five Day Fast
I’ve done all kinds of fasting over the years and most recently tried a more prolonged fast. I’ve now done three fasts of just over five days and will be sharing my experiences with you soon! Here is a video I made about my personal why for fasting and how going without food for 5 days feels.
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