You are here

Is Intermittent Fasting Good for Your Health?

With intermittent fasting becoming a popular approach to losing unnecessary fat, many have asked if fasting for short periods of time is good for their health. To answer this, let's take a look at what happens in your body when you begin to ingest nothing but water for a day.

After your cells use up the sugar that's in your bloodstream from your last meal or beverage, your body has to find another source of energy for your cells. The first places that it turns to are your liver and skeletal muscles. Both your liver and muscles store sugar in the form of glycogen, and when needed, glycogen can be broken down to glucose, which all of your cells can use to produce energy for their ongoing activities.

During a water-only fast, your glycogen stores are depleted within about 24 hours, give or take a few hours. After your glycogen stores are used up, most of your cells begin burning fatty acids for energy - these fatty acids come from your fat reserves, including fatty tissue that surrounds your organs.

Two groups of cells - your red blood cells and your brain cells - cannot use fatty acids to fuel their energy needs. Your red blood cells and brain require glucose, and once glycogen/glucose from your muscles and liver are used up, your brain and your red blood cells get their glucose from the following two sources:

  1. From glycerol, which is a component of your fat tissues.

  2. From your muscles - some of your muscle tissues get broken down, and the amino acids from your muscle tissues are used to produce glucose for your brain and red blood cells.

Clearly, it's not in your best interest to rapidly eat up your muscles to meet the energy requirements of your brain and red blood cells during a water-only fast. Your body knows this, and somewhere between the second and third day of water-only fasting, your liver begins churning out ketones, which, during a water-only fast, come primarily from the breakdown of fatty acids from your fat reserves.

Once your liver generates large numbers of ketones, your brain is able to use ketones to fuel itself. At this point, only your red blood cells require glucose that must still be derived from breakdown of your muscles, but with your brain no longer dependent on breakdown of your muscles for energy, the rate at which your muscles are catabolized will be such that your muscles are spared as much as possible - this state is called "protein sparing" - it's a survival mechanism that's built into human physiology to deal with times of famine.

Getting back to the big picture, it should be clear that from about the 2nd or 3rd day of a water-only fast, your body meets it energy requirements by burning through your fat reserves.

Since the bulk of the toxins in your body are stored in your fat reserves, the longer you fast on water only, the more fat you'll burn and the more toxins you'll eliminate from your system.

This is why we see elimination of lipomas, atheromas (accumulated waste in your blood vessels), and other conditions related to toxin accumulation during a prolonged water fast.

Put another way, your body does not experience significant detoxification during the first 12-24 hours of a water-only fast.

Your body begins to eliminate large quantities of toxins only after it begins to burn your fat reserves at a rapid rate. And this doesn't happen until you've used up the glycogen stores in your liver and muscles.

So when you fast for about 24 hours, you deplete the stores of sugar in your liver and muscles, and you begin to break down your muscles - these are the main things you accomplish during the first day of water fasting. Significant detoxification only begins to occur if you continue past day one of fasting.

This is not to say that there are no benefits to fasting for 16 to 24 hours at a time, or that you don't eliminate any toxins during a one-day fast.

You are eliminating toxins with every breath that you take. And your body will always increase its rate of ongoing detoxification whenever you get more rest and/or eat less food, because less digestive burden and more physical rest lead to more resources being available for detoxification.

Rather than fast for 16 to 24 hours at a time, I think it makes more sense to focus on eating minimally processed foods, and to eat only when you are truly hungry - with this approach, you will supply your body with fuel it needs for your everyday activities without depleting the sugar stores in your liver and muscles or causing breakdown of some of your skeletal muscle tissue.

If your goal is to give your body a period of rest and intense cleansing once in a while, you might spend a day having nothing but freshly pressed vegetable-based juices. Or you might eat only raw vegetables and perhaps a small quantity of antioxidant-rich fruits like berries. If this type of intermittent cleansing is of interest to you, please feel free to have a look at the following articles:

Full Body Cleanse Series

How to Enhance Your Health With Juices and Juice Fasting

Intermittent fasting can be an effective way of improving insulin sensitivity and losing unnecessary fat, but the most important long term determinants of body weight and health will always be eating minimally processed foods, not overeating, being physically active, getting sufficient rest, and striving to to be emotionally balanced.


Join more than 80,000 readers worldwide who receive Dr. Ben Kim's free newsletter

Receive simple suggestions to measurably improve your health and mobility, plus alerts on specials and giveaways at our catalogue

Please Rate This

Your rating: None Average: 4.3 (383 votes)
This question is for testing whether you are a human visitor and to prevent automated spam submissions.
Enter the characters shown in the image.


Thanks a lot for the information! I was contemplating on doing this but now I'm not so sure. I think I will have to read up on it more, maybe do a fast less often but maybe for 2-3 days? I'm new to fasting and cleansing the body though so I don't know where to start. Though I have been eating alot of vegetables lately. Plus I've tried a 2 day water fast as well.. Not fun with my acid reflux! Anyway, thanks alot again for the info, will keep it in mind.

This is not a medical advice. It may not be a safe option for you. Please contact your physician for personal advice. I am 50 year old 6’1″(185cm) male, medical doctor. Over years I had my weight gradually reached 310 lb. (141kg). I started very low carbohydrate diet in 2014. After initial weight loss of about 40 lb.(18kg) I gained it almost all back. In 2015 since I did water and salt fast for 30 days without significant effort since I was already in ketosis already for quite some time. I drank water whenever I wanted it and took salt from hand whenever I wanted. In order to retain that water I need to take salt. I also dissolved Centrum Silver multivitamins in water and drank it couple times a week. I continued to work as a physician in a very busy practice. After 30 days I stopped fasting because I felt weak and had difficulty concentration at work. To break my fast I avoided carbohydrates, (no sugar, no fruit, no juice, and no starch). People have complications from re-feeding because glucose raise in blood stimulate insulin production and severe electrolyte shift in the body that may cause irregular heart beat and very rarely death. I ate small amount of broth, eggs, cheese. I took Thiamin, Magnesium Oxide, Potassium Chloride and Potassium phosphate for the first several days.
In January 2016 I fasted for 35 days. I consumed water, salt, multivitamins. I broke my fast with broth, milk, heavy cream, eggs, very slow, small amounts, and several days. Again, I supplemented re-feeding with Thiamin, Magnesium, Potassium, and Phosphorous. During that fast my weight went from 321 lb. (146kg) to 266 lb. (121kg). Since then I started fasting every first week of every month. I did nothing to break a week long water and salt fasting, no vitamins, regular activity, no exercise. I ate anything I wanted, but kept very low carb diet. Every month I gained about 30 lb. (14kg), then lost 22 lb. (10kg) during wasting week. It is quite easy to fast if you are on low carb diet. But it is not enough. Every month I gain 8 lb. (4kg), approaching my initial weight 321lb (146kg). Starting next month I will do two 6 days fast. On the first and 15th day of every month. This is not a medical advice. It may not be a safe option for you. Please contact your physician for personal advice.

I believe the most reasonable intermittent fasting protocol is one developed by Dr Michael Mosley BBC medical journalist called the 5:2 Fast diet. Worth a look, esp. on the Facebook pages; inspirational.

You may be a doctor, but your results do NOT suggest anything positive about your regimen if you just keep gaining back weight you lost.

There are tremendous health benefits from fasting from beginning the 3rd day on. Read Dr Dom D'Agostino. As indicated the difficulty of fasting when transitioning to ketosis. It is possible to prepare for this prior to your fast and make the transition easier.

Aricept, respectfully, and out of concern, I ask you to consider that, while you are eating a very ketogenic diet in between fasts, you may be consuming more calories than your body can is, thus causing a weight gain of 8 lbs over what you lost while fasting. Perhaps it is time to consider monitoring, very precisely, your food intake, in order to determine whether you might be over-eating. An extra fast might not be your best long-term option. Fasting to lose weight is not as healthy an option if you are over-eating between fasts. It would perhaps be healthier to eat whole foods in moderation, at such a caloric level as to support continual, gradual weight loss.

A hight protein low carb diet will never work for weight loss. Everyone should instead do a hight fat no carb dieting. The way I do it is fast for about 20 to 24 hours then fry me some chicken with the skin on and fry on lard as well as fry eggs in lard. Good fat does not make one fat. But eating lard will satisfy the body's craving for fat. Will keep insulin low with no carbs or sugars to spike it. Nothing wrong with lard. It is a stable fat more stable than the commercially sold vegetable oils since they go rancid quickly. I experienced weight loss of 40 lbs. but reached my plateau and could not lose anymore weight being on a hight protein diet. The missing link is always "where is the fat"? Anyways, that's my two cents. I'm still on fasting and hight fat eating. Eat with lard, the real butter and coconut oil. Cheers.

Helobacter pylori is a bacteria of the gut. It causes ulcers and inflames the stomach lining. Apple cider vinegar (ACV) with "mother" 2 times a day would help your condition. ACV is a natural probiotic and benefits gut flora. It also helps to digest food. Plus, don't eat any foods and lie down afterward. Wait approximately 2 to 3 hours before bedtime. This will reduce acid reflux.

I can only relate my experiences with fasting - so your mileage may vary.

I used to do a 10-14 day water-fast every year. The one thing I would say regarding Dr. Kim's advice is that the first 3 days are the hardest by far. They are the "hump" you have to get over before you can get into the more pleasant part of fasting. In the first 36 hours, you will probably enter ketosis and start burning fat stores. The problem is that a lot of the toxins we ingest are fat soluble - so you will start burning this toxin-laden fat and those toxins will be dumped into your system all at once - making you feel pretty darn crappy ...for the next 36 hours. All this time, your a little voice will be saying to you "Hey, all this crappiness will go away ... if you just eat some food" ;)

At day 4 and beyond, I would feel very "stable" and clear - my rate of healing would dramatically increase (you heal like you did when you were a little kid). I didn't feel hungry any more and I would fall asleep and wake up in the same position - perfectly rested. No allergies. No headaches. No mood swings. You will realize what a profound effect that food has on your health and mental state. I would fast until I started feeling weak - which was dependent on how much adipose tissue I was carrying at the time - and carefully ramp out of the fast (very important).

The one part of fasting that I never did get around was that I would say "Duh" a lot while fasting - so I had to pick times when I wasn't heavily engaged in mentally strenuous work. They say that your brain will start using Ketones for energy, but mine always seemed to want to run on carbs. Again, this was my experience - so yours may be different.

I have been researching fasting and just completed my 2nd 24hr water fast in 2 weeks and I do find this really good for easing the pressure on stomach and bowel. I do feel quite light headed today (had dinner last night and breakfast this morning) and I am assuming this is a result of depleated glycogen. My research has certainly found and is supported in this article that a longer term fast of the kind mentioned in this article is far more beneficial especially if you do not want to loose muscle mass just body fat. As I understand it takes more than 24 hrs of fasting (3 to 4 days) for your cells to go into repair mode and start having any health benefit for disease. As mentioned here as well, I guess there is no quick fix to a cleanse and what we have taken years to pollute our bodys with may take years to undo with good healthy living and excercise.

Thankyou for this informative well written article.

I have fasted one day per week for extended period of time,once in a while I would do 2 days per week.What I found was my sense of smell,and my mind became extremely sharp I would experience a clearness in my head that was uncanny.Also I required less sleep and did not wake up groggy.The main thing that you have to be extremely careful about is how you break the fast ,it has to be broth,fruit,cooked vegetables something light and control the amount I found I had a tendency to over eat and the you will feel really awful it's almost like a drug induced stupor, I'm convinced that food is nothing more than a drug to the body because of how it makes you feel if you want to avoid that crappy feeling you can exist on a vita-mix machine my smoothies in the morning last me 5 to 6 hrs and consist of banana,raw almonds,chia seed,mixed berries,fresh spinach and cucumber , I use cashew milk instead of cows milk.I drink this in the morning and you can have a tiny lunch then for dinner I have another drink but smaller in portion an s piece of meat, I tell my wife that drink is my rocket fuel. I'm 66 and if I stick with this regime and along with my walk with Jesus I have been cured of some troubling problems and I don't get sick anymore even if my wife gets sick I won't catch it Fromm her as I used to which I find amazing.

You would be better off making fruit smoothies with the pulp. The fiber slows the digestion and mitigates the insulin spike. Juice is like soda with some nutrients in it. The nutrients are not enough to make it a good choice.

Thanks for really good article! I just began experimenting with fasting and was wondering if I should do one-day fast per week or longer fast less often. Your article answered my dilema and helped me a lot.

Fasting is best preceded by good low carb, low gluten high quality fat and reasonable non and soluble fibre habits of at least one week. If tolerated this will be your future.
If problematic, no bread but vary with some whole grain cereals or bean types to see which help comfort.
Finally water and milk kefir, fermented vegetables, Miso and berries and no omega 6 and you'll be like a spring chicken.

I usually love Dr. Kim's articles and advice and this is really no exception, but for one thing: In the article you did not give the contraindications for fasting. In other words, why someone should NOT fast. I tried water only fasting a while back - started out with fruits and veggies, then juices, then went into it, doing all the right things. By the second day, I could not get out of bed and I felt so weak and depressed. I have a history of low thyroid and adrenal fatigue, among other chronic ailments. I believe this is why I should not do fasting, at least until I have healed this condition. I can skip a meal using just fruit smoothies and that works well. I love knowing that fruit gives my brain the glucose it needs to keep me going. If I was that Dr. above, I would never break a fast with heavy cream, or eggs, etc. Fruits and veggies have everything we need and then some. Thanks for the great article, and for all you do, Dr. Kim. I only gave it a "2" for the reason I mentioned.

Thank you for this information, yet I have no fat on my bones, so I would faint most likely. What I would try is to fast while taking coconut oil to supply the energy.
Having a colonoscopy I had to follow a liquid diet of broth. So for 36 hours I was without any sugar and carbohydrate, it was much easier than the previous liquid fast when I had fruit juice as per the hospital's recommendations.

I also have no fat, maybe minimal amounts. I also have some kind of gut issue so I fast for a day at a time pretty regularly as it gives me great relief from the pain that food gives as it goes through my intestines. Seeds and nuts are such a no no for me, as is fibers, so soft cooked or raw blended seems to work the best. Broth dishes are of course my favorite. My staple is a cucumber, some radishes, avocado, maybe some carrot and celery, then steamed kale blended up in vitamix with maybe some sardines. I'm definitely existing on well under 2000 calories a day as is. I wish I knew how to get this mycotoxin, mycobacteria, permeability or whatever out of my guts, I got into growing garlic, lots of garlic so maybe this year I'll try a lengthy fast but eat copious amounts of garlic while on it and see what happens.

You should stick to a raw fruitarian diet - lots of bananas and high water fruits until your gut heals. Lots of nutrients and antioxidants. You will feel great! Celery juice first thing in the morning is great - lots of wild blueberries.

That's allot of fructose. I'm an O- with a high metabolism and I've pretty much put down the meat at this point. Fruits cause me to blow up inside, I can't even eat an apple. Celery always in my salads, I do feel great, after I go run a mile or two after work (works the gas all up), and lie in the bathtub for an hour afterwards or just don't eat at all, I've been this way going on 7 years. After G.I. talked me out of my perfectly healthy GB and sent me into a tailspin of pain and suffering, it's taken me 3 years just to recover from that. I've pretty much found myself to suffer from the same symptoms as SIBO/SIFO, but my large bowel also produces gas as well, gets me up all through the night. For a few weeks this summer after I went intense on antifungals I was doing pretty well, my weight had dropped considerably, wheat, dairy, sugar, oops, back to square one again. Thank you for your response, and wealth of info, the mobility exercises are superb.

Thank you Dr Ben! I have read a lot about this subject, but your simple , clear to understand explanation is best! Thank you again! Eva

I do fast as part of my religion practice, my question is can we stay on water only for 24 hours? because I do get headache anyway from fasting 13 hours but no water or food

Fasting 24 hours a day one day a week is very good for you. However, during the fast, you should drink as much water or fluids as you can- not to excess, but just enough. It's okay to drink water during a fast- you're cleansing out your system. I have been fasting off and on since I was 17 years old.

I appreciate your comments on fasting and they agree with much of the detox diet I am following from dr. Morris of Port Charlotte Florida. He thinks that fruit is much better than vegetables however because we are not designed to digest vegetables. He also says that your urine should be cloudy because that shows that your kidneys are filtering out junk. What is your opinion on these things?

I think any doctor who claims that we are not designed to digest vegetables needs his head examining.