The thought of purposely not eating may sound unrealistic and even terrible to you.
However, our bodies were not designed to eat constantly.
Until the last century or so, most people lived a "feast or famine" lifestyle. It was not uncommon for them to go a day or more between meals until they could afford to buy more food or until they could find another animal or plant to eat.
Eating multiple meals per day has resulted in a ton of health issues, including diabetes, obesity, hypertension, heart disease, and many others.
In this article, I will discuss the history of fasting and some false information we have been taught about dieting.
I will also discuss the medical benefits of fasting, some popular ways of fasting that have been shown to help with weight loss, and I will answer some common questions that people have about fasting.
Here we go...
Before we discuss fasting and its benefits, we need to understand why we gain weight in the first place.
Society is riddled with false information. We have been sold a bill of goods when it comes to understanding nutrition and how and when we should eat.
Even doctors get minimal training and education on diet and nutrition. Even some nutritionists in my experience are still teaching information from 20 or more years ago that has since been debunked.
Much of this false information comes from the food industry. The more we eat, the more their business prospers. They want us to eat as much as possible!
As a result, some ideas have spread through advertising and other means that are patently false, yet they have nonetheless become ingrained into our way of thinking.
- You must always eat breakfast.
- Never miss a meal. It will cause you to overeat when you do eat.
- You should eat multiple times a day, including snacks.
For those of you that were born in the 1970s or earlier, think about your childhood.
You probably only ate 3 meals a day and that was usually it. If you tried to eat a snack when you got home from school, your mother probably said, "Stop that! You'll ruin your dinner!"
By the way, the incidence of obesity in those days was MUCH lower than today.
Compare that to today: The average American now eats 5 to 6 times per day. That means you are eating in a 16-18 hour window and only fasting for 6-8 hours a day, the reverse of what it was 30-40 years ago.
Should we be surprised then that obesity is such an epidemic?
To look into this further, let's discuss some of the common beliefs that most of us have about diets and why they are wrong:
Most of us have been taught this theory and it is core to our beliefs on weight loss and obesity.
We look at our body like a large container with a hole in the top and a hole in the bottom.
As long as the container is draining more out from the bottom than is being poured into the top, we will lose weight.
If someone is obese, then obviously they must be consuming more energy than they are burning everyday. Right?
What is the first thought that comes to your mind when you see an obese person in public? Most likely it is something like this - "If they would just quit eating so much they would lose weight."
Haven't we all thought this before?
If this were true, then why do up to 99% of people fail long term when they try a low calorie diet?
Even if you do lose weight on one of those diets, why do you always seem to plateau after a few weeks and the weight loss slows and even stops?
There has to be more going on, doesn't there?
Over the past decade or so, many people in our society have evolved their thinking into more of this concept.
If you put on a few pounds over the holidays, what thought comes to your mind? Probably something like this - "I need to quit eating so much and get my rear back in the gym."
Yes, if you are consuming massive amounts of food, you need to back off. If you are a total couch potato, you need to start doing some exercise.
However, it's just not that easy, is it? How many of you feel like you are eating very little and exercising as much as you can, yet your weight doesn't budge?
The fact is, there is no correlation between obesity and calories.
A study from 1997 showed that between 1980 and 1990, Americans consumed about 4% fewer calories than previously, yet the incidence of obesity increased by 31%. They called it the "American Paradox."
Losing weight just isn't as simple as reducing the number of calories you eat and burning more by exercising.
A United Kingdom review from 2015 showed that the probability of achieving a normal weight by just reducing calories was 0.8% in women and 0.47% in men. In other words, losing weight by using the conventional calorie-counting methods had a failure rate of 99.2%!
In fact, eating too few of calories for an extended period of time can actually be harmful.
If you reduce your calorie input too much for too long, it can damage your metabolism for years!
This was shown in the Biggest Loser Study. 14 contestants from the 2009 show were studied. All but one of them had regained most or all of the weight they had lost while on the show plus sometimes more six years after the show.
The severe calorie-restricted diets that they were on during the show had severely damaged their metabolism. The damage was still present 6 years later.
In other words, instead of burning the typical ~2000 calories per day that they burned before being on the show, their resting metabolic rate had dropped by about 1000 calories per day.
Dr. Jason Fung, widely considered one of the world's foremost experts on fasting, brilliantly discusses why these concepts of weight loss are wrong in this video plus he also explains the importance of fasting:
So if these 2 concepts are wrong, what is the real reason that we gain weight?
The underlying cause of obesity is an imbalance of your hormones, not an imbalance of your calories!
The real root issue is insulin resistance!
If you want to lose weight, you HAVE to lower your insulin (you must also reduce leptin resistance).
The body responds to excessive amounts of anything by developing resistance to it.
Have you ever seen anyone that takes pain medication on a regular basis? Typically, after a period of time, it begins to require more of it to get the same effect. That is true for alcohol and other medications as well.
The same thing happens with insulin.
When we eat a diet high is sugar or processed carbs, our pancreas is signaled to release insulin. The insulin attaches to the sugar in our blood and transports it into the cells where it is used immediately for energy.
The insulin also carries the sugar into the liver which links it into long chains called glycogen. The glycogen is stored in the liver where it can be accessed as a source of glucose when needed.
When the storage limit of glycogen is reached in the liver, the glucose is then converted into fat (called lipogenesis). That fat is stored in the liver and around the body to be used for energy in the future.
When this process occurs in excess, we gain weight.
It only makes sense that if we want to reverse this process, we have to lower insulin.
Anything that reduces your intake of sugar and carbs (and protein, actually) will result in less insulin being secreted by the body.
No diet will do this as well as not eating anything at all.
Yes, fasting is by far the most potent tool we have to lower the insulin levels in our body.
Let's discuss it further...
Fasting has been practiced by virtually every religion and culture on earth. It is considered a vital part of Christianity, Judaism, Islam, Hinduism, and Buddhism.
The ancient Greeks noticed that both animals and humans avoided eating when they became sick. They realized that fasting is a natural treatment for illness.
They also realized that fasting improved their mental capabilities. These times of fasting were called "purification" or "cleansing."
The father of modern medicine, Hippocrates, recognized the benefit of fasting as a treatment for obesity over 2000 years ago.
Benjamin Franklin was a strong believer in the power of fasting, as his quote above shows.
Mark Twain, one of America's foremost philosphers and writers, wrote "A little starvation can really do more for the average sick man than can the best medicines and the best doctors."
Fasting began to appear in medical literature in the early 1900s. There was a renewed interest in it in the 1950s when Dr. W.L. Bloom began recommending it, but that interest faded by the late 1960s.
Fasting was rarely if ever mentioned again until the last few years when people such as Dr. Jason Fung reignited interest.
Fasting has been shown to:
Fasting also has many practical benefits:
It is free to fast. It will not add any expenses to your budget.
Fasting increases your amount of free time. No food preparation or running to the grocery store or a restaurant is needed when you fast.
Fasting is also very flexible. Depending on your work schedule or other activities, fasting can be modified to be used however and whenever you need it.
You can also incorporate fasting with any diet. I believe that combining fasting with a whole food, low carb diet is one of the single most effect ways for someone to lose weight, but it can be used with any diet you choose.
Fasting refers to abstaining from something. In this article, I am referring specifically to abstaining from food for a period of time.
A true fast would mean that no food was consumed during the time of the fast. However, there are some types of fasting that I will discuss that allow a small amount of food at times. Even though they are technically not a true fast, I will consider them a fast for the sake of this article.
With all of these fasts, the fluid consumed should be water, tea or coffee. I am also okay with drinking kumbucha and bone broth, both of which can provide some nutrition without increasing caloric intake substantially or raising insulin levels.
Soft drinks or artificial sweeteners should be avoided because they can raise insulin levels.
Fasts are primarily differentiated by the amount of time involved when food is withheld or at least reduced.
Let's discuss the various fasts that are commonly used and in what situations they are helpful:
This was how most Americans ate prior to the 1970s. In this fast, all meals (usually 3) are eaten typically between 7am and 7pm.
This balances the time of fasting and feasting to 12 hours for each.
Who Benefits From This Diet? - Anyone trying to maintain their current weight and prevent insulin resistance.
This type of fasting is most likely not powerful enough to cause weight loss or to reverse insulin resistance that is already present but is a good maintenance diet.
In this diet, the window when you eat is reduced to only 8 hours of the day. Basically, you skip the breakfast time of eating.
A typical person will eat dinner at 8 pm and not eat again until noon the following day. Typically either 2 or 3 meals are consumed during the 8 hour window.
This diet is commonly called the 8-Hour Diet.
It has also been called the LeanGains method.
Who Benefits from This Diet? - Anyone that needs a simple way of fasting to incorporate into their life.
It works best when combined with a low carb diet.
Weight loss occurs, but it is slow and gradual.
Ori Hofmekler popularized this diet in the early 2000s. He argued that knowing when to eat is just as important as knowing what to eat.
This diet is inspired by ancient warrior tribes such as the Spartans and Romans, who typically battled all day then would feast in the evenings.
With this diet, all meals are eaten in the evening in a 4 hour window of time. It also stresses the importance of eating whole, natural foods and perfoming high-intensity interval training.
Who Benefits From This Diet? - Anyone wishing to lose weight that is willing to incorporate a longer time of fasting with a whole food diet and intense exercise.
With this fast, eating is limited to only once per day. You can fast from morning to morning, evening to evening, or mid day to mid day.
This is helpful for people that take medication that should be taken with food (iron, aspirin, etc).
This diet is also easy to incorporate into daily routines. You can still plan on eating whichever meal is important in your life - dinner with the family, lunch at work, etc.
Nutrient deficiencies are rare since food is still being consumed daily.
The fast was popularized by the book "Eat, Stop, Eat." The author, Brad Pilon, recommends doing a 24 hour fast twice a week.
You could do this fast up to 3 days per week, but I would not recommend any more than that for fear of damaging your metabolism. Doing it 2 days consecutively is particularly effective in my experience.
Who Benefits From This Diet? - Anyone needing powerful weight loss and reduction of insulin resistance. Also, anyone that is on medication that needs to be taken with food.
Diabetics should consult with their physician before attempting this diet.
This is really not a true fast. With this diet, calories are just reduced for a period of time.
It consists of 5 normal eating days. On the other 2 days, women may eat up to 500 calories and men may eat up to 600 calories. The "fasting" days can either be done consecutively or split up during the week.
The calories can either be consumed in a single meal or spread out over the entire "fasting" day. The diet is continued indefinitely, even after your target weight is reached.
Who Benefits From This Diet? - Anyone that is hesitant to try fasting. This diet can be a good starting point, then you can transition to a stricter fast if needed.
This diet is similar to the 5:2 diet, except the "fasting" days occur every other day. Therefore, there are 3-4 days that are low calorie every week rather than just 2.
On the "fasting" days, women are allowed to eat 500 calories and men 600 calories.
The diet should be continued until your target weight is reached. The fasting days can then be reduced as long as you are maintaining your desired weight.
The effectiveness of this protocol has been shown in several studies to be effective for weight loss.
Who Benefits From This Diet? - Anyone that has had success with the 5:2 diet but needs a slightly more aggressive approach. Also, anyone that struggles with fasting completely and feels they need to eat at least some food daily.
With this diet, you do not eat for an entire day.
You eat dinner at 7pm, eat nothing the next day, then eat breakfast on the NEXT day at 7am.
You typically do this fast 2-3 times per week.
The diet is continued until the desired results are achieved - weight loss and/or reversal of insulin resistance and diabetes. The longer you have had diabetes, the longer you will need to do be on this fasting protocol to reach your goals.
The frequency of the fasting is then reduced to allow for maintenance.
This is the diet of choice for Dr. Jason Fung and his Intensive Dietary Management (IDM) program.
Who Benefits From This Diet? - Type 2 diabetes and other patients with severe insulin resistance.
Blood sugar should be closely monitored multiple times per day. Diabetic medications will need to be reduced or stopped under the supervision of your doctor.
This fast is similar to the 36 hour fast except the first meal at the end of the fast is not until lunch instead of breakfast.
This makes it easier for you to follow a 16:8 fast and diet on the regular days.
This fast is done twice per week.
Who Benefits From This Diet? - Type 2 diabetics and others with severe metabolic syndrome and insulin resistance.
Again, this must ONLY be done under the close supervision of a physician.
There are ample historical records of people surviving and even doing well during extended day fasts.
The longest recorded fast in history is 382 days! A Scottish man started at 456 pounds. His only intake was noncaloric fluids, a daily multivitamin, and various supplements.
At the end of the 382 days, he weighed 180 pounds. He had no significant medical issues during the fast. Five years later he still only weighed 196 pounds.
Several experts, including Dr. Fung, routinely start patients with severe type 2 diabetes on 7-14 day fasts. They typically see rapid improvement and reversal of insulin resistance with only rare medical complications.
Some oncologists are also beginning to recommend a 7 day fast once per year because it is appears to help lower cancer risk.
I personally have never tried an extended fast. I would also be hesitant to prescribe it to a patient, although I admit that my experience is extremely limited.
If you decide to embark on an extended fast for either health or religious reasons, it is critical that you involve your physician and monitor your glucose and other lab tests closely. Medications may need to be reduced or even stopped during the fast.
I would also recommend consuming bone broth throughout the fast, perhaps even with sea salt added to it.
I have concerns about the possibility of damaging your metabolism if fasting is done to excess. Like most things in this world, moderation is usually best.
Fasting isn't for everyone.
There are certain minerals, vitamins, and other nutrients that are obviously not ingested when you fast.
Most people can tolerate the lack of those nutrients for a period of time, but there are some people that should never attempt fasting. They include:
People who should not attempt fasting without first consulting with their doctor include:
This caution is especially true for diabetics. Fasting is so potent at lowering insulin that it can cause a rapid decrease in the blood sugar which can be very dangerous, particularly if the person is on diabetic medications.
Those people can still consider fasting, but it is crucial that they discuss it with their doctor beforehand and closely monitor their blood sugar throughout the fasting period. It may be necessary to reduce or even stop their diabetes medications during that time.
When I suggest fasting to patients, I usually initially get a shocked stare followed by a nervous giggle and a bunch of questions.
I have answered the most common ones below:
When used correctly, fasting is very safe.
Remember, our bodies were designed to have periods of time when we eat and periods of time when food is not available. There are mechanisms in place that will maintain our energy, electrolytes, and other vital bodily functions during times without food.
The biggest risk is dehydration. Most of us do not drink enough fluids and exist in a state of mild dehydration on a daily basis.
Many times our brains will confuse our thirst with hunger. The next time you feel hungry, try drinking a glass of water instead. You may be surprised that the hunger resolves.
It is therefore important to drink a lot of fluids when you fast. Water is great, kumbucha or bone broth are even better because of the nutrients they contain.
Again, special precautions should be made if you are taking prescription medication (particularly diabetic meds). Do not fast without discussing it with your doctor first.
I have also found that fasting can be difficult for people that are dealing with low cortisol levels (adrenal fatigue). Those folks should get their cortisol levels normalized first before attempting fasting or else it could exhaust them.
No! In fact, most people feel more energized.
How in the world does that happen?
Norepinephrine (adrenaline) is secreted when you fast. Its job is to release the glycogen stores in the liver. It also increases your metabolism.
After a 4 day fast, one study showed that the resting energy expenditure of the people in the study increased by 12%.
It makes sense if you think about it.
This is a survival response that has developed in our bodies over thousands of years to help us survive times when food is scarce.
If someone hasn't eaten for a few days, that increase in energy is critical to help them get up and find some food. Otherwise, if the lack of food continued to drop their energy levels, they would eventually just lay down and die.
No. Studies show no significant loss in muscle mass even when people fast for prolonged periods of time.
The primary reason for this is fasting causes huge surges in human growth hormone (HGH) secretion.
The elevated HGH maintains muscle mass by raising the levels of lipoprotein lipase and hepatic lipase which trigger release of fat to be used for fuel.
In one study, HGH secretion doubled after a 5 day fast.
In another study, HGH secretion increased 5 fold after a 2 day fast.
During a religious 40 day fast, one man showed an increase of HGH secretion of 1250%!
And the opposite is true as well.
Overeating has been shown to suppress HGH secretion by up to 80%. If you eat all day long, you will almost completely shut down your HGH secretion.
Just like with anything else is life, fasting can be overdone. However, when used correctly, fasting can actually repair our metabolism, not damage it.
The body will adjust to a constant low calorie diet by reducing the amount of calories we burn (our metabolic rate). Remember, our body is designed to maintain itself whenever possible, including our weight (even when that weight is excessive).
A prolonged exposure to a low calorie diet will trigger this downward adjustment in our metabolism.
Since intermittent fasting is done intermittently, the body never has a chance to make that adjustment. As a result, the metabolism isn't reduced. If anything, it goes up!
That is why it is good to change up your fasting routine. On one week you might want to have two consecutive 24 hour fasts, while on the next week you might try three 16-18 hour fasts.
Avoid developing a routine that your body has an opportunity to detect and make adjustments.
Yes. In fact, many experts suggest that the best time for you to exercise is at the end of a period of fasting.
Exercising during fasting trains your muscle to burn fat. Instead of relying on the limited glycogen stores, you can teach your body to use an almost unlimited supply of fat stores.
Remember, fasting triggers an increase in norepinephrine which increases energy. Also, the increase in HGH will help you build muscle faster.
In one study, fasting had no negative impact on strength, aerobic capacity or endurance.
Another showed that fasting before exercise improves insulin sensitivity.
At first, some people may notice a decline in their athletic performance until their body adjusts to burning fat instead of glycogen. That will typically resolve within a couple of weeks.
No. Studies have shown that fasting has no negative impact on brain function.
Another study showed no effect from fasting on attention, focus or memory.
In fact, one study showed that memory improved in elderly patients when their calories were restricted. This was felt to be secondary to the reduction in insulin and inflammation.
In my personal experience, fasting seems to improve focus and concentration in myself and in my patients.
Have you ever had a hard time focusing at work after eating a large meal? Food worsens, not improves, our brain function.
1. "The Complete Guide to Fasting" - Jason Fung MD. Copyright 2016.
2. "Eat, Stop, Eat" - Brad Pilon. Copyright 2007.
3. The 8-Hour Diet - David Zenczenko and Peter Moore. Copyright 2015.
5. "The 5:2 Diet - Feast for Five Days, Fast for 2 Days to Lose Weight and Revitalize Your Health" - Kate Harrison. Copyright 2013.
Fasting may seem scary or even ridiculous, but it has been around for thousands of years.
When done correctly, fasting is the single most effective tool available to lose weight and reduce insulin resistance.
Severe low calorie diets can damage the metabolism, whereas intermittent fasting does not and is even more effective.
Fasting should be started with caution, especially if you have health issues such as type 2 diabetes. Consulting with your physician is essential before starting and while on the fast.
There are many different ways to fast. While some are more potent than others, all can be helpful and should be considered as part of a healthy lifestyle.
Now its your turn...
Have you tried fasting?
If so, how did you feel during the fast?
What were the results of the fast?
Any advice that you would give for others considering fasting?
Leave your questions and comments below.
Dr. Whelchel is a family physician who specializes in functional medicine, especially hormone optimization. He has over 20 years experience in private practice managing patients with various medical issues. His passion is helping patients reach their full potential of wellness and quality of life. He grew up in the Texas Panhandle where he currently lives. He is married and has 3 awesome children.