What is Intermittent Fasting?
Written by Ioannis Nikitidis MD, MSc, BSc and Medically Reviewed by Ana Reisdorf, MS, RD on January 23, 2020.
Intermittent Fasting is an eating pattern where you cycle between periods of eating and fasting.
In other words, it’s a periodic energy restriction that accelerates weight loss and leads to various health benefits, such as reduced inflammation, regulated blood sugar and cholesterol levels, improved brain health and many others. Most popular type of fasting is 16:8, which means eating during an 8-hour window and restraining from food for the remaining 16 hours everyday.
Why should we fast?
Take a good look at human history and you will find that fasting is deeply rooted in our biology. Back in the day, access to food was limited, which led to natural periods of fasting and changes in human evolution. The human body, including the brain, became accustomed to regular food scarcity.
Fasting is the most time-honored healing tradition in the world. It’s a part of every culture and religion on Earth. People from civilizations like Egypt or Ancient Greece would introduce periods of voluntary fasting into their personal lives. They observed that fasting had many health benefits, which is why they engaged in the practice even when the periods of involuntary starvation had ended. They called it “cleansing,” “purification,” and “detoxification.” If you think about it now, these civilizations were much more advanced than we credit them for.
However, in recent years, everything has changed. Today, we have access to unlimited amounts of food and our bodies find it difficult to handle the load. That’s where the need for Intermittent Fasting comes from. It’s only when we are not eating or drinking anything with calories in it that our bodies can begin the process of repair and create space for new cells to grow. Intermittent Fasting doesn’t prohibit people from eating but encourages them to eat only the food we need, when we need it, so we don’t eat more than our bodies are capable of processing.
Humankind has a long history of fasting, but the aspects of how and why we do it have drastically changed.
Modern Intermittent Fasting consists of periods of fasting that occur regularly between periods of eating. You are still able to eat a healthy amount of food, but you do it in a smaller time frame, which is called your ‘eating window.’
Today, you have the freedom to choose from many different methods of fasting. Fasts can last from 12 hours to 24 hours. You can choose to fast once a week, once a month, or even once a year. You can do shorter fasts daily, or longer ones (24-36 hours) two or three times per week.
There is no single best way for everybody – it is always a personal preference. What’s the difference between fasting and starvation? Well, for starters, people who are starving are not doing it voluntarily. They cannot tell when they will eat next. It can happen during war and famine when food is scarce. However, those who choose to fast are entirely in control of their eating patterns.
Fasting is a voluntary abstention from eating for health, weight loss, or other reasons. It can be simple, practical, and very effective.
Basics of intermittent fasting
Most of us already “fast” every day while we are sleeping. Intermittent Fasting can be as simple as extending that fast for a few hours longer. However, if you’re thinking about fasting as a long-term lifestyle change, you can choose from several Intermittent Fasting methods that are easy to implement into your daily routine.
Here’s one example: Skip your breakfast and have your first meal at noon. Then have your last meal at 8 p.m. That’s it! You’ll have fasted about 16 hours in a day.
Generally speaking, women should fast for about 13 to 14 hours a day. Men should fast up to 16 hours a day. One of the most important aspects of Intermittent Fasting is timing. In order to adjust the release of hormones in your body, it’s better to fast around the same time each day. This way, you will be able to ease your body into this new eating pattern and, of course, stick to the plan for the long term.
Benefits of Intermittent Fasting
Slows down the aging process
Intermittent Fasting has helped numerous people who suffered from skin-related problems, such as acne. The secret is, when you give your body an extended break from its digestion routine, it kick starts your body’s detox mechanism too.
Those who allow their body to adjust to this new digestion pattern claim to enjoy clear, glowing, healthy skin – an unexpected, long-lasting perk that comes with IF.
Benefits your skin
Intermittent Fasting has helped numerous people who suffered from skin-related problems, such as acne. The secret is, when you give your body an extended break from its digestion routine, it kick starts your body’s detox mechanism too. Those who allow their body to adjust to this new digestion pattern claim to enjoy clear, glowing, healthy skin – an unexpected, long-lasting perk that comes with Intermittent Fasting.
Improves resistance to oxidative stress
A number of observations prove that intermittent fasting effectively reduces oxidative stress. Reducing it is extremely important as long-term oxidative stress eventually leads to the destruction of important molecules in the body through reactions with other unstable molecules, such as free radicals. It also plays a role in the development of a range of conditions, including cancer, diabetes, Alzheimer’s and Parkinson’s disease and can also accelerate the aging process.
New studies have concluded that intermittent fasting reduces inflammation, a condition that can lead to various diseases such as diabetes, multiple sclerosis, and inflammatory bowel syndrome. It might also exacerbate symptoms of certain conditions you already have. The reduction of inflammation occurs due to a decrease in cells that cause inflammation — called “monocytes” — in the blood. Fasting for prolonged periods, coupled with ample water consumption, can help you eliminate inflammation issues.
Improves your brain health
Fasting improves metabolic factors that are important to our brain health. This includes less oxidative stress and inflammation, as well as a drop in blood sugar levels and insulin resistance. Eventually, it also increases the levels of brain-driven neurotrophic factors – a brain hormone that, when deficient, is directly linked to depression and other mental health issues. In addition to that, people who try intermittent fasting confess to be feeling more sharp-minded, focused and tend to be more productive at work.
Reduces risk of diabetes and heart disease
Generally explained, a higher-than-normal blood sugar level puts you at risk for developing diabetes and heart disease. Intermittent fasting helps to balance out blood sugar and insulin. It can reduce blood sugar by 3-6% and insulin levels by 21-30%. Studies have shown that intermittent fasting has helped people with type 2 diabetes to limit their insulin intake, dramatically lower insulin levels and significantly improve insulin sensitivity, as well as significantly lower blood pressure. It also reduces “bad” LDL cholesterol, blood triglycerides, and inflammatory markers.
Helps to lose weight
This might be an obvious benefit, but it’s important to know about some of the less obvious reasons behind weight loss. Intermittent Fasting increases “lipolysis” (fat burning) in the body and changes hormone levels to facilitate weight loss. By lowering insulin and increasing growth hormone levels, it also increases the release of the fat-burning hormone called norepinephrine (noradrenaline). Due to these hormonal changes, fasting increases your metabolic rate. Studies show that people who practice Intermittent Fasting can expect to lose up to 7% of their waist circumference, which indicates a significant loss of harmful belly fat that builds up around internal organs and causes disease.
The other mechanism that makes fasting so effective for weight loss is the fact that it provokes the secretion of HGH— hormone that regulates body composition, body fluids, muscle and bone growth, sugar and fat metabolism, and possibly heart function. The hormone has many well-recognized “anti-aging” health and fitness benefits.
Simplifies your meal-prep
When you eliminate the number of meals you eat in a day or weak, you also find this amazing added benefit: meal preparation becomes much easier. During eating windows, you can simply eat your regular food and then drink water, tea, coffee, or bone broth while fasting.
With other diets, you actually need to replace your daily food with expensive alternatives or fancy ingredients that are difficult to find. Fasting does not require replacing food. You are simply eliminating some of what you eat, and not replacing anything if you don’t want to.
Types of Fasting
This type of fasting may be a good option for beginners. A 12-hour fast means that you eat within the first 12 hours of the day and abstain from food for the next 12 hours.
The easiest way to do the 12-hour fast is to include the period of sleep in the fasting window. For example, if you choose to fast between 7 p.m. and 7 a.m. you have to finish dinner before 7 p.m. and wait until 7 a.m. to eat breakfast. This method will allow your body to start turning its fat stores into energy, which releases ketones into the bloodstream and will encourage weight loss.
Small piece of advice from us will guide you through basic steps how to make it easier:
First of all, prepare balanced, nutritious, healthy meals and snacks. Split your meals into three big and one or two small snacks per day. Your daily calories per meal should split into: 25% of daily calories for breakfast, 10% for snacks, 30% for lunch and 25% for dinner. Make sure you have a routine and eat every 3 hours. Your last meal should be consumed 3 hours before bedtime.
Make sure you drink enough water and avoid fast food, sweets, cookies and other junk food.
This fasting type is a bit more challenging than 12:12. A 14-hour fast means that you eat within chosen 10 hours of the day and abstain from food for the next 14 hours. In order to make it easier, we advise you to include your sleeping time in the fasting window.
During fasting your body enters into a state called gluconeogenesis, marking the body's transition into fasting mode and it not only helps you lose weight, but it can also improve digestion, give you more energy, help you focus, and even slow brain aging. It can also help regulate levels of insulin and lower blood sugar, which is helpful for people who are insulin resistant or prediabetic. Fasting influences metabolic processes, boosts metabolism, improves fat burning mechanisms, controls appetite, cardiovascular function, enhances detoxification. You can use this fasting method every day.
Keep up with healthy and nutritious meals during the day. The main rules for your general meals during fasting window should follow the same steps and rules as described in 12:12 method.
Make sure you drink 8 glasses of water every day. Avoid fast releasing carbohydrates, such as: crackers, cookies, white bread, sweets and etc., sugars, and trans-fats.
As said in the very beginning, 16:8 is one of the most popular styles of fasting. As the title implies, you should hold off from any food for 16 hours and eat during the remaining 8 hours of the day. 16:8 cycle can be repeated as often as you like — from just regularly once or twice per week to every day, depending on your personal preference.
You can experiment and pick the time frame for eating and fasting windows that best fits your schedule, but we highly advise you to include the period of sleep in the fasting window, and finish your last meal 3 hours before bedtime, for example: you can finish your evening meal by 8 p.m., then skip breakfast the next day and start eating again at noon. It’s important to note that during your eating window, you have to eat several small meals and snacks spaced evenly throughout the day to help stabilize blood sugar levels and keep hunger under control. Try to incorporate foods that are high in fiber. Stick to nutritious whole foods and beverages during your eating periods.
Keep your body hydrated by drinking water, bone broth, coffee and other calorie-free beverages. Avoid all kinds of junk food. It'll negate the positive effects associated with fasting.
While fasting for 16 hours, you’ll protect your body from aging, obesity, inflammation, diabetes, and liver disease.
20:4 IF (known as The Warrior Diet)
This type of intermittent fasting is based on the eating patterns of warriors in the days of old when they would eat very sparingly during the day, then feast at night. For this reason, the 20:4 intermittent fasting method is also known as the Warrior Diet.
How do you do the 20:4 fast?
To effectively do the 20:4 fast, you need to abstain from any foods for 20 hours. You can drink calorie-free drinks like water, plain black coffee, tea, and calorie-free sports drinks. This might sound intimidating but ideally, you’d be sleeping for at least 8 hours of those 20.
Then you have four hours at the end of your day that you can eat all you want. If you are a shift worker, or just would rather eat during the day, you can tailor that four-hour window to fit your schedule.
How to eat during the 20 hour fast?
The simple answer is - you don’t. There shouldn’t be any eating during the 20 hours of your fasting window. If you do, it should be a bare minimum. Keep in mind that this type of intermittent fasting is one of the hardest to stick to, as you tend to get really hungry abstaining from food for 20 hours.
This can also be difficult if you’re used to having breakfast and drinking coffee with cream and sugar. In this case, you can choose morning to be your four-hour window. But, that means you’ll be going to bed hungry and that can make falling asleep difficult.
A tip from us would be to play around with the schedule and find one that is the most doable for you.
How to eat during your 4-hour feast?
Obviously, you won’t be eating for four hours straight. That’s not what the concept entails. It just simply means that during those four hours, you can eat as much as you want.
Some people eat one meal at the beginning of four hours and one at the end. Others choose to eat one big meal and snack for the remaining time. You want to make sure that you don’t eat too close to bedtime so that you won’t be too full to sleep. You also don’t want to overeat. Eat until you are full and as early as possible.
Because the fasting period is very long, it is recommended that you stick to low carbohydrate foods when you eat. This helps the body to reduce insulin levels and keeps your hunger at bay during your fasting hours. You would almost want to adopt a keto type diet, although you don’t have to be that strict.
It is vital to include moderate amounts of foods that contain complex dietary fibers that are found in vegetables, whole grains, seeds, fruits and legumes. That'll help you feel full for a longer time.
Right foods to include in your feasts would be meat, fish, eggs, cheese, nuts, and vegetables. Make sure you get plenty of liquids that are sugar-free. Another good idea is to drink bone broth for sodium and its other health benefits.
24:0 IF (known as OMAD)
A 24-hour fast means that you fast from breakfast to breakfast, from lunch to lunch, or from dinner to dinner – whichever you prefer.
For example, if you have dinner at 7 p.m. on Day 1, you should fast until 7 p.m. on Day 2. Regardless of what the title suggests, you don’t have to fast for 24 hours. You can have one meal during that time to tide you over and use it to take medications that must be taken with food. Same as The Warrior Diet mentioned above, it’s not for everyone.
You can try this fasting method for a short period of time if you need to lose weight fast because it really brings outstanding results. But stay smart, don’t go too harsh on yourself at once. If you’re deciding on trying OMAD together with restriction of carbohydrates or even going full-blooded keto, do it in small steps and listen to your body, or even better to discuss it with your family doctor or nutritionist.
There are also psychological aspects of sticking to a diet: if you go too hard, you might lose your will in the first couple of days or even cheat the same day only to feel bad about yourself.
Thus you might consider starting with some of the traditional routines, such as 16:8, and then building up yourself towards OMAD.
36:12 IF (fasting on alternate days)
This plan allows you to eat every second day. If you start on Monday, you can eat anytime between 7-8 a.m. and 7-8 p.m. and fast on Monday night and all day and all night on Tuesday. Those who follow this method should choose healthy food on fasting days and eat whatever they wish on non-fasting days.
Choosing this method for two weeks per month, will increase your cellular repair, stimulate autophagy, fat burning and also will boost your metabolism. There are many theoretical benefits, including appetite suppression and greater mental clarity. After this fasting method you should return to normal healthy eating patterns starting from breakfast and having other 4 meals (snacks, lunch and dinner). Try to keep yourself from overeating, stay calm, sleep well and begin your new non-fasting day.
After the fast one of the most important part is to break the fast correctly! It’s important to gradually reintroduce food. Have a first small meal full of nutrient-dense foods like fruits and vegetables, for example: bone broth-based soups, vegetable smoothies or a small salad.
Focus on foods that are easy to digest — try cooked, steamed, stewed meals over raw, grilled or deep fried.
Have 3 main meals every 3 hours and one-two small snacks in between main meals.
Drink enough water. At least 8 glasses per day. Don't rush into eating. Overeating right after fasting may lead to stomach discomfort. It’s important to wait between meals until you are actually hungry.
After your body adjusts you can resume a balanced "clean" diet, only try to keep it simple and easily digestible.”
A very popular method of intermittent fasting is the 5:2 diet. With this type of intermittent fasting, it's not about blocks of time during your day that you fast, but days during your week that you are fasting.
How do you do the 5:2 fast?
In a 5:2 fast, you would pick two days out of your week that you dedicate to fasting. That doesn’t mean that you don’t eat anything all day, you simply lower your calorie intake to about 500-600 calories per day. You don’t always have to fast the same two days every week, but you shouldn’t choose two days back to back.
For example, the first week you are implementing the 5:2 fast you decide to fast Monday and Wednesday. In the second week, you have a big event on Wednesday and you decide to move your fasting day to Thursday. That switch in days won’t have any negative effects in your fasting efforts.
Some people prefer to fast for two days in a row. Although this isn’t suitable for most. You won’t really get any more benefits from it and it might make you feel very weak. But, if you’ve been fasting for a while now and your body has gotten used to it, it might not affect you so hard.
Some people also take the word “fasting” for all its worth and don’t eat during their fasting days at all. If your body is used to extreme calorie restriction, you might be willing to try it. However, is not recommended for beginners.
How to eat on fast days?
Since you’ll be cutting your calories well below your regular eating habits, feeling hungry is expected, especially during the first few days of fasting. To minimize your hunger as much as possible it’s best that you eat nutritious and filling foods.
Think about foods that give you plenty of fiber with as little calories as possible. Vegetables are a great choice. You might want to include some lean meats, boiled eggs, baked fish. You can eat fruits but remember that some fruits are high in calories. Berries are your best choice since they are low in calories and high in fiber.
Clear based soups are great, especially if they are stocked with vegetables. You get the filling sensation from a warm meal, plenty of liquid, and fiber from the vegetables.
You should stay away from empty calories like crackers, cookies, white bread and pasta. Rice is fine if you choose brown, black, or wild rice. Steer clear of candy and sweets, especially sweet drinks. No junk food, fried or processed foods.
When it comes to drinks, it is important to stay hydrated on your fasting day. You should be getting plenty of water in your system. It's also important to drink water with minerals, especially sodium. You can also have black coffee, tea, or calorie-free sports drinks.
Meal timing on fasting days isn’t too important as long as you stay within your calorie limit. It should be noted that most people find that it is best to eat one or two meals on their fasting days. It is more convenient, and some evidence suggests that it can be more beneficial.
How to eat on your non-fasting days?
The best way to explain how to eat on your non-fasting days is to act like you aren’t fasting at all. The idea is to cut your overall weekly calorie consumption so that you lose weight. If you eat more than normal on your non-fasting days, you won’t see any weight loss.
You don’t need to count calories, but you should be practicing mindful eating. Focus on savoring your foods and paying attention to how your body feels. Stop eating when you are full. You are free to eat whatever foods you want but don’t eat as much as you want.
Commercial Intermittent Fasting Methods
Intermittent fasting is unique for having so many different variations. We already covered the most common intermittent fasting methods and now we’re going to share some of the methods popularized by intermittent fasting enthusiasts.
You can see they are not too much different from the classical ones, however, interestıng twists they have that might be worth your attention.
As the name suggests, a water fast is a type of fast where you don’t consume any food or drink except for water. This time herbal teas or black coffees don’t count. This may sound tough but the health benefits can be profound.
There is no set time for a water fast but generally, they can last anywhere from 24 hours to three days. This duration is considered safe for most people. However, it’s down to the individual and their needs as to how long the fast can be. Water fasting for over a week isn’t uncommon.
The Dubrow Diet
You may have heard about the Dubrows before hearing about their diet.
Heather Dubrow first came into the spotlight through "The real housewives of Orange County" show and now has her own podcast. Her husband, doctor Terry Dubrow is a plastic surgeon, and a TV star featured on the reality show called "Botched" on the "E!" channel. The Dubrow Diet consists of three phases.
Phase one lasts for about 5 days. It includes 16:8 fasting but has carb and alcohol intake limitation.
Phase two lasts until the desired weight is reached. The fasting window in this phase lasts somewhere between 12-16 hours per day. The diet is similar to phase one, but a bit less restrictive, as you can eat more healthy fats and complex carbohydrates
Phase three is all about maintaining the results. A 12 hour fast is done for 5 days a week and a 16 hour fast for two days a week. Phase three also allows to include cheat-meals every now and then, which will help to stay motivated and stick to the new eating pattern.
Leangains is a style of intermittent fasting created by Martin Berkhan, who is a personal trainer and a nutritionist. Leangains is a daily routine centered around structured eating, fasting and resistance training. The fasting method that is recommended is 16:8.
Since in the fasted state the body is already burning fat for energy, exercise is incorporated into the fasting period in order to boost weight loss. By exercising, fat-burning rate is increased, hence why the leangains methods advise that you workout in a fasted state prior to your first meal.
As for the diet - leangains protocol typically recommends a diet that cycles macros throughout the week. Carbohydrate and calorie intake should be highest on the active days, with lower carbohydrate and higher fat intake on rest days.
Post-workout meal is recommended to be the largest meal of the day. During rest days, first meal of the day should be the largest, accounting for around 40% of your daily calorie intake.