What is Intermittent Fasting(IF)?
Intermittent fasting (IF) is voluntarily abstaining from food in a non-continuous manner.
Intermittent fasting is shown to have many health benefits but it has been labeled as a fad diet (1). This label may not hold for long if intermittent fasting continues to show promising results.
Using modern and current research, this primer on intermittent fasting will discuss the forms of fasting, the benefits or harms of intermittent fasting and frequently asked questions(FAQs).
Fasting is not a new concept. It has been practiced for thousands of years for religious and medical reasons ( 2).
In recent years, the obesity epidemic has ignited interest in fasting. In modern times, fasting has been practiced predominantly for obesity and weight management (3)
How intermittent fasting promotes weight loss?
Human and animal research has shown that in fasted state energy utilization behavior is changed in our bodies (4).
Instead of glucose, the body uses fat cells and fat cells derived from ketone bodies as the energy source for the brain and body during fasting by a process called glucose to ketone (G-to-K) switchover (5).
During extended periods (12-36 hours) of food deprivation when the primary source of energy i.e. stored glucose (glycogen) is used up and the fat cells start producing energy-rich fat molecules (6).
These fat molecules are transported to the liver where they are further processed to ketones which are the source of energy in the fasted state.
Ketones are transported to high function cells, for example, the muscle cells and the brain cells. Here ketones are used as an energy source and protect the high functioning cells.
In summary, in a fasted state, instead of fat storage and synthesis, fat cells promote the production of fat-derived ketone bodies which become the preferred source of energy requirements of our bodies (7,8).
This is one of the reasons, fat burning (lipolysis) in fasting has been proven to promote weight loss, fight obesity, and diabetes type 2 (9).
Interestingly, research shows that intermittent fasting promotes fat loss even though the calorie intake during non-fasting and intermittent fasting may be the same (10).
Common Fasting Methods
Over the years several methods of fasting have been practiced but they can be broadly categorized into two types.
|Continuous energy restriction (CER)/non-intermittent Calorie restriction/ Daily calorie Restriction (CR)
|Intermittent energy restriction (IER) aka Intermittent fasting (IF)
|This form of dietary restriction has been very popular for weight management and obesity control. Continuous energy restriction dictates that you reduce your energy intake by 15–60% of baseline needs every day.
|The other approach to calorie restriction is called intermittent energy restriction (IER) involves 24 hours of ad libitum (as much or as often as necessary or desired) eating and the next 24 hours of complete removal or reduced calorie intake
Continuous energy restriction (CER)
Also known as /Non-intermittent Calorie Restriction/ Daily Calorie Restriction (CR)
In Continuous Energy Restriction (CER), you reduce your calorie intake by 15-60% of the normal calorie intake every day. Hence, by continuously reducing the calorie intake the overall body mass is reduced (11).
Intermittent energy restriction (IER) AKA Intermittent fasting (IF)
The other approach to calorie restriction is called intermittent energy restriction (IER) or simply Intermittent fasting that involves 24 hours of ad libitum (as much or as often as necessary or desired) eating and the next 24 hours of complete or reduced calorie intake (12).
There are several forms of intermittent fasting which are explained below.
Continuous Energy Restriction or Intermittent fasting. Which is better?
The question of which method is better is a crucial one. Here are some of the reasons why intermittent fasting is the preferred method.
- Research on adherence to dietary regimen has shown that the more you restrict your daily calories, the harder it gets to adhere to continuous energy restriction.
- In a study, the absolute dietary restriction (800 kcal) per day leads to a reduction in adherence to that diet regimen (13). It is logical to assume that people will be less willing to live their entire lives on a 15-60% reduced diet.
- The Intermittent fasting provides an alternative that focuses on WHEN to eat rather than HOW much to eat hence it is easier to follow intermittent fasting regime. Intermittent fasting takes the calorie counting out of the equation.
- The intermittent fasting is shown to be as effective as continuous energy restriction with regard to weight loss, insulin levels, and other health biomarkers. The studies did not find any significant difference in weight loss between continuous energy restriction and intermittent fasting (14,15).
Thus, Intermittent fasting can be a substitute for continuous energy restriction (CER).
- However, research also found that Intermittent fasting leads to increased hunger feeling compared to continuous energy restriction (16). This could be due to a decrease in leptin levels observed during intermittent fasting (17).
- Another research article showed that Continuous Energy Restriction is superior to intermittent fasting but continuous energy restriction is a difficult regimen to follow (18).
- Contrary to it, other studies have found that intermittent fasting is superior to CER in reducing postprandial lipaemia (19). There is strong evidence that postprandial lipemia increases the risk of atherogenesis which is a condition characterized by the formation of fatty plaques in the arteries (20).
- Studies in clinical settings have shown that intermittent fasting has a higher rate of attrition and weight regain following the cession of a 6-month weight-loss period.
- However, Intermittent fasting is more effective at reducing body weight and improving some cardiometabolic risk factors in the short term (21).
- One other reason for intermittent popularity could be that it supports the retention of lean muscle mass.
- A study shows that Intermittent fasting and continuous energy restriction provide similar weight loss outcomes in overweight and obese individuals, however, intermittent fasting was better in lean mass retention (22).
In summary, the research suggests that intermittent fasting could be an alternative choice for continuous energy restriction because:
- Both diet strategies effects have similar weight loss benefits
- Dietary adherence is difficult in CER
- Intermittent fasting may induce hunger but it is shown to be better for certain health biomarkers and most importantly it proves to be better in lean mass retention compared to continuous energy restriction.
3 major forms of Intermittent fasting
Intermittent fasting is shown to be a viable and effective method for calorie restriction.
3 most popular forms of Intermittent fasting studied in laboratories are (24):
- Complete Alternate Day Fasting (ADR): This regimen involves alternating fasting days (no or low energy-containing foods or beverages) with eating days (foods and beverages consumed ad-libitum).
- Modified Fasting Regimens: The most popular form is 5:2 (5-day feeding, 2 non-consecutive days no or reduced calorie intake)
- Time-restricted feeding: It involves ad libitum feeding periods within a specific “feeding window” of time every day.
Alternate day fasting (ADR)
This form of Intermittent fasting is also known as alternate-day energy restriction (ADER), alternate-day fasting(ADF), alternate-day modified fasting (ADMF), or every-other-day fasting.
During Alternate Day Fasting, you fast every other day. On the fasting day, you can take no-calorie fluids and on feed days you can consume food and calorie-containing drinks ad libitum.
Modified Fasting Regimens: 5:2 Intermittent fasting
5:2 intermittent fasting is a modified version of Alternate day restriction fasting. In a 5:2 intermittent fasting regimen, you fast 2 non-consecutive days of the week (only consume 20-25% of calorie requirement) and eat ad libitum on the rest of the 5 days (25,26) It is that simple.
For example, your daily energy calorie requirement is 2000 calories. On a fast day, you should aim at getting only 400-500 kcal/ day in women. Men generally go for 800 kcal per day.
Here is one example of 5:2 week.
The 5:2 intermittent fasting regimen is explained in detail by Dr. Michael Mosely and Mimi Spencer in their book “The Fast Diet”. Dr. Mosley is also the author of The Fast 800: How to combine rapid weight loss and intermittent fasting for long-term health.
Time-restricted feeding (TRF)
As the name implies, Time-restricted feeding entails restricting the feeding period to 4-10 hours and fasting for 14 to 20 hours in a 24 hour day (27).
During the feeding period, you can eat ad libitum i.e. however much you want, and during the fasting hours consume calorie deficient liquids.
Here, is what a time-restricted feeding would look like on a weekly basis. At least more than 12 hours of fasting is required.
There several modified versions of time-restricted intermittent fasting and more popular variations of time-restricted intermittent fasting are given below.
16:8 intermittent fasting
In recent years, 16:8 intermittent fasting has become very popular among sports enthusiasts and gym-goers. The book “The 8-Hour Diet: Watch the Pounds Disappear Without Watching What You Eat!” by David Zinczenko and Peter Moore played a role in its popularity.
16:8 intermittent fasting is a simple regimen. You have a “feeding window” of 8 hours during which you can eat whatever you want and you don’t have to count calories.
You fast for the rest of the 16 hours out of 24 hours. It is recommended that you make healthy food choices during feeding hours.
One of the reasons for 16:8 popularity is that it is easy to follow, thus providing a good alternative to 5:2 alternate day fasting. If you stop eating at 8 pm, you only have to delay your breakfast to 12 pm the next day.
Then you will have an 8 hours (12 pm-8 pm) window to eat. Since, you will be spending approximately 8 of your 16 hours sleeping you can use the rest of the 8 hours working, doing daily chores, exercising, drinking water, tea and coffee.
Remember you can only take these drinks without any sugar or milk.
You don’t have to follow the 8 pm-12 pm schedule. You can choose any 16 hours out of 24 hours of the day for your fasting.
Two recent research studies analyzed the role of 16:8 intermittent fasting on weight loss, lean mass, and metabolic diseases. They noted a significant role of intermittent fasting in influencing these parameters in the human body.
In a study, 34 individuals who regularly took part in resistance training were divided into an 8-hour feeding window and a non-fasting regimen.
Both groups consume similar levels of calories. 8 weeks of the program resulted in reduced fat and maintenance of muscle mass in the 16:8 intermittent fasting group (28).
A similar study on obese individuals showed less calorie intake and weight loss in the fasting group compared to non-fasted individuals (29).
Therefore, 16:8 could be beneficial for weight loss while keeping lean muscle mass.
20:4 intermittent fasting is another modification of time-restricted fasting. In fitness circles, this is also known as warrior dieting.
It is an extreme version of intermittent fasting since you’re only eating for 4 hours of the day. 20:4 is similar to OMAD fasting.
OMAD (One Meal A Day) fasting
OMAD is a more extreme form of intermittent fasting than 20:4. Although not strictly a time-restricted feeding regimen. It involves eating in a very small window of time i.e. one sitting or 1 hour.
You aim to eat a 2000 calorie diet in one sitting. During the fasting period, you can drink calorie-free drinks (30).
This type of fasting strategy may not be suitable for everyone, yet it has become popular over the past few years.
Benefits of intermittent fasting
Moreover, other concurrent benefits include a reduction in blood pressure, LDL level, and glucose.
Diabetes type 2 management: Not many studies have studied the role of intermittent fasting in diabetes management. But new research is starting to show a relationship between intermittent fasting and diabetes type 2 management (35). Reduced glucose levels and insulin sensitivity are observed in patients with intermittent fasting (36).
Cardiovascular health: Studies in both animals and humans have shown a beneficial role of intermittent fasting. Intermittent fasting is shown to protect the heart, reduce cholesterol and blood pressure, and other metabolic markers important for heart function (37,38,39).
Brain health: Intermittent fasting is shown to improve cognitive function. Since intermittent fasting mimics the ketogenic diet at the molecular level, the brain benefits of ketogenic can be expected on intermittent fasting as well (44,45,46).
Quality of life: In small animals such as rodents, intermittent fasting can prolong life (47). In humans, lowering, disease risk, obesity, and diabetes will ultimately help in improving life quality and better aging (48).
Fasting has been used for centuries by different religions and for medical purposes.
The current data and studies are a mix of animal and human studies.
The animal data support the use of intermittent fasting for health benefits.
Human studies have shown support for animal studies, however, there is a need for more studies to fully understand the potential of intermittent fasting.
Frequently Asked Questions (FAQs)
Is intermittent fasting safe?
Yes, intermittent fasting is safe. Several studies have shown that intermittent fasting is a safe method for weight loss and can provide other health benefits (49).
What are the side effects of intermittent fasting?
Although safe, Intermittent fasting can have some side effects:
Intermittent fasting can improve eating behaviors and mood in overweight and obese individuals but could trigger unrestrained and binge-eating on fasting and non-fasting days among normal-weight individuals.
Also, Intermittent fasting could interfere with the frequency and length of menstrual cycles. However, the effects depend on a number of variables, such as starting weight, energy levels, and the type of intermittent fasting (50).
Intermittent fasting could potentially increase the release of cortisol, a stress hormone (51).
It is important to note that the observation was done in competitive athletes, and intermittent fasting could be one of the contributing factors in addition to the high-intensity competition.
Intermittent fasting lowers blood pressure and heart rate (52). Patients with low blood pressure and heart conditions should do fasting with caution.
The following people should talk to a physician before starting intermittent fasting
Pregnant women or new moms
People with low blood pressure
People on medications for metabolic or other conditions
Will intermittent fasting work for me?
Intermittent fasting has proven weight loss benefits.
However, intermittent fasting studies have shown variability in results due to differences in fasting methods, age, the weight of the subjects, and study models.
Therefore, one particular approach may not work for everybody due to individual health status, current weight, and eating habits (53).
In order to maximize your chance for success with intermittent fasting, try different types and methods to find out what works for you and supplement it with healthy eating and regular exercise.
What can I eat during my feeding time?
Research on the type of food to be consumed during intermittent fasting shows that combining the Mediterranean diet and intermittent fasting showed weight loss and reduction in insulin resistance. Moreover, this combination also helped in weight maintenance (54).
The Mediterranean diet is composed of olive oil, olives, fresh fruits, vegetables, grains, legumes, and moderate amounts of dairy, fish poultry red wine, and limited red meat (55).
Having mentioned that, the Mediterranean diet and intermittent fasting may not be the best strategy for you. You must experiment with different diets to see what works best with your intermittent fasting regimen.
The idea of combining the Mediterranean diet and intermittent fasting is to increase whole foods and minimize the consumption of processed foods e.g., sugary drinks, fried or roasted food, pizza and, pastries cakes which are high in simple carbs.
What Can I drink during the fast?
Any liquid without calories. For example, Water, Sparkling water, coffee and tea with no sugar and milk.
Can I work out during Intermittent fasting?
Yes, you can! At least in time-restricted fasting 16:8 resistance training resulted in weight loss, reduced fat mass and, maintenance of lean muscle mass (56).
Working out, especially in the morning before breaking the fast could be beneficial as your body is in a low glucose state and will use fat as an energy state.