In this article:
- History of fasting
- The science behind intermittent fasting (IF)
- How does IF work?
- Popular IF formulas to follow
- Health benefits of intermittent fasting
- Q&A: Answering all your questions on IF
Whether you’ve tried intermittent fasting and decided it wasn’t for you or you’re just considering it, any significant change to your nutrition deserves some careful consideration. What works for others may not be the best choice for you, and we’re not just talking about personal preference. As popular and beneficial as intermittent fasting (IF) can be, just like any other lifestyle or diet trend, it’s useful to understand the risks.
With so much conflicting information regarding IF online, it is completely understandable that you might be confused. In this detailed guide, we will explain its basic principles, introduce different forms of IF, and tackle the most talked-about benefits, while addressing possible risks that are less talked about among the hype.
Whether you’re just a beginner or a faster for life, it never hurts to learn more about the most popular nutrition lifestyle du jour. Let’s dive right in!
Let’s start with a little bit of history behind the intermittent fasting phenomenon. IF was introduced as a form of a diet back in 2012, when a BBC broadcast journalist, Dr. Michael Mosley, shared his idea of fasting and eating periods. But when you think about it, fasting is nothing new, as it remains a staple in numerous religions and cultures.
Fasting as a form of both spiritual and physical cleansing has been around since ancient times. Proof of beneficial effects of fasting originates from ancient Greece when Hippocrates would advise his patients to fast when sick, as he believed that “to eat when you are sick is to feed your illness”.
In ancient Greece, fasting was also referred to as the “physician within”, and was believed to promote cognitive sharpness. However, it is not only ancient medicine and philosophy that supported fasting as a healing method. Benjamin Franklin, one of the founding fathers, recognized fasting as the best of all medicines, in addition to resting.
Fasting is the key to spiritual awakening in many religions, worshipped as a form of purification and a true connection with one’s spiritual side. Even though fasting is practiced differently in all major religions, its essence and purpose remain the same. Muslims fast during the holy month of Ramadan, from sunrise to sunset every day.
While some Buddhists eat their food in the morning, and fast from noon until the next day, Orthodox Christians, such as myself, engage in numerous fasting periods throughout the year. The fast in my religion entails not only depriving yourself of certain foods, but also avoiding negative thoughts and making amends with loved ones, as a form of both physical and spiritual detoxification. Who knew?! Turns out I’m trendy. :)
We often hear that we should perceive food as fuel, and select the diet plan that will provide the body with enough energy. But what does this process actually look like? Let’s start with the basics.
There are two types of fuels in the body: glucose and fats. When we eat, glucose is produced and used as fuel. However, since we don’t get to use all the fuel produced, it is converted into glycogen. If we’re not physically active and we don’t use up the glycogen we have left, it will be transformed into triglycerides and stored as fat - hence excess weight or weight gain.
And this is where fasting comes into play. When we’re in a fasted state, our body is forced to utilize the remaining glycogen, and once it has been used up, the body starts to use the stored fat as a source of fuel. To be more specific, during the fasting period, the body breaks down fat into triglycerides, which are then broken down in order to produce fatty acids.
Finally, the liver is able to convert these fatty acids into ketones (you’ve probably heard all about these if you’ve tried keto), and they’re ultimately used as fuel. In order for the body to produce ketones and achieve this metabolic switch, there typically has to be a fasting period of 8 to 12 hours - which is the basis of many IF types.
Advocates of IF will tell you that you shouldn’t perceive it as a diet per se, but more of an eating pattern, and definitely a lifestyle. Why is that? Well, it simply doesn’t limit your food choice and there are no “forbidden” foods or ingredients. How nice! IF relies on scientifically-backed fasting practices, and you can choose the pattern that fits your needs and daily routine the best (which we’ll discuss in a bit). There is no one-size fits all when it comes to intermittent fasting. Perhaps that is part of the appeal.
Basically, intermittent fasting consists of periods where you abstain from food (fast), and periods when you eat. The eating stage is also known as the feeding window, and if you’re wondering what’s the best way to break a fast - here’s our detailed guide.
Many people choose IF over other popular diets due to one important factor: there are NO restrictions. While dieting, it can be challenging both mentally and physically to consciously reduce your caloric intake, while also managing “blacklisted” and “approved” foods. Restrictive diets often lead to binge eating, yo-yo diet weight-gain, as well as mental exhaustion.
Based on the science behind IF, no deprivation is necessary when it comes to your food choices, so you can leave the “good and bad foods” mentality behind. However, it is certainly advised to work towards more balanced nutrition, whether you’re trying to lose weight, gain more muscle, or simply lead a healthier lifestyle.
Okay, now that you’ve decided that you want to give intermittent fasting a shot, it’s time to talk about your daily routine, goals, and current eating habits. Regardless of the fasting method you choose, you’ll certainly need time to adjust. This is healthy and normal, by the way. Due to its lack of restrictiveness, IF is a perfect long-term and sustainable choice for nutrition, but as you know, every routine takes time.
The 5:2 diet is touted as the best choice for newbies to the fasting world, as it allows you to plan your nutrition weekly instead of daily. During the 5:2 diet, you eat normally for 5 days of the week, and decrease your calorie intake and feeding windows for only 2 days a week. This means that during the week, you’ll have 5 “normal”, non-fasting days, and only 2 fasting days.
For instance, you could fast on Tuesdays and Thursdays, while the rest of the week would involve no fasting periods. The recommended calorie intake on fasting days is approx. 500cal for women and 600cal for men, but this is definitely something you adapt to your daily schedule, workout regime, and overall energy levels.
The 16:8 method is the most popular IF format and, according to many people, the most sustainable form as well. As the name itself implies, the daily fasting period should last anywhere between 14 and 16 hours, while the feeding window (i.e. the time when you should have your daily meals) should last 8-10 hours. Fitness expert Martin Berkhan is responsible for the expansion of this method which is also known as Leangains protocol.
If you haven’t tried this method yet, you may wonder, how is it considered one of the simplest IF methods? Well, here’s a little hack: if you have your dinner, go to sleep, and then just skip breakfast in the morning - that’s your fast! It’s that simple: if you eat your dinner at, say, 8 pm. (and avoid any late-night snacking), and then fast until noon the next day, that’s your 16:8 IF plan sorted. For many modern lifestyles, this keeps things manageable and simple.
For best results, you want to make sure that you’re consuming few to zero calories during fasting periods. While your usual snacks are off-limits, you can definitely enjoy zero-calorie beverages such as coffee, tea, and of course - water (lots of water!). If you’d like to know what you can consume during your fast, and if your favorite supplements will break your fast, check out this guide.
Alternate-day fasting is pretty self-explanatory, as you’re supposed to fast every other day. However, since most of the variations of this method limit fasting day caloric intake to 500cal, alternate-day fasting isn’t exactly everyone’s cup of tea. In fact, because it is rather strict, it is not recommended for beginners.
The eat-stop-eat method must be one of the most challenging types of IF, as it involves a full-day fast. If you’re following this method, you’re supposed to have a 24h fast once or twice a week, which is definitely something that doesn’t work for many people. An example of a 24h fast would be fasting from breakfast on Monday till breakfast on Tuesday, or any other meal for that matter.
The face behind the Eat-Stop-Eat movement is Brad Pilon, a health and fitness enthusiast, who based his theory on his own experience and research, and even wrote a guide on this IF method.
If eat-stop-eat is your preferred method of fasting, you should make sure that your eating window is filled with the right nutrients. It is important to plan your meals wisely and fuel the body to prepare it for these extreme forms of fasting.
If you want a more spontaneous routine, this is the best IF method for you. There’s no need to think about hours, fasting periods, and eating windows. All you need to do is skip a meal here and there, whenever you feel like it!
This method relies on natural body instincts. If you’re into intuitive eating for example, you might naturally do this already. For instance, if you woke up and you’re not craving food, it is okay to skip your breakfast. If you had a nutritious lunch and you’re still feeling full by the time you’re supposed to eat dinner - call it a day and skip your dinner. Contrary to popular belief, skipping a meal will not interfere with your muscle gain goals - as long as your other meals are balanced. At the end of the day, we don’t always have to eat three square meals a day to stay healthy. Sorry Grandma.
The warrior diet has a specific name as it was created by a former member of the Israeli Special Forces, Ori Hofmekler. Apparently, Ori based the concept of this diet on the eating pattern of warriors from the ancient times, who didn’t consume a lot of food during the day, but indulged in a real feast at night. The core principle of this diet is to have a 20h fasting period, when you consume a small amount of food such as raw fruits and veggies, and then eat whatever you desire. You might have overheard this diet referred to as the OMAD diet as well, which simply stands for “One Meal A Day.” Some cross-fitters swear by it.
The warrior diet proposes a fasting period of 20 hours and an eating window of 4 hours - which is more of an “overeating window” as you’re encouraged to eat as much as you want. You might need to in order to regain energy.
Intermittent fasting is different from all other popular diets on many levels - one of them being the beneficial effects. While the majority of diets focus strictly on weight loss and fat loss, the benefits of IF stretch far beyond the scale, which makes this eating pattern suitable for many more of us, even if you’re not looking to lose any weight at all.
It is safe to say that IF is suitable for a variety of health and fitness goals, including weight loss, toning your body, maintaining your current shape, or even gaining “healthy” weight by increasing lean muscle mass.
The basic concept of intermittent fasting, which is to eat less during the day, is already a great start for weight loss (unless you’re planning on compensating for the lost calories during your eating window). What’s more, IF has been found to boost metabolism by 3.6-14%. Increasing your metabolic rate means allowing your body to burn fat and more calories, resulting in weight loss. Therefore, by reducing your calorie intake and promoting the rate of burning calories, IF is guaranteeing serious weight loss results, even when managing obesity.
So, how drastic can weight loss be, using an IF diet? According to a study comparing the efficacy of IF and daily calorie restriction, IF can result in weight loss of 3-8% over the course of 3-24 weeks. In addition, yet another study comparing IF to calorie restriction found that, even though both methods are extremely effective when it comes to weight loss, intermittent fasting allows you to retain lean mass and “target” fat loss more precisely.
Speaking of “targeting” body fat, you’ve probably been told a thousand times that weight loss and fat loss cannot be targeted per se and that it is largely determined by genetics. If your problem area is your waistline, you’ll be happy to learn that a reduction in stubborn abdominal fat is one of the most remarkable effects of intermittent fasting. In addition to weight loss, people who followed an IF regime for 3-24 weeks lost approximately 4-7% of their waist circumference, a study conducted by A. R.Barnosky suggests.
According to an aforementioned study conducted by A. R.Barnosky and colleagues, intermittent fasting has been associated with reduced insulin resistance and decreased sugar levels. Namely, sugar levels in individuals who followed this regimen were lowered by 3-6%, followed by a 20-31% reduction in insulin levels, hence the potential of IF in helping many to decrease the risk of type 2 diabetes.
Intermittent fasting has been shown to promote sensitivity to insulin (which is the blood-glucose-lowering hormone). In addition, IF promotes an eating pattern that has been associated with lower levels of fat deposits in the pancreas, hence its potential in decreasing the risk of fatty liver. Because fatty liver is one of the possible triggers of diabetes type 2, this is yet another promising finding for its management.
We already know that oxidative stress can be very harmful to our health. In simple terms, oxidative stress occurs when the body is unable to keep oxygen reactive species (ROS) under control. There are many factors that can influence the production of ROS and aid in their proliferation, some of them being environmental pollutants (such as UV rays and heavy metals) and xenobiotics.
Why are ROS considered so dangerous? Well, the term oxidative stress stands for a process in which cell tissues are damaged, the cell structure is disrupted, and cell membranes are compromised. A study conducted by J. B. Johnson et al. finds that intermittent fasting reduces markers of both oxidative stress and inflammation.
Just like oxidative stress, inflammation is one of the main underlying causes of numerous conditions. There is one significant difference between the two though since inflammation can be both good and bad. On one hand, normal inflammatory processes are desirable, as they help the body heal injuries and put up walls once foreign invaders have been detected. On the other hand, in the case of “false alarms” or bad inflammation, this process can trigger a variety of disorders, including asthma and arthritis. According to a study conducted by F. B. Aksungar et al., prolonged intermittent fasting can be linked to “positive effects on the inflammatory status of the body”.
Because IF induces ketogenesis (the development of ketone bodies in the bloodstream), it triggers various cellular signaling pathways. The activity of these pathways promotes the body’s immunity against oxidative and metabolic stress, while removing or repairing the damaged molecules. What’s great about this process is that the benefits continue even during the eating window, improving stress resistance, lowering inflammation, and managing glucose.
While heart health is determined by a complex mix of genetics and lifestyle, dietary changes can make a significant difference. Intermittent fasting has been shown to improve different heart health-related factors, including blood pressure, cholesterol, triglycerides, blood sugar levels, and inflammatory markers.
Furthermore, intermittent fasting supports the natural process of autophagy, which is essentially cellular detoxification. Overall, detoxification, especially of environmental toxins, has been linked to complex cardiovascular issues such as irregular heart rhythm.
When you combine reduced oxidative stress, low inflammation, managed insulin resistance, and optimal blood sugar levels - you get improved cognitive health! All these vital metabolic processes are necessary for your brain to function properly.
What’s more, aside from improving brain-related functions, intermittent fasting seems to promote the development of brand new healthy cells, as well as the levels of brain-derived neurotrophic factor (BDNF). This brain-derived neurotrophic factor is the key characteristic in overall brain health and memory.
In addition, intermittent fasting has been found to improve overall mental clarity, boost mood, and increase the brain’s ability to slow down the aging process. Furthermore, ketones produced during fasting periods play a significant role in stabilizing mood, as well as neuronal protection.
How many times has it happened that you’re unable to sleep after a large meal? Diet can certainly have a serious effect on sleep quality and duration - especially if we’re making dietary choices that don’t agree with our digestion. It appears that IF truly has a positive impact on sleep, as it regulates the circadian rhythm - which benefits your sleep pattern. How is that? Well, if you’re following a method that requires you to have your last meal of the day hours before bedtime, you’ll have enough time to digest food and go to bed without that heavy feeling in your stomach.
Also, if you’re going to bed after a large meal, you may compromise your digestion which could not only affect your sleep, but also cause acid reflux or heartburn.
According to a study on intermittent fasting and gut microbiota, IF may directly affect the gut microbial composition, function, and interaction. The gut microbiome has a vital role in the human body, and not only digestion. It takes part in vitamin production, immune system development, the gut-brain axis, skin conditions, allergies, as well as digestive tract disorders.
Good bacteria in the microbiome is highly reactive to the presence and absence of food, so in this regard, fasting can largely affect its composition. According to Amir Zarrinpar, MD, Ph.D. at Health.com, periodic eating and fasting may aid in reinstating and maintaining the natural, optimal cycle of the microbiome bacteria production.
What’s more, fasting as an eating pattern has been associated with more positive changes in the digestive system, including lowered intestinal inflammation and a healthier gut barrier. However, in order to maintain the balance and diversity of your gut bacteria, it is advised to avoid calorie restriction during your eating window, as well as prolonging your fast.
While this is not necessarily a health benefit, it is definitely one of the biggest advantages of intermittent fasting. IF is a great addition to your time management strategy, as it will cut your meal prep time in half. Even if you’re a passionate cook such as myself, day to day responsibilities can get in the way of your passion.
Planning and making 3-5 healthy, balanced meals can be quite challenging on a daily basis, and once it becomes a stressful burden - is it even healthy anymore? While keeping your body healthy, it is also necessary to take care of your mental health, and obsessing about three meals and their micronutrients throughout the day could even push you into your old habits (as it may seem like a more stress-free option at the moment).
With intermittent fasting, you’ll usually have 1-2 meals a day - which means less time in the kitchen and more time for other things in life. Also, if you’re meal prepping once a week, there’s no need to spend an entire afternoon in the kitchen like you used to.
There are definitely many celebrities who have shared their IF experiences and results with their methods of choice. Jennifer Aniston is on-board with 16:8 IF, so she prefers to skip her breakfast and start her day with a cup of black coffee and a celery juice instead.
Reese Witherspoon who has also been an advocate of fasting, skips breakfast and eats her first meal around noon, and then dinner around 8 pm. Halle Berry follows a 16:8 format of fasting as well.
Rosie Huntington-Whitely has also embraced an IF diet and swears by its benefits for her figure and digestive health. In Rosie’s case, she never extends her feeding window beyond 6pm, which allows her body to digest and reset for the next day.
Nicole Kidman has also been following the 16:8 fasting diet for years. Nicole likes to fit all her meals into an eight-hour window that starts around 10am and finishes at 6 pm. Margot Robbie also chooses intermittent fasting over restrictive diets to stay in shape.
Besides following keto and the 16:8 IF method, Kourtney Kardashian participates in a 24-hour fast once per week. On the other hand, JLo (Jennifer Lopez) has discovered that the best method for her crazy schedule is the 5:2 diet, which means that she limits her calorie intake to 500cal two times per week.
Dr. Jason Fung is one of the medical community’s most celebrated resources alongside the growing popularity of intermittent fasting around the world. Instead of focusing strictly on visible results like weight loss, Fung puts emphasis on the numerous health benefits of IF lifestyles, including cognitive health and blood insulin levels.
If you’re accustomed to eating often throughout the day, IF may initially result in tiredness and extreme cravings. To avoid these symptoms, it is necessary to start fasting little by little, slowly introducing this new routine, and allowing both the body and the mind the time needed to adapt. Find the IF method that works for you and that doesn’t leave you craving food every single day. Of course, it is normal for you to have “off” days when you simply indulge and give yourself a break.
Just like with any other diet regime, deprivation may cause mood fluctuations - especially if IF is a drastic change to your lifestyle. Once again, you should take it step by step and avoid strict rules. Part of IF is indeed intuitive, allowing yourself to eat what you like during feeding windows. Make sure to listen to YOUR body. What works for other people may not always be the best solution for you!
Yet another risk of IF is over-indulging or binge eating during feeding windows or on non-fasting days. Moderating your eating habits during feeding windows initially might be extremely difficult if you find yourself hungry. It’s important to drink plenty of water during fasting periods and in some cases allow yourself to consume a small amount of calories during fasting windows so that you don’t crash.
Dehydration is also one of the potential issues, since there are many people who associate drinking with eating, and forget to drink their water while fasting. Staying hydrated is always important, but it is certainly a priority in IF in order to avoid fatigue, brain fog, and similar side effects.
Unfortunately, for some women intermittent fasting is a no-go. Female hormones responsible for important processes such as ovulation, metabolism, and even mood management largely depend upon energy intake. Apparently, even short-term fasting may cause a hormonal disbalance in some women, especially due to the production of estrogen.
If estrogen (one of the key metabolic regulators) is in any way compromised by fasting, many aspects including hormonal cycles and even periods can be disrupted. What’s more, as a result of an estrogen decrease, females may experience weight gain and fat storage on an IF diet. Needless to say, it is necessary to track any changes, listen to your body, and consult with your physician.
Intermittent fasting is not recommended for breastfeeding and pregnant women. The nutrition in these sensitive life phases is quite specific, and IF is not recommended during maternity due to the risk of compromising both the mother and child’s health.
In general, IF diets are not recommended for individuals who have suffered from disordered eating such as anorexia, bulimia, or binge-eating disorder. It’s imperative to speak with your health professional or psychologist in such cases to establish whether or not IF might be a safe option for you.
For those suffering from low-blood sugar or hypoglycaemia, an IF lifestyle can be very risky to your energy levels and health. It is not recommended that these individuals partake in intermittent fasting without medical authorization.
Any person with medical conditions should certainly consult with their healthcare provider prior to making any drastic lifestyle and/or nutrition changes.
You certainly shouldn’t go straight for the most extreme forms of intermittent fasting. The best introduction method is the 5:2 diet which allows you to continue your normal routine for 5 days and then reduce your calories to 500-600cal for 2 days of the week.
It could take time to discover the best method for you, but we promise it’s all worth it! You could also keep a food journal in order to discover what kind of method works best for you and which meals are your favorite for breaking a fast. Also, having someone on board to share this IF journey with makes it that much easier.
Consult your doctor prior to starting an intermittent fasting plan to establish what your personal risk factors are.
Don’t be too hard on yourself! Yes, fasting means depriving yourself of food during longer periods than you might be used to. If you find it too hard to wait until your next feeding window, allow yourself ~200cal during the fast and give yourself a break. It is also important to optimize your feeding window by consuming nutrient-dense, high protein meals, and choosing the right supplementation to support your goals.
Finding the right fit for a new lifestyle takes time, so do it little by little, and don’t be afraid to make small changes and tweaks along the way to optimize for your specific lifestyle.
Avoid comparing yourself to others, as your progress is completely individual. With so many different IF methods, it shouldn’t be hard for you to find the best option (and even customize it if needed!). At the end of the day, your nutrition should make you feel good and energized, and it should never be a burden.