Intermittent Fasting: A Complete Guide on How to Intermittent Fast

Intermittent fasting has become a wellness trend. In 2019, it was Google’s most searched diet, overtaken by the keto diet in 2020 (although it still had over 12 million hits). 

Some call it a fad, others the best thing since sliced bread. Everyday you hear about a new celebrity like Jennifer Aniston or Jay-Z who endorse the diet, or are rumoured to privately stick to its tenets. Whatever the consensus, there’s no denying the history and context of intermittent fasting goes far beyond best-selling wellness books or temporary trends.

A growing amount of scientific research shows the benefit of occasional breaks from food. But what’s the truth behind intermittent fasting? And is it something you should try? 

In this complete guide, we’ll explore the historical context of fasting, look at different intermittent fasting plans, and give you all the fuel you need to consider whether to integrate intermittent fasting into your lifestyle routine.

What is intermittent fasting?

One definition of fast is “voluntary abstinence from food and drink or from certain kinds of food.” Intermittent fasting is occasional abstinence from food or drink. It’s a pattern of eating that oscillates between spells of eating and fasting. 

These patterns typically vary; there’s no one way to follow intermittent fasting, and it’s more complex than not eating at all. Some styles of intermittent fasting include restricted calories or windows of eating.

There are three main categories of intermittent fasting: 

• Alternate-day fasting 
• Periodic fasting
• Time-restricted feeding 

The first involves, unsurprisingly, alternate days of fasting, with calories restricted one day followed by a standard day of eating the next. Periodic fasting involves entire days of fasting, with the “5:2 diet” being the most popular. Time-restricted feeding consists of eating windows, such as not eating before or after certain times.

A brief history of fasting: It’s not all about weight loss

Most of us eat breakfast without taking much consideration to the meaning behind the word itself. Breakfast is the first meal of the day: you break an overnight fast. 

Technically, you could argue that, no matter what time you eat your first meal of the day, it’s breakfast! Language often illuminates historical context. Indeed, fasting has a rich history, and has been practiced across cultures for a multitude of reasons.

The religious history of fasting

Fasting has strong connections to sacred rituals and religion. In ancient civilizations, priests and priestesses would fast before facing deities. Native Americans fast for four days and four nights at a sacred location, before embarking on a rite of passage known as a  vision quest. Hellenistic religions of Ancient Greece believed gods passed on divine teachings only after disciples practiced fasting.

Most major religions to this day have some aspects of fasting involved. Judaism, Christianity, and Islam all mark periods of fasting — from Lent to Ramadan. In Eastern religions, from Buddhism to Hinduism, fasting is seen as an act of repentance that primes the spirit for higher levels of consciousness and spiritual growth. The Buddha himself went through a period of asceticism, a form of extreme abstinence, which included a lack of food.

Medical history of fasting

There’s evidence from as early as the 5th century BCE that fasting was used for medical purposes. The Greek physician Hippocrates, widely viewed as the father of modern medicine, recommended patients with certain diseases to abstain from food or drink. Another healer, Paracelsus, once wrote that “fasting is the greatest remedy, the physician within.”

Fasting is also a key practice in Ayurveda, the ancient Indian medical system. Fasting is seen as a purifying process that cleanses the body of Ama (toxins or impurities), helping to regulate Agni, the “fire” that helps to digest food. Emphasis is on portion size, eating only enough food you can fit in cupped hands, and making space for digestion between meals. Zero-fasting for longer periods of time is sometimes recommended, although rare.

Naturopathic medicine, using natural remedies for the body to heal itself, has also moved through the times, and remains popular in modern-day. In the US, the Natural Hygiene Movement of the 19th century looked to fasting as a form of medical treatment. Fasting was also popularised in the UK, under the “Nature Cure” movement, with many similar approaches towards fasting across Europe.

Benefits of intermittent fasting: How to lose weight and feel great

You don’t only have to look far back in history to uncover the health benefits of intermittent fasting. The practice has found a resurgence in recent years, becoming a modern trend, accompanied by a rich body of research. Research has shown that benefits of intermittent fasting include a host of other benefits.

Aids with weight loss

According to the Harvard School of Public Health, a review of 40 studies into intermittent fasting has shown it to be effective for weight loss and to help the body burn fat, with an average loss of 7-11 pounds over 10 weeks. These studies explored various forms of intermittent fasting and calorie restriction diets, with people of different demographics.

Boosts metabolism

By restricting eating periods, the body exhausts sugar stores and turns to burn fat, a process known as metabolic switching. “When changes occur with this metabolic switch, it affects the body and brain,” Johns Hopkins neuroscientist Mark Mattson, Ph.D., says.

Prevention of diseases

“Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers,” Mattson adds. Research also suggests intermittent fasting reduces the risk of heart disease, cancer, and stroke.

Improved brain power

Intermittent fasting’s benefits on the brain make sense when you consider our hunter-gatherer background. Humans were used to going long periods without food, and they had to be mentally sharp while on a scavenge. “Going without food for even a day increases your brain’s natural growth factors, which support the survival and growth of neurons,” neurosurgeon Rahul Jandial, MD, Ph.D., writes.

Increased cell regeneration

MIT biologists discovered that the stem cell’s ability to regenerate was “dramatically improved” following a 24-hour fast. Not only that, but fasting helps “flip a switch” that encourages self-renewal in stem cells responsible for immune function and blood generation, according to a study by the University of Southern California.

To enjoy the benefits of intermittent fasting, the practice has to be fully understood and applied. It’s not as straightforward as skipping meals, as it’s essential for the body to be nourished in a healthy way. Fasting requires a mixture of discipline and the application of knowledge to make sure the effects are desirable.

Intermittent fasting isn’t for the lighthearted, either, and it’s a decision to make after plenty of consideration. The process itself is challenging, despite its benefits, and requires self-control. “When people do intermittent fasting, they are kind of stressing their body,” notes PhD researcher Laura Corrales-Diaz Pomatto, “but it can actually be protective.” Just like a solid fitness routine, being smart about fasting is key.

When intermittent fasting isn’t recommended

Like all diets, intermittent fasting isn’t ideal for everyone, particularly those who need to eat regularly or have other underlying health issues. 

Knowing whether fasting is right for you is part of the decision-making process. Harvard School of Public Health note different instances where people should avoid intermittent fasting, which include:

• People with diabetes: some studies demonstrate that intermittent fasting may be beneficial for those with type-2 diabetes, by improving glycemic control. However, there is a risk of enhanced hypoglycemia and hyperglycemia (raising or lowering blood sugar beyond recommended limits) due to fluctuations in blood sugar levels when fasting.
• People with eating disorders: “Intermittent fasting has a high association with bulimia nervosa, and as a result, individuals who are susceptible to an eating disorder should not undergo any diet associated with fasting,” write eating disorder specialists, the Center for Discovery.
• Those on certain forms of medication: anyone who takes medication that involves eating beforehand probably won’t suit the spells of fasting required.
• Younger people: those who are still actively growing aren’t recommended to restrict calories or limiting their eating window.
• Pregnant or breastfeeding women: with a little one on the way, women who are pregnant need all the calories they can get — in fact, putting on weight can be healthier than losing it. “Low blood sugar in combination with the natural drop in blood pressure in pregnant women could lead to lightheadedness and fainting,” Dr. Jennifer Wu, told Women’s Health.

Ultimately, intermittent fasting is a personal choice. It won’t work for everyone, and its application varies. But, if you’ve looked at the benefits and feel ready to give it a go, what are the steps to follow?

How do I do intermittent fasting?

Before exploring the technical breakdown of intermittent fasting, it helps to begin by considering your goals. Are you looking to lose weight? Give your digestion time to rest? Improve your self-discipline? Encourage healthier eating habits by removing processed foods? Or even spend some time fasting as part of a sacred routine?

Having a clear goal for why you’re choosing to begin an intermittent fasting plan is a valuable starting point. Not only will it provide you with clarity, but it’ll also give you more chance of sticking to your healthy diet plan, and making intermittent fasting a new habit.

Personally, I’ve found the best way to integrate intermittent fasting is to make the process as easy as possible. Years ago, when I was really into weight training and trying to get as muscular as possible, I’d eat huge amounts of calories, with big dinners, snacks before bed, and a big breakfast when I woke up (even when I woke up at 5 am!). I never gave my digestive system time to rest.

Slowly, as my habits changed, I started to delay eating breakfast. I found I had more energy and felt less sluggish than on my high-calorie diet, and my digestion improved, too. All I had to do was postpone breakfast by a few hours (now, I eat my first meal around 10:30 or 11 am). I typically fast around 14 hours during the week, which is less than some plans, but it works for me. On weekends sometimes this stretches on for longer.

Trust yourself when contemplating how you can integrate intermittent fasting. I encourage you to consider: Is an intermittent fasting program something you wish to fully integrate into your routine, each and every day? Or something you’d like to try once in a while as more of a conventional “cleanse,” and less of a continuous calorie restriction?

4 popular ways to do intermittent fasting

As we’ve mentioned, intermittent fasting is broken down into three main categories: alternate-day fasting, periodic fasting, and time-restricted feeding. Unsurprisingly, due to the popularity of intermittent fasting, there are streams and streams of tailored plans that you can choose from.

Ultimately, the best way to do intermittent fasting is one that you will stick to, one that suits your lifestyle and caloric needs, and one best suited for your overall health. Keep the goals you’ve explored in mind, whether weight loss or improved brain function or simply giving your body a break from digesting food (five to 15 percent of your body’s energy expenditure goes to digestion alone!).

Consider your current eating habits, too. Too extreme a shift will likely make intermittent fasting harder to stick to. Do you generally skip breakfast? Or is food the first thing on your mind when you wake up in the morning? Do you eat out for dinner most nights or prefer brunch? Getting clear on your current habits gives you a great platform to put together an intermittent fasting plan that will work for you, long-term.

For extra guidance, below are the most common intermittent fasting plans, ones that can be used for inspiration.

The 16/8 method

One of the most common forms of intermittent fasting is time-restricted feeding, including a 16 hour-fasting period (14 for women) and eight hour eating period. 

This method, also referred to as “leangains”, was popularised by fitness guru and self-proclaimed “god of intermittent fasting” Martin Berkhan. It’s one of the recommended forms for those into weight training and packing on muscle mass.

Due to its flexibility, the 16/8 method is one of the most achievable forms of fasting. There’s nothing particularly enlightening about it, as most people fast through the night and delay eating breakfast. That’s it! For example, if you eat your last meal at 7 pm, you’ll essentially skip breakfast, and eat again at 11 am.

The 5:2 diet

Another well-known plan is a form of periodic fasting involving two days a week of restricted calories. This was one of the catalysts for the rise of intermittent fasting in the mainstream, following the release of doctor and journalist Michael Mosley’s book, The Fast Diet, in 2013.

When fasting, it’s recommended to cut your regular calorie intake to a quarter of your usual amount — for women that’s 500 calories, and for men 600 calories. Though this also varies depending on BMI and activity level throughout the week. When not fasting, you consume your normal level of calories, and there’s flexibility with what you eat.

Eat Stop Eat

One of the more intense forms of intermittent fasting involves one or two days per week with zero-eating across a 24 hour period. 

The alternate-day fasting plan was designed by weight-loss guru Brad Pilon, who wrote a book titled Eat Stop Eat. “Rather than just looking at it as fasting, which can have a negative connotation, it’s more about breaking the cycle of thinking you can’t take the occasional break from eating,” Pilon told NCBI.

On eating days, there are no restrictions in terms of calories or what foods you eat. More than other plans, this requires a lot of self-control — not only to avoid eating foods when going longer periods without but to avoid the temptation to binge once the fast is over.

The Warrior Diet

Fitness expert Ori Hofmekler created the Warrior Diet, which consists of restricted calories throughout the day (consuming raw fruit and vegetables) followed by a feast of food in the evening. It’s based on the eating patterns of ancient Roman and Spartan warriors, hence its name. It follows an eating period of around 20:4, with just four hours of eating, within which you eat one huge meal.

Though popular online, this diet has little scientific backing around its benefits ahead of other styles of intermittent fasting. “The average office worker isn’t a warrior,” director of the Naked Nutritionist Daniel O’Shaughnessy told Men’s Health. “An ancient warrior wouldn’t have the opportunity to gorge on processed food like we do. This diet seems likely to promote an unhealthy relationship with food.” Maybe one to avoid, then.

Meal skipping

The typically Western lifestyle consists of low levels of physical activity, and regular eating. A basic and easily achieved form of intermittent fasting is to skip the occasional meal, rather than eating based solely on times of the day. The key here isn’t that you skip meals unconsciously, due to being too busy or not wanting to make the effort to cook. Instead, it’s a conscious process where you tune into your body’s hunger signals, and go without food when it’s not fully necessary.

With the above layouts in mind, consider what is right for you. It’s worth noting that a large part of this depends on your overall attitude and approach to food. Leagains, for example, was developed with athletes and fitness enthusiasts in mind. If you’re someone who already understands nutrition to some degree, and knows how to take care of your caloric and nutritional needs, you’ll likely find the transition easier.

A note on your overall diet

That leads us to a very important caveat to all of the above. Before starting an intermittent fasting plan, it’s essential to build nutritional knowledge. I personally wouldn’t recommend jumping into intermittent fasting if your current eating habits are poor. Fasting alone won’t do much if you’re used to eating junk food, takeaways, and little in the way of fruit and vegetables.

Rather than look for quick fixes, it’s best to focus on your overall lifestyle. Be honest with yourself in terms of your knowledge of nutrition. Are there things you need to learn before embarking on a fasting journey? All of the above plans emphasize eating in moderation, with high-protein consumption and a diet that is mostly whole, unprocessed foods.

Some basic pointers are to understand how to balance carbohydrates, fats, and proteins. Understanding your basic caloric needs, and what foods to eat for a balanced diet. Along with what foods to avoid, such as processed sugars, dairy, or wheat. Again, this is a subjective experience, and all of this is part of an exploration into your body, one which is best achieved through compassion and curiosity.

Understanding eating patterns and habits

As well as objective knowledge about what foods are good for you, having self-awareness around your personal eating patterns and habits is crucial. This gives you the best insight into how your natural rhythms work, when you’re more likely to snack, when you lose your appetite. For example, some people snack when they feel anxious, others crave carbohydrates when sad, others forget to eat when stressed.

Intermittent fasting is an opportunity to better understand these habits and patterns. And, because it involves spells without eating, it’s worthwhile understanding the difference between hunger, appetite, and cravings. 

Hunger is physiological. It’s your body's way of sending you a signal that you need to eat for energy. Appetite is the desire to eat, that is largely influenced by emotional state. And cravings are the desire to eat a specific food, whether hungry or not. 

Ready to start your intermittent fasting plan?

With certain health practices, there is a tendency to place attention on one area, such as fasting, and feel like that’s job done. It goes without saying that intermittent fasting alone isn’t a magical formula for improved body and mind. 

Your overall diet, and the quality of food you consume, is just as important as how you time your meals. Intermittent fasting is another tool to add to your lifestyle routine.

It’s not for everyone, either. If you have underlying health issues, it’s worth speaking to your doctor before starting any significant change in diet. If you’ve been eating a certain way over a long period of time, there will naturally be a spell of adjustment. So consider slowly increasing your fasting period until you feel comfortable going for longer. To recap:

• Consider your goals: why are you looking to incorporate intermittent fasting into your lifestyle? Get clear about this before you begin, as it’ll help with both intrinsic motivation and guide you on what intermittent fasting plan to follow.

• Be sure fasting is right for you: don’t jump in because it’s popular, but take time to consider if it’s a healthy option for you. Is your general relationship with food healthy? Are you free from conditions that make fasting unadvisable?

• Improve your nutritional knowledge: what’s your starting point? Do you eat without much conscious thought, or are you a fitness enthusiast who understands terms such as macros, micros, and protein synthesis? Before starting intermittent fasting, make sure you understand enough about healthy diets in general, so your foundation is solid.

• Explore different intermittent fasting plans: with the examples above and your goals and current lifestyle in mind, consider what type of plan works best for your specific needs.

• Start slow: any significant change in diet requires an adjustment stage. If you currently eat at regular intervals, including an early breakfast, late dinner, and snacks, it makes sense to slowly increase your fasting period to give your body time to adjust.

Stick to it, but be flexible: once you’ve decided on a plan, experiment, and see how you go. Remain committed for long enough to adjust, but don’t overcommit if it’s not working for you. Be flexible. If one day doesn’t work, don’t panic. You’ve got what you need to try again tomorrow!