What is intermittent fasting, and where did it come from?

Intermittent fasting is an eating routine that involves switching between periods of fasting and periods of eating on a regular schedule. The timing and length of these fasting and eating windows can vary depending on your specific plan. Still, all intermittent fasting plans focus on when you eat rather than what you eat. 

Yep, that’s right—no need for exhausting calorie counting! Or being reminded of why you hated math class … 

Extensive research has linked intermittent fasting to weight loss, reduced body fat, lower systolic blood pressure, and reduced inflammation, but everybody’s different. So how do you know if and what type of intermittent fasting will be a good fit for you? 

You might be asking yourself, “What’s intermittent fasting all about? Is this type of eating routine right for me?” Keep reading our expert guide to learn about intermittent fasting’s origins, benefits, and risks. You’ll be a pro in no time. Already on board with the basics? Skip straight to our guide on intermittent fasting for beginners to decide which type of fasting plan is right for you.

Key takeaways

  • Intermittent fasting involves a repeated pattern of eating and fasting that focuses on when you eat rather than what you eat. 
  • There are several different approaches you can take, and you can tailor the approach to your lifestyle. 
  • Intermittent fasting may help you lose weight and improve your health, but it’s not for everyone, so consult your doctor or dietitian before trying it.

What is intermittent fasting? 

As far as eating habits go, intermittent fasting is super simple: it’s eating, then fasting, on repeat.  

It’s like HIIT for your eating routine: you eat during a certain period of time, pause for a while, then do it again. 

There are several different types of intermittent fasting plans, and your specific eat-fast-repeat routine can vary. Some plans are daily, while others are based on alternate days or weekly schedules. Some also involve different fasting and eating windows. 

Whatever type of intermittent fasting you do, the fundamental component of each plan is the same: for a defined period, you take a complete break from eating. 

Although intermittent fasting can have some of the same effects as calorie restriction, it’s not a diet in the traditional restrictive sense of the word. It’s a way of eating that dictates when you eat rather than what you eat. So while some types of intermittent fasting can involve additional elements of calorie restriction or recommendations around what foods to eat and what foods to limit, at its core, intermittent fasting is all about the timing of your eating routine.

Intrigued? Since everybody is unique, intermittent fasting may have different impacts on different people, and it may not be right for everyone. 

To see if intermittent fasting might help you achieve your health goals or reach/maintain your happy weight, take our SIMPLE quiz. We also always recommend consulting your doctor or dietitian before making any significant changes to how you eat.

How does intermittent fasting work? 

An eating routine that doesn’t dictate what you eat but still might benefit your health overall—sounds great, right? But what does intermittent fasting do to your body? And how does it all work? 

First, let’s dig into how your body reacts to food. 

When you finish a meal, your body gets to work on digesting and absorbing all the food you just ate. It calls on insulin to help—because insulin is a hormone that shuttles nutrients where they need to go, like into your muscle and fat cells—and over the next few hours, your body turns your meal into useful body stuff, like energy, fat, muscle, and brain power. 

At this stage, it’s not burning fat. 

Next, your body enters what’s called the post-absorptive state—your internal version of flopping on the couch in front of the TV after a big feast. Your internal couch potato is content in this state for the next 8–12 hours. Afterward, insulin levels drop, and you enter a fasted state. 

Now your body is primed to burn fat to use as its fuel.

On a typical day, you probably don’t go twelve hours without eating, so you likely wouldn’t naturally land in this fasted state.

That’s where a fasting diet comes in: an intermittent fasting plan triggers this natural fat-burning phase in a simple, straightforward way. Keeping an eye on the clock is a lot easier than trying to keep track of macros!

How does intermittent fasting affect the body? 

An intermittent fasting diet uses timed eating breaks to get your body into a state that’s primed for fat burning. Having already used up its sugar stores, your body effectively flips a “metabolic switch” to start burning fat instead. 

Based on current research, you can think of intermittent fasting + an active lifestyle as the trigger. It’s an area that definitely needs further study, though, so take this with a grain of salt.

By flipping this switch, intermittent fasting can help your body dig into its fat stores and subsequently, potentially lose weight and improve your body’s overall functioning.

The origin and history of intermittent fasting 

So, how did intermittent fasting become a thing?

If you’re someone who likes to do a deep dive into an internet rabbit hole to get the full backstory, don’t worry: we’ve got you covered.

While scientists have studied the health effects of restricted diets on animals since the 1930s—and more recently, the impact of intermittent fasting on both life span and health span—intermittent fasting really took off for people when it took the fitness and lifestyle world by storm in the 2000s and 2010s. Books like Brad Pilon’s Eat Stop Eat and Gin Stephen’s Delay, Don’t Deny became bestsellers, and intermittent fasting transitioned from something only athletes or bodybuilders discussed to a common household concept. 

Even Google clocked its popularity: by 2019, intermittent fasting was deemed the most-searched diet. 

However, the practice of fasting has been around for millennia, and not just during periods of food scarcity. Using fasting therapeutically can actually be traced to some of the earliest known societies: during the fifth century BCE, famous Ancient Greek physician Hippocrates recommended fasting as a possible way to heal certain illnesses.

Intermittent fasting and religion 

You’ve also probably heard about intermittent fasting in the context of religion.

Maybe you or someone you know participates in Ramadan, a month of the Islamic calendar defined by fasting and reflection, or the 25-hour fast of Yom Kippur. Or maybe you went to Burning Man once and heard something about a vision quest that would bring you closer to nature.

From widely practiced religions like Judaism, Islam, Christianity, Hinduism, and Buddhism to forms of worship like vision quests, seasonal ceremonies, and meditation, fasting has been seen by some as a way to strengthen faith and reflection, atone or repent for their past, or feel more connected to a higher power. 

So, people will abstain from food for set periods—whether within a specific day or for alternating days. For example, during Ramadan, participants won’t eat or drink during daylight hours.

If you’re motivated to fast by religion or spirituality, remember: fasting still affects the physical body. One review of research that looked into the impact of religious fasting on health found some positive effects—like reduced cholesterol, blood pressure, and insulin sensitivity—though many individual circumstances could influence results. 

And for the environmentally motivated out there, fasting may actually have a positive impact on the planet, too. While the specific foods that can be consumed during eating windows vary by practice, many of them involve a version of vegetarianism, which is more sustainable.

How common is intermittent fasting? 

Intermittent fasting definitely isn’t right for everyone, but a Real Research Media 2022 survey of 50,000 people worldwide found that 80% of participants had heard about intermittent fasting before, and 75% had tried it. 

What does science say about intermittent fasting? 

This all sounds great in theory, but how do you know if intermittent fasting is legit?

Although there is still a lot more work to do on studying the longer-term effects of intermittent fasting on health, existing research largely supports the idea that intermittent fasting could improve metabolic health and promote weight loss, at least in the short term (up to 12 months).

We’ll dig into these potential health benefits—and potential risks—shortly, but for now, it’s worth keeping in mind that fasting may not be effective for everyone, and some smaller-scale research suggests that time-restricted eating may not create these benefits without other lifestyle changes.

However, even though some conflicting results have left the jury debating whether science fully backs intermittent fasting for overall health, there is a lot of compelling evidence to suggest it does.

The proof is in the (consumed-during-a-timed-eating-window) pudding.    

Intermittent fasting diet plans 

Not sure how to start intermittent fasting or what possible approaches there are? Our experts have outlined the most common methods and how they work to give you an idea of which one(s) might fit your lifestyle best. 

  1. Time-restricted eating 16/8

On a 16/8 fasting schedule, you alternate between 16 hours of fasting and an eight-hour eating window. There’s no calorie counting needed, and you can structure your eating window any way you want. If you adore breakfast, you can start your fast earlier in the evening, and if you can’t imagine life without dinner at the end of the day, you can skip breakfast. What you eat and when you eat is up to you, as long as you stick to the eating window timing. But for best results, we don’t recommend feasting on pizza and ice cream, FYI. Not sure what to eat? Check out our blog here.

If you’re skeptical or concerned about how to do intermittent fasting, this plan might be a good place to start, as it simply extends the natural daily fast that happens when you’re sleeping. While research most often studies the 16/8 time-restricted eating schedule, our users have also had weight loss success by following 12/12 or 14/10 plans. 

  1. Warrior diet

The Warrior Diet involves a 20-hour fast that begins overnight and continues into the next day, followed by a four-hour eating window.

You may have accidentally found yourself having a Warrior Diet day if you were running around nonstop and barely had time to grab a cup of coffee, much less a meal. However, sticking to this plan intentionally and safely requires effort and careful planning to ensure you get all the nutrients you need in that four-hour window. It’s not an ideal starting point for beginners, but if you’re up for a more rigorous fasting-based regimen, this might be a good fit.

  1. 5:2 diet

To follow the 5:2 diet, you eat as you normally would five days a week and then restrict your intake to 500 or 600 calories on the other two days. 

Some people find it easier to restrict calories for just two days of the week rather than trying to stick to a daily fasting schedule. However, others may find that this very limited calorie intake—especially when they’re mostly used to following their normal eating routine—can cause too much hunger, fatigue, and stress and be difficult to plan around daily schedules.

  1. Eat stop eat

Following the Eat Stop Eat plan means eating as you normally would one day, then fasting for 24 hours until the following day. With this fasting plan, you get at least one full meal daily, which you choose. For example, you could fast from dinner to dinner or breakfast to breakfast. 

A 24-hour fast is a hefty undertaking, and it can be pretty tricky to meet your nutritional needs. Our experts tend not to recommend it, especially without medical supervision, since, over time, it can add up to some serious nutrient deficiencies. 

  1. Alternate day fasting

Similar to 5:2 but slightly more intense, Alternate Day Fasting involves alternating days of eating normally and fasting (eating 500 or 600 calories). That means each week has three or four fasting days. 

Since this plan involves quite a bit of fasting—which means not only extra careful planning around your nutrient intake but also more challenges around managing eating routines in your daily life—it’s not an ideal plan for beginners. But if you’re someone who does better with on/off approaches, this plan could be a good match. 

  1. Water fasting

Water fasting involves going 24–72 hours drinking only water and going completely without food. 

We don’t recommend this intermittent fasting plan unless you’re going to have medical supervision, so let’s strike this fasting routine from our list of options. 

What can I eat & drink while intermittent fasting? 

While there’s no one-size-fits-all “how to intermittent fast” plan, one of the most appealing aspects of intermittent fasting is that you can still eat and enjoy a wide variety of foods during your eating windows. 

Food is never inherently “good” or “bad,” either. Some foods are just more nutrient-dense and more health-promoting than others, and no matter what eating routine you follow, you can and should still enjoy less nutrient-dense foods without feeling guilty. 

After all, whether you eat to live or live to eat, food is one of our biggest supports.

What you eat and drink during your intermittent fasting protocol can vary, but use your health goals, nutritional needs, preferences, budget, and schedule to help plan what works for you. 

For a general rule of thumb, here are some tips on what to eat and drink and what to avoid to maximize the potential health benefits of intermittent fasting.

 What to eat during intermittent fasting  

  • Whole grains (like oats, brown rice, quinoa, and whole wheat pasta)
  • Legumes (like lentils, peas, chickpeas, and beans)
  • Fruits and veggies
  • Lean protein (like fish, tofu, and poultry)
  • Healthy fats (like avocados, eggs, nuts, and seeds)
  • Dairy products or calcium-fortified plant-based alternatives (like yogurt, cheese, and fortified soy milk)

 What you can drink while fasting

  • Water and mineral water (if you’re not a big water fan, you can add a slice of fruit such as lemon, orange, or cucumber, or try some mint)
  • Tea (without milk, sugar, cream, or sweeteners)
  • Coffee (without milk, sugar, cream, syrups, or sweeteners) 
  • Apple cider vinegar (a few tablespoons diluted in water)

What foods to limit during intermittent fasting 

  • Ultra-processed foods (like cookies, chips, and fast food)
  • Foods high in refined sugar (like candy, cakes, and desserts)
  • Foods high in sodium (like cured meats, pizza, and fried food)
  • Foods high in saturated and trans fats (keep an eye out for hydrogenated oil)

What drinks to limit during intermittent fasting

  • Sugar-sweetened soda (keep the diet stuff to two cans per day, as we still aren’t sure what impact consuming lots of diet drinks has on health yet!)
  • Fruit juice
  • Alcohol

Health risks and benefits of intermittent fasting 

Your body is your body. How do you know how intermittent fasting will affect you specifically?

There are so many different variables involved in a person’s health. This means that some intermittent fasting benefits we see in studies may not always translate to everybody looking to take up this way of eating. 

The short story: it’s complicated.

But the good news is that we’ve rounded up the science, so you don’t have to. The findings may not be a guarantee for your body, but they can help you understand what to expect.

Lower inflammation 

Inflammation contributes to many chronic and acute illnesses, like high blood pressure and heart disease. Some studies have linked intermittent fasting to reducing inflammation and inflammatory diseases. These findings may be more likely for those living with overweight and obesity.

Lower risk of type 2 diabetes

Intermittent fasting can improve insulin sensitivity, reduce insulin resistance, and decrease both cholesterol and blood sugar for individuals living with overweight and obesity—all of which are factors that can lower your risk of developing type 2 diabetes and generally improve your metabolic health. 

A healthier heart

By lowering inflammation, cholesterol, blood sugar, and insulin resistance, intermittent fasting may also support our cardiovascular health and increase our heart’s resistance to possible pathogens

A healthier brain

Some studies suggest that intermittent fasting can promote neuroplasticity; stronger, more durable brain cells; and healthy brain aging

Hormonal changes

Some research suggests that assigned-at-birth men and women may respond differently to intermittent fasting because it may decrease certain reproductive hormones while not affecting others. A reduction in androgens (such as testosterone) could be beneficial for women with polycystic ovarian syndrome but detrimental to men. However, existing research is very limited and fails to explore the effects of fasting specifically on trans men and women, nonbinary people, and individuals currently on hormone replacement therapy. 

Mood changes 

While some studies suggest intermittent fasting may reduce symptoms of anxiety and depression, others also suggest it could increase negative emotions and decrease positive emotions. When you initially start fasting, you may also be more likely to feel hungrier, crankier, or grouchier. But don’t worry! You won’t be hangry forever. After a few weeks, these feelings will pass, and you’ll be enjoying all those intermittent fasting health benefits. 

Decreased concentration

Your brain needs certain nutrients from food to function and perform cognitive processes. Following a fasting-based food routine can lead to depriving your brain of these nutrients if you aren’t careful in planning your meals. 

Higher risk of disordered eating 

Intermittent fasting helps bring more conscious awareness to your eating habits and what you eat. While this mindful focus can be a helpful motivator for people looking to introduce more nutrient-dense foods into their routines, it can also lead to disordered relationships with food or exacerbate disordered eating tendencies. This is why we always emphasize that fasting is not suitable for anyone with an eating disorder history.

Intermittent fasting and weight loss 

Not all intermittent fasting journeys are motivated by the same destination, but many of our users at Simple consider trying intermittent fasting as a weight loss plan.

There’s a reason “intermittent fasting” and “weight loss” are often talked about in the same breath—quite a bit of research connects intermittent fasting to various physiological benefits that may individually or collectively lead to weight loss. This approach isn’t just potentially beneficial in the short-term, either; some research also suggests intermittent fasting could likewise be used to help maintain your happy weight

There are several ways intermittent fasting can support weight loss in addition to priming your body for fat burning. It can: 

Since cutting calories typically slows metabolism—so our body can preserve its energy stores for as long as possible—we often get asked questions like, “Does intermittent fasting slow metabolism, too?” While intermittent fasting can lead to a natural reduction in calorie intake, it may actually speed up your metabolism—and therefore support weight loss—by increasing the production of metabolism-boosting hormones like norepinephrine and epinephrine

One caveat to keep in mind about intermittent fasting and weight loss: some research suggests that, while intermittent fasting may be beneficial for weight loss, it may not necessarily be better than calorie-restriction diets. But if you’ve been there, done that, and need something new, it’s still worth a try. 

Why should you intermittent fast? 

Why you intermittent fast is up to you.

Maybe you want to lose weight, improve your heart health, or lower your risk of certain metabolic disorders. 

Maybe you want to practice more mindful eating and improve your concentration and brain health.

Your reasons are your reasons, but we get it: it helps to know what other people have gotten out of it. 

So we asked our SIMPLE users and community for some insight:  

Melina: “Started intermittent fasting six months ago, and [I’m] down 23 [pounds]!” 

wiselittlerock: “I am more focused and energized.”

mertola: “I always thought I was hopeless when it came to really losing weight, but I was surprised how much I lost in just a few days, then weeks, then a month … it’s not that much, but I [am] proud of it.”

Sebastiano: “8 month[s on] 16/8. Lost 10 kg. Blood exams are perfect. Still losing weight, slowly but all over the body. I didn’t lose muscle.”

How to begin intermittent fasting 

Okay, you’re convinced. Intermittent fasting has legs. 

But now you’re not sure how to start intermittent fasting—or you’re wondering how do you do intermittent fasting within the context of your lifestyle. 

The first question to ask yourself is if intermittent fasting is right for you. 

We recommend speaking with your doctor before starting any fasting method since it can be dangerous for some people. You should be particularly cautious if you: 

  • Are pregnant, breastfeeding, or trying to conceive; 
  • Have a history of disordered eating;
  • Have diabetes or another chronic medical condition;
  • Have poor or irregular digestion;  
  • Are under 18 or over 65 years old; or
  • Are prescribed medication (for example, anti-hypertensives, diabetes medications, any that need to be taken with food)

If you don’t identify with any of these categories, your next step is to figure out which plan is your best fit. Our SIMPLE quiz is here to help you figure out the best plan for you!

Think about your goals, nutritional needs, preferences, schedule, budget, and any other commitments, situations, or people you need to factor into your eating plan. You’re in the driver’s seat here. Be realistic rather than overly ambitious—you can always adjust as you get more comfortable with the process, so start with something that feels simple and doable.

When you’re ready to get started, think about how to meal plan according to the approach you’ve chosen and all the other factors you’ve considered. 

Finally, don’t forget to build in regular self check-ins to keep an eye on how things are going. Particularly if you’re fasting for the first time, you want to ensure your body and mind are coping with any changes and that you’re giving yourself extra doses of love, appreciation, and compassion. Celebrate little wins, toss any guilt in the trash, and if what you’re doing isn’t feeling right, reassess without judgment. No matter what, you’ve got this!

Simple’s expert opinion and final thoughts 

The team here at Simple all agree: the best eating routine is the one that is right for your unique body—and one that you can stick to.

All intermittent fasting approaches can benefit your body and mind, but everyone responds to fasting differently, and fasting may not be a good fit for you. Especially if you’ve never fasted before, we recommend trying out a limited time-restricted approach before committing to a longer, more intense one. 

To learn more about which intermittent fasting protocol might work best for you and get tips on when and how to get started, stay motivated, and achieve your goals, head to our SIMPLE quiz today.

Frequently asked questions about intermittent fasting 

What are the intermittent fasting rules?

Like any eating routine, intermittent fasting should be flexible according to your individual needs. There aren’t any set-in-stone rules to intermittent fasting, but to set yourself up for success, pick a fasting schedule that works for you, enlist some support, stay hydrated, and focus on when you eat rather than what you eat—as long as you keep your nutritional needs in mind. And remember: hunger doesn’t last forever! 

How long should you do intermittent fasting? 

How long you do intermittent fasting depends on your goals and how your body responds to the eating patterns involved. We always recommend starting small with as few changes as possible and trying out different methods to see what feels best to you. If at any point you’re experiencing consistent or severe side effects, stop and consult a medical professional immediately.  

What happens after one week of intermittent fasting?

Everybody is unique, so what happens after one week of intermittent fasting can vary significantly. What fasting approach you take, what foods you eat, and how much exercise or physical activity you do can also impact how your body responds to different eating patterns. The first week or two is always the trickiest, so plan for some choppy waters, and rest assured, the process will be much smoother sailing after that. 

How much weight do you lose in a month with intermittent fasting? 

The amount of intermittent fasting weight loss you have at the end of a month depends on your starting weight and medical conditions, the food you eat during your eating windows, and other factors like your lifestyle, age, and activity level. While some people experience more rapid weight loss, the healthiest—and most enduring—weight loss is gradual. Losing one or two pounds a week is safe and effective. 

Why am I gaining weight while intermittent fasting? 

It may sound counterintuitive, but you may not be eating enough during your eating window if you’re gaining weight while intermittent fasting. If you’re suddenly and severely restricting calories, your metabolism may slow down and actually make it harder for you to burn fat.

On the other hand, you also may be consuming significant amounts of ultra-processed foods. Intermittent fasting isn’t about calorie counting, but you want to ensure you’re getting enough minimally-processed, nutrient-dense foods. Protein may help to preserve muscle and support your metabolic rate. Drinking plenty of water will also help you avoid mistaking thirst for hunger. It’s also good to remember that if you’re working out hard and building muscle while losing fat, you may be gaining weight according to a scale since fat loss and weight loss aren’t the same thing.