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What Is Intermittent Fasting? Basics for Beginners

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intermittent fasting for weight loss
Jonathan Valdez post Reviewer Jonathan Valdez post Reviewer
Verified by Jonathan Valdez
Registered Dietitian Nutritionist, Certified Personal Trainer, Certified Dietitian/Nutritionist in NYC, Certified Diabetes Care and Education Specialist

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With the increasing popularity of intermittent fasting as a dietary pattern for weight loss or maintenance, you may wonder if and how you should safely and healthily incorporate it into your lifestyle. 

Intermittent fasting comes with both potential benefits and drawbacks to your physical and emotional health. Therefore, carefully consider the factors covered in this article before choosing whether or not to begin intermittent fasting.

Read on to dive into the truth and the science behind intermittent fasting and see if it is right for you and your health. Before taking action, consult a registered dietitian nutritionist to see if this is appropriate for you.

What is intermittent fasting?

Intermittent fasting (IF) is a time-based eating pattern in which you cycle between periods of eating and fasting. 

Whether you fast for a set number of hours each day or week, intermittent fasting is a popular method for weight loss or weight maintenance. Rather than restricting based on calories or macronutrients, intermittent fasting restricts eating for a period of time only. 

what is intermittent fasting

Many turn to intermittent fasting to manage their weight.

Overall, it can be effective for weight loss and at decreasing your health risk from obesity-related diseases, such as diabetes or certain types of cancer. However, this effectiveness is not any higher than a regular healthy, balanced diet that reduces calories. 

What to drink and eat while intermittent fasting

There are no particular food restrictions during the eating phase of IF. However, it’s better to keep your nutrients under control to avoid health issues and hunger during the fasting window.

During the eating window, you can resume eating a balanced and healthy diet rich in nutrient-dense foods, and, therefore, in vitamins, minerals, and beneficial nutrients. 

During the fasting window of intermittent fasting, you should abstain from food. However, you can (and should!) drink plenty of water to stay hydrated.

Additionally, you can consume any beverage that is free of calories, such as black coffee or unsweetened tea. Any food breaks fast, so even shacking is not allowed during IF.

To learn more about what you can eat or drink while intermittent fasting, check out our recent article on the subject.

What is the science behind intermittent fasting?

Have you been wondering how intermittent fasting really works inside the body? Let’s cover some metabolism basics.

Typically, when you consume food, your body uses carbohydrates or sugars as the primary fuel source. When we have excess carbohydrates paired with an excessive caloric intake (that our bodies can’t use for energy anymore), they are stored in the body as fat, contributing to weight gain.

Intermittent fasting and ketosis

Intermittent fasting can influence this process and help the human body use fat cells instead of carbohydrates as an energy source. After only 12 hours of fasting, which is quite a mild regimen, the human body may go into ketosis. 

Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood, and ketone bodies generally become the preferred source of energy for the brain and body during the fasting state. Increased ketones in blood actually signal that your body uses fat for energy.

After hours of not consuming any food, your blood glucose and insulin levels decrease. Then the body will deplete glycogen stores from organs and muscles to maintain blood glucose levels while slowly metabolizing fat for energy. Your body will not have any sugars left for energy. 

Therefore, after 10 to 36 hours of fasting, depending on your liver’s sugar stores and energy expenditure, your body will use all of its stored sugar for energy and begin burning fat, which is a process known as metabolic switching.

According to research, IF has a strong potential to improve obesity or overweight due to metabolic switching.

Main types of intermittent fasting

While there are many different types and patterns of intermittent fasting, a few of them are quite popular.

Main types of intermittent fasting visualisation

The time-restricted methods (16/8, 14/10, and others)

Time-restricted methods go by hours of the day. For the 18/6 intermittent fasting method, you would fast for 18 hours and then have an eating period of six hours. The 14/10 method follows the same principle—you would fast for 14 hours and eat during a 10-hour period.

This is a popular method of intermittent fasting, especially for beginners, as you typically fast when you are sleeping anyway. You would simply extend the fast from overnight by skipping breakfast and not eating until lunchtime. 

Even a simple 8-hour fasting window reduces the concentration of insulin and glucose that are often associated with chronic disease. 

A more preferential eating pattern may be eating a small meal no later than 6 PM. Then you fast until 8 AM (14 hours) or as late as noon, depending on your time-restricted eating pattern window.

Many modify this method of intermittent fasting by only completing it once or twice a week. 

The twice-a-week or 5:2 method

The twice-a-week or the 5:2 method involves including two days of consuming very small numbers of calories per week and “sensible eating” for the other five non-fasting days. 

On those two fasting days, you consume only about 500 to 600 calories in total throughout the entire day.

The two days of fasting can be consecutive or non-consecutive. 

Alternate-day fasting

It is a “modified” method of IF that often includes calorie restriction every other day. 

For example, one option is to only consume 500 calories every other day or about 25% of your energy requirement needs. Those calories can be consumed at any time throughout the day.

On non-fasting days, you can eat as usual and not restrict your calorie intake.

The 24-hour method or eat-stop-eat

For this method, you consume no food for 24 hours twice a week.

Too long without food slows your metabolism and may lead to binging. Ultimately, that can lead to weight gain. 

Going 24 hours or more without food can be dangerous, especially if you have a medical condition, so in that case, it is best to avoid this intermittent fasting method. 

The warrior diet

The warrior diet is a lesser-known form of intermittent fasting.

It involves fasting for 20 hours of the day and consuming the entire day’s food in just four hours.

This method of intermittent fasting is quite extreme and can cause intense discomfort. Therefore, this method of intermittent fasting is not often recommended.

Do consult a physician before beginning this method of intermittent fasting if you choose to try it.

Benefits of intermittent fasting

Let’s dive into some of the core benefits of intermittent fasting, such as weight loss, heart health, and more. 

Promotes healthy weight loss

Intermittent fasting can help you reach the state of ketosis within 12 hours, which is when the body is deficient in glucose used for energy and therefore breaks down fat to use instead. 

In addition, you typically consume fewer calories while you are intermittent fasting. 

Since weight loss generally relies on the principle of consuming fewer calories, that can consequently lead to successful weight loss as well.

Fortunately, unlike other popular diets such as the ketogenic diet, weight loss is very slow (often only a half of a pound or one pound of weight loss each week) but will not contribute to muscle loss. 

Improved blood sugar and heart health

Next, according to a 2017 study, intermittent fasting is effective for more than just significant weight loss. 

IF offers many different metabolic health benefits, including improved insulin resistance and lowered risk of heart disease. Although these are promising results, more research is warranted to establish intermittent fasting for general health benefits and preventing and managing different diseases.

Another study found that intermittent fasting benefits blood pressure regulation and atherosclerosis development. However, most of the diet’s successes in the research were in compliance with the circadian rhythm.

Sustainable and nutritious

Another main benefit of intermittent fasting is the potential to consume a fully nutritious, whole food diet. 

Many popular diets, such as the ketogenic or paleo diets, involve a form of dietary restriction. For example, the ketogenic diet involves severely restricting carbohydrates, and the paleo diet avoids legumes, dairy, grains, refined sugar, and more. These forms of restriction often lead to poor compliance and ineffective results, such as weight gain after stopping the diet. 

IF is more sustainable long term since you can consume your favorite foods and the foods you crave instead of restricting them, which will lead to more satisfaction and better compliance.

That means you can include all foods and food groups while intermittent fasting. However, to reap the most benefits from IF, you should consume a healthy, balanced diet rich in fruits and vegetables, lean meats, low-fat dairy, healthy fats, and whole grains.

Overall, intermittent fasting is a more sustainable method of weight loss since it is slow and steady and does not require any restriction of certain foods or entire food groups. 

Potential to benefit cancer prevention

Finally, research has shown the potential for intermittent fasting to prevent certain types of cancer in mice. This decreased risk of developing certain types of cancer is likely related to weight loss or weight management. However, it is unknown whether intermittent fasting itself plays a role in metabolic pathways and cancer.

It is essential to note that there is no sufficient evidence to support the recommendation of intermittent fasting for those with cancer or who are undergoing cancer treatment. 

Downsides of intermittent fasting

Now, it is also essential to consider the downsides and possible risks of intermittent fasting. Let’s discuss some of the key ones.

Does not follow intuitive eating

Intermittent fasting does promote some hormonal changes in the body. According to a study in the Public Library of Science journal, individuals who fasted during Ramadan experienced a decrease in the hunger and fullness hormones called leptin and ghrelin.

Therefore, a significant downside of intermittent fasting is that it goes against intuitive eating, which is the process of listening to your body’s natural hunger and fullness cues to dictate when and how much to eat. 

It may be challenging to follow your hunger and fullness cues while following intermittent fasting, even if you fast for just two days a week. This study mentioned above found that fasting decreased the presence of those hunger and fullness signal hormones. 

Increases the risk or maintenance of eating disorders or disordered eating

Additionally, it is crucial to note that intermittent fasting can increase the risk of eating disorders and disordered eating or exacerbate existing eating disorders or disordered eating symptoms. 

According to a study performed in the Journal of Abnormal Psychology, the act of fasting predicted the onset of eating disorders. 

Possible side effects

Finally, intermittent fasting can also cause some unpleasant side effects, including poor work and activity performance, persistent hunger, dehydration, irritability, low energy, and temperature sensitivity. 

Consider the potential of these side effects when deciding whether you should begin intermittent fasting. 

When you should try intermittent fasting, and when you should not

First of all, it is important to check with your physician and a registered dietitian nutritionist before beginning an intermittent fasting dietary plan. This recommendation is especially crucial if you have any chronic diseases.

Try an IF regimen if you areAvoid an IF regimen if you are
👉 Looking for a sustainable diet regimen👉 Have a history of an eating disorder or
disordered eating 
👉 Fine with losing weight slowly👉 Pregnant
👉 Want to preserve your muscle mass👉 Underage
👉 Thrive with more structure in your dietary patterns👉 Have diabetes (as IF may increase the risk
of hypoglycemia or low blood sugar)

Overall, intermittent fasting can be an effective weight-loss method, but it is not for everyone. If you try intermittent fasting and find yourself havocked by side effects and ultimately unhappy, it may not be right for you.

How to get started with intermittent fasting

Consult with your doctor

When beginning intermittent fasting, it is essential to consult with a physician or registered dietitian nutritionist first. This is especially important if you have a chronic disease such as diabetes, or if you are taking medication that is for heart health or for lowering high blood pressure.

Start with less restrictive regimens

The time-restricted eating methods, such as the 16/8 or 14/10 methods of intermittent fasting, are the best for beginners. To recap, these methods involve fasting for 16 or 14 hours and then eating a healthy diet for the remaining 8 or 10 hours of the day, respectively. 

You can also use a fasting tracker to keep up with your IF regimen and receive diet recommendations.

Start by fasting for a few days a week

Additionally, if you do not want to begin with this regimen every day of the week, you could start by fasting for a few days of the week, such as every other day, and gradually work up to every day. 

Monitor side effects

When you’re just getting started with intermittent fasting, monitor for side effects, including fatigue, nausea, headache, anxiety, and others. Speak with your healthcare provider if you experience any of these symptoms. 

Also, it is important to monitor your mood. Ask yourself some of the following questions: 

  • Is intermittent fasting impeding my happiness or overall well-being? 
  • Is it causing more stress in my life?

If you answer yes to either of these questions, you should perhaps reconsider if you would like to continue with intermittent fasting.


Let’s cover some commonly asked questions about intermittent fasting. 

How long should I follow an intermittent fasting diet?

Since intermittent fasting promotes slow but sustainable weight loss, you can follow an intermittent fasting diet as long as you feel happy with it.

Besides any individualized advice you may receive from your physician or registered dietitian nutritionist, there is no set amount of time it is recommended to follow an intermittent fasting diet. 

However, it is important to monitor for side effects while you are intermittent fasting. If you experience nausea, fatigue, headaches, or any other adverse effects of beginning intermittent fasting, speak with your physician and possibly reconsider whether you would like to continue with intermittent fasting at all.

Your well-being and quality of life are important, too. Consider whether intermittent fasting is interfering with your happiness or even your social life, such as needing to cancel meal plans with others because of your fasting schedule. If you find that you are more stressed or unhappy due to beginning intermittent fasting, it would be more difficult to sustain IF as a lifestyle change. Therefore, perhaps a regular healthy and balanced diet with a calorie deficit is a better method for you to lose or maintain your current weight.

What is the most effective intermittent fasting schedule?

The 16/8 or 14/10 methods may be most effective if you are a beginner to intermittent fasting.

However, alternate-day fasting has been found comparable in effectiveness to calorie-deficit weight-loss diets.

Can I eat whatever I want during intermittent fasting?

Yes, a benefit of intermittent fasting versus other weight-loss methods is less restriction on the foods you can eat. However, it is preferable to eat a balanced and healthy diet rich in whole grains, low-fat dairy, lean protein, healthy fats, and fruits and vegetables. 

Consuming a nutritious diet, along with increased physical activity, can promote weight loss with intermittent fasting as well as improve your overall metabolic health, such as blood pressure, blood sugar control, and inflammation.

What is dirty intermittent fasting?

While it is a term that is not used in the medical field and has not been studied, many individuals use the term “dirty intermittent fasting” to describe consuming a small amount of food during their fasting window.

This small amount is often about 100 calories, usually from coffee creamer or bone broth.

However, it is important to note that there is no standardized definition of dirty intermittent fasting. 

The idea of this modified version of fasting is that consuming such small numbers of calories, such as just 100 calories, is not enough to actually break the fast and it simply helps the individual make it through the fasting window. However, there is no research to support or deny this at this time. 

Final words

  • IF has been shown to be effective for weight loss and reducing the risk of obesity-related chronic diseases, such as diabetes and certain types of cancer. 
  • IF is not any more effective than a regular healthy, balanced diet that reduces the number of calories you consume. 
  • Studies show the benefits of intermittent fasting for insulin resistance and blood sugar control, weight management, heart health, such as high blood pressure and atherosclerosis, and more.
  • IF can have unpleasant side effects, including headaches, nausea, fatigue, and persistent hunger. These adverse effects can increase stress and dissatisfaction and should be seriously considered when deciding to begin intermittent fasting.

With all these considerations in mind, you can decide if intermittent fasting is suitable for you and your lifestyle.

As always, it is recommended that you speak with a registered dietitian nutritionist who can help you safely and healthily lose weight or maintain your weight and improve your overall health. 

Disclaimer This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!

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