Intermittent Fasting 101: The Complete Guide to Fasting

You have probably heard of intermittent fasting by now.

It is a different way of eating that has gained popularity in the United States in recent years, with some people claiming that they have had amazing results from this approach.

Proponents of intermittent fasting claim that this dietary change helps them burn fat, improve mental clarity, build muscle, and simplify their lives without following a typical calorie restriction diet plan or eating any specific types of foods.

Are you interested in intermittent fasting but aren’t sure if it’s a good option for you? Or are you curious but don’t know the best way to get started?

The following article will give you a comprehensive overview of intermittent fasting basics, the different types of fasting plans, and the pros and cons you should consider before getting started.

Think of this as the complete guide to intermittent fasting for beginners.

What Is Intermittent Fasting?

The first thing to know about intermittent fasting (IF) is that it is an umbrella term used to describe various fasting diets that cycle between periods of fasting and eating.

Unlike most traditional diets, such as weight watchers or the keto diet, IF does not specify what or how much food you should eat, but rather, it is about when you should eat.

For this reason, many people classify IF as more of an eating pattern compared to usual dietary plans, which generally count calories or macros.

In theory, you could eat unlimited junk foods during the eating time period since IF does not have a traditional calorie restriction. However, we’ll discuss why that might not be a good idea later in this article. 

There are multiple different approaches to IF, with the 16:8 fasting diet being the most popular variety of fasting protocols. 

The 16:8 plan involves eating all meals and snacks during an 8-hour window while abstaining from food for the remaining hours of the day. 

Most of the time, this plan involves skipping breakfast, having your first meal at lunch time, and not eating anything after a specific point for the rest of the day.

A typical 16:8 schedule looks like the following:

• 7:00 am – Alarm clock goes off
• 7:30 am – Skip breakfast, but non-caloric, unsweetened beverages are permitted 
• 12:00 pm – First meal 
• 3:00 pm – Snack
• 6:00 pm – Last meal
• 7:45 pm – “Dessert” or small meal
• 8:00 pm – Begin 16-hour fasting

A sample meal plan on the 16:8 method may look like this:

• Meal #1: Avocado and poultry salad over a bed of leafy greens. Served with a bowl of broth or soup
• Snack #2: Smoothie made with fruit, leafy greens, and honey for flavor
• Meal #3: Teriyaki chicken, broccoli, and bell peppers over quinoa
• Snack #4: Greek yogurt topped with oat cereal, blueberries, and sliced almonds

While the 16:8 IF is one of the more popular plans, there are different types of intermittent fasting.

Depending on which intermittent fasting method you choose, the fasting period can be anywhere from a few hours per day to a full day. We will discuss the different fasting protocols in more detail later in the article. 

Intermittent Fasting for Weight Loss

There is growing research that intermittent fasting is at least as effective for weight control as other weight loss programs (1).

There is also evidence that fasting may help boost the metabolic rate and be a useful tool to combat obesity (2, 3).

One review study found that intermittent fasting plans resulted in a 3 to 8% reduction in body weight over 3 to 24 weeks.

Study participants also had a 4 to 7% decrease in waist circumference, meaning that they lost belly fat (4).

A more recent systematic review found that intermittent fasting resulted in a weight loss ranging from 0.8% to 13% of starting body weight.

Participants also had significant changes in body composition.

In particular, waist circumference, aka fat mass around the waistline, decreased by 3 to 8 cm (or about 1 to 3 inches) among intermittent fasting participants in the studies that had a duration of four weeks or longer. 

Additionally, the researchers noted that most of the body weight loss from IF was from body fat loss (3).

The reason why an intermittent fasting diet can help with weight change is two-fold.

The main reason that intermittent fasting works to help you lose weight is by decreasing your daily caloric intake to create a calorie deficit.

A caloric deficit occurs when you consume less calories in a day than you burn. 

In general, people practicing IF eat fewer meals and snacks, to some extent, because they limit their food intake to a defined eating window.

Therefore, you consume fewer calories than usual, as long as you are not overcompensating during the eating periods. Overall, you decrease the number of calories you can ingest in a day when you limit your eating window.

Additionally, your body also adjusts hormone levels during periods without food to make fat stores more accessible to use as energy.

To understand these changes, we first need to discuss the impact eating has on the human body and blood glucose levels. 

When you eat, your digestive tract or gut releases enzymes to break down food into smaller molecules that can be absorbed into the bloodstream.

During digestion, carbohydrates, or carbs, are broken down into glucose, a preferred energy source for your body tissues and brain.

This causes glucose levels in the bloodstream to rise.

In this fed state, glucose enters your cells with the help of a hormone known as insulin, which is secreted by the pancreas to regulate blood glucose levels.

Basically, insulin signals to your cells to absorb glucose and burn it as fuel. Insulin also communicates with fat cells and tells them to absorb excess glucose to be stored as body fat. 

During periods without food, blood glucose levels decrease, which also lowers the insulin response. This decrease in insulin sends signals to your body to utilize stored glycogen and body fat for energy instead of blood sugar. 

Additionally, the stored fat molecules are broken down into ketones, which can then be used by the brain for energy.

In fact, some fasters may enter a form of mild ketosis during the fasting window. However, unlike the keto diet, IF relies more on stored body fat and lipids to maintain energy levels versus dietary fat. 

To sum it up, when your body is cut off from the energy found in food, it will switch to fat-burning as its primary source of fuel.

Therefore, you are not only losing weight, but a portion of this loss will be from body fat storage.

Potential Health Benefits of Intermittent Fasting

The idea of time-restricted eating is not new. In fact, the history of fasting dates back to the dawn of our species.

Some proponents of IF even claim that this dietary approach is an “ancient secret” to wellness, although there is not enough evidence to fully back this claim. 

During ancient times, hunter-gatherers did not have access to unlimited food supplies and likely went through starvation periods.

As a result, humans evolved to survive without food for extended time periods. 

Periodic fasting is also a part of religious and spiritual traditions and is practiced in Islam, Judaism, Buddhism, and some denominations of Christianity.

An example of fasting for spiritual reasons is Ramadan, where practicing Muslims abstain from food and drink from sunrise to sunset. 

Additionally, in your own personal experience, there may have been specific times when you “fasted” by intentionally or unintentionally skipping a meal or eating less food.

This is all to say that the human body is equipped to handle short periods without food. 

However, the concept of regular fasting as a health and fitness fad is relatively new, and there are various reasons that someone might choose to try actual fasting. 

What Are the Benefits of Intermittent Fasting?

Fasting periods have been shown to impact the human body in several ways and may provide potential benefits. Some changes that occur in the fasted state include:

• Lower insulin levels and increased insulin sensitivity – During the fasting period, insulin levels and blood sugar levels decrease, resulting in lipolysis—the breakdown of lipids or fat. This makes stored fat more accessible to use as energy, potentially resulting in more significant fat loss (5).
• Increased human growth hormone – Human growth hormone (HGH) or growth hormone has been linked to higher fat burn and increases in muscle mass and muscle gain. Research has shown that HGH levels increase during a fast (6-8). One study found that two days of fasting caused a 5-fold increase in growth hormone secretion rate among participants (8). 
• Changes in adrenaline and norepinephrine – During a fast, norepinephrine levels rise in the body. Norepinephrine is a hormone that tells the body to release stored body fat (2, 9).
• Changes in gene expression – Some preliminary research has suggested that a fasting diet may change gene expression. 

In particular, fasting has been linked to the expression of genes associated with longevity, immune system function, the aging process, and the circadian rhythm, which helps with sleep quality. 

Additionally, periodic fasting is associated with greater cell autophagy, a type of programmed cell death, where parts of an old cell are broken down and recycled. 

This process may protect against certain chronic diseases such as Alzheimer’s and Parkinson’s disease (10-12). 

One study conducted on rodents found that IF may slow the onset of Alzheimer’s disease- a form of dementia (12).

Additional animal studies suggest that this dietary approach may protect against other neurodegenerative diseases, including Parkinson’s disease. 

However, further research is needed to explore the relationship between regular fasting, gene expression, and human physiology. 

These changes at the molecular and cellular levels may lead to health benefits, including:

Weight Loss

Weight loss relies on creating a calorie deficit. Basically, you lose weight when you eat fewer calories (kcal) per day than your body burns.

An intermittent fasting diet makes it easier to eat fewer daily calories without strict meal plans by limiting the amount of time you spend eating. 

Decreased Risk of Type 2 Diabetes

Periodic fasting may reduce insulin resistance by lowering blood glucose and insulin levels. A reduction in insulin resistance may help prevent type 2 diabetes and improve blood glucose levels. 

One review study found that IF decreased blood sugars by 3 to 6% and insulin levels by 20 to 31% in people diagnosed with prediabetes (4). 

Improvements in Heart Health and Metabolic Health

Intermittent fasting may reduce the risk factors associated with heart disease. Research has shown that fasting protocols may lower blood pressure, “bad” LDL cholesterol, blood triglycerides, blood glucose levels, and insulin resistance (13, 14). 

Reduced Inflammation

Research shows that fasting protocols may reduce oxidative stress, which could decrease inflammation (10).

Chronic inflammation has been linked to several potential health problems, including cardiovascular disease, arthritis, Alzheimer’s, impaired immune function, cancer, and elevated cortisol levels.

Lower Blood Pressure

IF has been shown to decrease blood pressure. High blood pressure is associated with an increased risk of heart disease.

Potential Anti-Cancer Effects:

Preliminary scientific research trials conducted on rodents suggest that IF may slow the growth of tumors and prevent the development of certain cancers (15, 16). 

Possibly Slows the Aging Process and Increases Longevity

Animal studies have shown that an intermittent fasting diet helps rodents, particularly rats, live longer. One study found that fasted rats had an 83% increase in life expectancy compared to those on a normal diet (17). 

It’s important to keep in mind that scientific evidence using animals is considered preliminary research, and many questions still need to be answered using high-quality human studies.

Potential use as a therapeutic approach to manage metabolic syndrome: 

Metabolic syndrome is the term used to describe a cluster of health problems that increase chronic disease risk. These conditions include high blood pressure, elevated blood sugar levels, excess body fat around the waistline, and high cholesterol or triglyceride levels. 

A recent review study published last year found that IF was at least as effective as calorie-controlled plans in terms of weight management and blood sugar control among study participants with metabolic syndrome, prediabetes, and type 2 diabetes (18). 

Intermittent Fasting Protocols

There are various ways that you can fast. Some involve fasting on a daily basis, while others recommend abstaining from food for longer periods. Below, you will find a list of some of the common fasting protocols.

The 16:8 Plan

The 16:8 diet sometimes referred to as the Leangains protocol, is a popular form of intermittent fasting.

Many people find that the 16:8 routine is the easiest way to get started, and it may be a good first step if you are a beginner or are trying a fasting diet for the first time.

It involves eating all meals and snacks during a designated 8 hours of the day while abstaining from food for a 16-hour window.

A typical eating timeframe on this intermittent fasting regimen is from 12 pm to 8 pm. Generally, this plan involves skipping breakfast and not eating anything after a certain point at nighttime until the following day.

24-Hour Fasts

This form of fasting is sometimes referred to as the eat stop eat plan or one-day diet. On the 24-hour program, you abstain from eating for a 24-hour period one or two days of the week.

For example, you could finish dinner at 6 pm one day and not eat again until 6 pm the next day- an entire day. You then eat a normal diet for the rest of the time.

The 5:2 Method

The 5:2 form of intermittent fasting restricts your intake to 500 to 600 calories for two fasting days per week while following a typical eating pattern on the other five days of the week.

Alternate Day Fasting

For the alternate-day plan, you fast alternate days of the week. Several different versions of this intermittent fasting regimen exist, with one plan allowing 500 calories during the fast days and others recommending to abstain from food for the 24 hour period.

Completing longer fasts every other day can be a challenge, and this method may not be ideal for long-term success. Side effects of alternate-day fasting include feeling hungry multiple days a week.

This strict fasting regimen could also result in nutrient deficiencies if followed for extended periods.

The Warrior Diet

This version of IF entails eating small meals of fruits and veggies throughout the day and one large meal in the evening.

OMAD

OMAD stands for one meal a day. This is a more restrictive version of the warrior diet and is likely not among the greatest diets. Most people will find that eating only one big meal per day is not sustainable.

Water Fast

Water fasts sometimes referred to as water fasting, is a fad diet that restricts everything but water for 24 to 72 hours. Proponents of water fasts claim that this type of fasting can help promote autophagy and remove body “toxins.”

Additionally, someone may try water fasts for a medical procedure or for spiritual reasons. However, water fasts come with potential health risks, including imbalances in electrolytes such as potassium and sodium, and should only be undertaken under medical supervision.

Popular detoxification diets are based on water fasts. These detoxification programs’ claims are plentiful and include ridding the body of toxins and fixing hormonal imbalances.

However, there is little scientific evidence to support the use of these fads for disease prevention.

Remember that the best intermittent fasting regimen is the one that you can sustain for the long run. As a disclaimer, some examples on this list, like water fasts, are not recommended unless prescribed by someone with a medical degree.

Additionally, some are more restrictive than others, including OMAD and alternate-day fasting, and may not be the best option for all fasters.

Before starting any intermittent plan, you may want to consider your current eating pattern and select the plan that fits your lifestyle to achieve the best results.

As with all types of diets, IF should not be an all-or-nothing approach. Perhaps you are interested in the benefits of fasting but are hesitant to commit to a full 16 hours of fasting? If that is the case, try starting with 12 hours and increase your fast period to the next level a little bit at a time if it feels right.

Another option is to skip a meal the next time you do not feel hungry. There is no “true method” or true fast, and even engaging in sporadic or occasional periods without eating will create a caloric restriction for most people. The important thing is to find the type of diet that works for your life and your own experiences.

Who Should Avoid Intermittent Fasting?

Many people find that IF is an effective way to lose weight and simplify their eating habits.

However, this diet has some potential downsides or cons and is not appropriate for everyone.

As you might imagine, the most common side effect of intermittent fasting is hunger pangs.

Additional potential side effects include weakness, fatigue, food cravings, discomfort, irritation, and headaches during the fasting window, especially when starting an intermittent fasting plan. 

Additionally, digestive system issues may initially occur as you give the body time to adjust to a new eating schedule. This may include diarrhea, constipation, nausea. Eating large meals can also cause heartburn.

Binge eating during the eating window is another potential con of IF and can happen when you get overly hungry. It can also be a sign that you are not getting enough calories during the eating period.

Intermittent fasters may notice that these side effects are most common during the first week or weeks of the new diet.

In general, IF is considered safe for the general population. However, it may not be right for all people. This includes individuals who are underweight or who have been advised to gain weight by a healthcare provider.

Additionally, people with a history of an eating disorder may want to avoid IF. This regimen may not provide enough flexibility and is likely not the best thing for treatment and recovery from an eating disorder.

You may want to avoid fasting diets in the first place if:

• You are breastfeeding or pregnant
• You are trying to become pregnant   
• You have a history of an eating disorder 
• You have low blood pressure 
• You have a history of hypoglycemia
• You are underweight, or you have been advised to gain weight by a healthcare provider or nutritionist
• You have a history of amenorrhea 

Additionally, make sure to seek medical supervision or medical advice before starting IF if you have any of the following health conditions:

• You are taking certain medications
• You have certain medical conditions or health issues, such as diabetes or high blood pressure (aka hypertension)
• You have a history of hypoglycemia or low blood sugar

Intermittent fasting plans are only one of many types of diets that can help people improve overall health.

If you find that you are uncomfortable with the idea of going a long time before your next meal, then this may not be an effective therapeutic approach for you. Ultimately, actual fasting isn’t for everyone, and that is not a bad thing.

The best dietary plan is the one that you can maintain in the long run.

What Should I Eat After Fasting?

Unlike a normal diet, intermittent fasting plans do not have any food rules. 

However, it is still recommended to limit portion sizes and junk food, such as sugars, soda, sweets, and refined cereal grains, like those found in bagels, cookies, and muffins.

These foods are associated with weight gain, obesity, and excess calories. 

Highly processed foods are also linked to an increased risk of certain health conditions, including heart disease, high blood pressure, and cholesterol levels, and insulin resistance.

Ultimately, the prevailing dietary recommendations still hold true and include eating plenty of whole foods for good health. This includes:

• Fruits and veggies, including broccoli, leafy greens, spinach, cucumbers, kale, tomato, peppers, and bell peppers
• Whole grains and complex carbohydrates, such as brown rice, whole grain pasta and cereals, oats, barley, quinoa, and more
• Protein sources: Lean red meat, poultry, fish, beans, tofu, nuts, seeds, and eggs (including egg yolks)
• Legumes, including lentils
• Nuts and seeds, such as almonds
• Dairy products, including unsweetened yogurts
• Healthy fats: From fatty fish, olive oil, avocados, coconut oil, nuts, and seeds
• Unprocessed, whole foods

Try focusing on real food and foods rich in nutrients, including potassium, magnesium, copper, and manganese. 

In addition to a healthy diet, behaviors such as exercising on a regular basis, and getting enough sleep are also important for overall health and wellness.

Got Questions Before You Start?

Do you have additional questions about IF? Below, you can find answers to the FAQs.

How to Get Started

As previously mentioned, IF is associated with potential health benefits, including a reduced risk of type 2 diabetes, decreased inflammation, and improved heart health.

Intermittent fasting can also help you lose weight by reducing calorie consumption and has been shown to be at least as effective for weight loss compared to a calorie restriction(1). 

If you feel comfortable with the idea of a fasting diet and find it to be a sustainable way to approach better health, then it can be a good weight loss program and may improve your overall health and wellness. 

So are you ready to try an intermittent fasting plan?

Begin by selecting a fasting approach that will set you up for long term success. Consider your weight loss goals, lifestyle, current eating pattern, and appetite when choosing the fasting schedule that best suits your needs. 

Additionally, keep in mind that IF is not a replacement for a healthy lifestyle or a healthy diet.

It is still recommended to limit processed and junk foods during your eating windows, as these foods are associated with obesity, weight gain, and increased risk of certain health problems.

Even while on an intermittent fasting journey, you will want to focus on moderation and healthy foods for optimal health.

The bottom line is that the most successful nutritional health program is the one that you can maintain for a long period of time.

At the end of the day, this means that intermittent fasting will be a natural fit for some people but a major change for others. Perhaps the best way to find out if it is right for you is to try a plan and see if it gives you positive results that are sustainable for the long-term. 

A great way to get started is by using the tools and tips outlined in this article or by speaking with a healthcare provider or registered dietitian to determine if an intermittent fasting journey is right for you.