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Intermittent Fasting: A Detailed Beginner’s Guide

By Brandi Givens, RDN

Last Updated on August 24, 2021

Medically Reviewed by Ana Reisdorf, MS, RD

Intermittent fasting is an umbrella term for several calorie-restriction diets that continue to gain popularity, that may have benefits and risks. Having a good understanding of how it works can help you decide if intermittent fasting is right for you.

Written by
Brandi Givens, RDN
Registered Dietitian Nutritionist
Brandi Givens has been a registered dietitian nutritionist (RDN) since 2010, and an internationally board-certified lactation consultant (IBCLC) since 2016.
Medically Reviewed by
Ana Reisdorf, MS, RD
Registered Dietitian Nutritionist
Ana Reisdorf is a registered dietitian nutritionist with 14 years of experience in the field of nutrition and dietetics. She graduated from UCLA in 2002 with a degree in psychology and women’s studies and completed her master’s degree from Central Michigan University in 2010
Intermittent Fasting: A Detailed Beginner’s Guide
Photo credit: iStock.com/hxyume


  • May improve health conditions of certain at-risk groups
  • May help with weight loss
  • Simple to follow compared to many weight loss plans


  • May not be safe for some people including those who have diabetes
  • It’s more difficult to get recommended nutrients on fasting days
  • Hunger is uncomfortable and may not be sustainable or safe for long-term weight loss

The practice of fasting, the complete or partial abstinence from food, is not new. People have been fasting for its health benefits for thousands of years. (1)

Perhaps the world’s most famous faster was Gautama Buddha, who was said to encourage monks to enjoy better health by avoiding evening meals. (2)

Hippocrates, an ancient Greek physician, recommended fasting to remedy certain diseases. (1)

And countless doctors over the centuries since then have prescribed fasting with vast degrees of success, from regained wellness to death by starvation, depending on the extremity of their recommendations.

Moderate fasting, or intermittent fasting, has gained popularity in recent decades, as contemporary research has validated its benefits.

What Is Intermittent Fasting?

Intermittent fasting (IF) is a trending diet plan that tells you when to eat, not what to eat.

The meaning of the word intermittent is sporadic or alternating. IF is a general term for several ways of restricting food periodically, not for extreme lengths of time.

Many people like IF because there’s no need for special food preparation. Dieters simply go from cycles of eating what they normally eat to much lower calorie intake or forgoing food altogether.

The length of fasting time and calorie restriction depends on which method the dieter chooses.

Some people practice long-term IF for its reputed health benefits, but most people try it for a shorter time to lose weight.

For weight loss, intermittent fasting works at least in part because dieters eat fewer calories overall.

As long as participants don’t binge eat and consume extra calories after each fasting window, the calorie deficit may result in weight loss.

Success for using IF as a weight-loss tool is similar to traditional calorie restriction diets in some studies. (3)

The diet may also work for weight loss and increased health by allowing the body’s circadian rhythms to work their best. (4)

The simple act of putting food in our mouths sets a cascade of hormones and body functions into motion. Abstaining from food may allow for better rest and repair of our cells, organs, and endocrine system.

A third reason IF may work is because our body burns fat for fuel once it uses its glucose store. This use of fat for energy is called ketosis, sometimes referred to as metabolic switching. (5)

Fasting allows your body to tap into fat stores, having burned through the calories of your last meal.

When you are able to switch to burning fat for energy, you start to effortlessly lose weight, and your hunger decreases.


Intermittent fasting may have some benefits for health, but science is still working out the best and safest methods.

Intermittent Fasting Methods

There are three popular methods of IF that go by countless names, which can get confusing. Some human research has been conducted on all three methods.

Choosing the one which may work for you depends on your preference and warrants a discussion with your doctor about whether IF is right for you.

The three most popular methods are:

24-Hour Fasts (or Eat Stop Eat)

24-hour fasts are exactly what they sound like – fasting for 24 hours.

Some people choose to do this once a month or once every few months because of its possible health benefits.

For weight loss, some people choose to fast twice a week on non-consecutive days.

This method of IF is more extreme, more uncomfortable, and potentially unsafe.

The 5/2 Diet

In this method, dieters choose two non-consecutive days to restrict calories to 25% of normal intake, or around 400–600 calories.

On the other days, participants eat the way they normally would. For example, the person might choose to fast on Monday and Thursday.

A sample diet of 500 calories in one of those days might look like this:

  • Breakfast: 1 cup blueberries + 6 oz plain yogurt
  • Lunch: 1 boiled egg on 1 slice whole-wheat toast
  • Dinner: 2 oz chicken breast, 1/4 cup brown rice with cooked spinach

Time-Restricted Eating or Feeding (TRE or TRF)

In this method, dieters choose the number of hours they want to fast each 24-hour period, leaving the rest of the hours as their eating window.

TRF is also known as 16/8, 14/10, or 12/12, depending on fasting versus eating hours.

For weight loss, many people are successful with fasting for 16 hours (example: 6 pm–10 am) and eating normally for the other 8 (example: 10 am–6 pm). (6)

For weight maintenance and health benefits, people generally choose a shorter window of fasting, like 14 (example: 6 pm–8 am) or 12 hours (example: 7 pm–7 am)

The TRF method is considered a gentler, more sustainable IF method by some nutrition researchers, with less uncomfortable hunger and side effects during waking hours. (7)

Plus, it’s based on both circadian rhythm science and the theory that daylight hours are when humans are naturally programmed to consume food. (8)


There are several variations of intermittent fasting that have the potential for weight loss and health benefits.

Potential Health Benefits

Most of the evidence supporting intermittent fasting comes from animal studies; however, human research is gaining. Here’s some of what has emerged from human studies so far:

Weight Loss

Overweight and obesity statistics continue to remain high in the United States, exacerbating other health conditions, including cardiovascular diseases and type 2 diabetes. (9)

Health care professionals and overweight individuals continue to seek effective and sustainable ways to manage weight successfully.

A systemic review of 27 studies using various intermittent fasting methods reported that all studies found success with weight loss. (10)

There were no adverse effects reported by the participants. In the analysis, the success of weight loss ranged from 0.8% to 13% body weight.

The study with the highest weight loss, 13% body weight, was a 2018 12-week randomized control trial involving 35 people.

The participants fasted 3-day per week, only consuming 25% of recommended calories, and normally ate the other 4 days of the week.

The control group in the study used a traditional calorie restriction diet and had similar results.

The length of the studies also varied, from 2 weeks to 1 year. The year-long studies conducted in 2018 and 2019 used 2-day per week 25% calorie restriction, with 5.6% and 6.8% weight loss, respectively. (11, 12)

Their results were similar to calorie restriction control participants.


Intermittent fasting is a weight-loss tool that seems to have similar success to traditional calorie restriction plans.

Diabetes and Pre-Diabetes

Because blood sugar control is a delicate balance, it may be dangerous for diabetic patients to try intermittent fasting without being carefully monitored by a doctor.

Human studies on diabetic patients are limited, but the small studies show promise with glycemic control.

For example, a 12-week study in 2016 of 63 men with type 2 diabetes compared glycemic control results in IF participants to calorie restriction patients. Both groups lost weight, and both groups had improved glycemic control. (13)

In most studies testing diabetic patients, it isn’t clear whether the improved blood sugar control is because of the fasting method or the weight loss.

A small, 5-week study published in 2018 involving 8 pre-diabetic participants showed that with the time-restricted eating method of IF, their glycemic control improved even without weight loss. (14)


Intermittent fasting may be a useful tool for diabetes management. However, more human trials are needed to see which methods are most effective in glycemic control.

Cardiovascular Disease

There are no large, long-term studies available to determine the effects of intermittent fasting on cardiovascular health.

However, a few human studies have shown that intermittent fasting improves high blood pressure and cholesterol. (15)

The reason for these improvements is unclear and may be related to weight loss during the studies.

In a large study of almost 2000 people published in 2019, researchers analyzed the longevity of cardiac patients who became routine fasters. (16)

Results showed that fasters had a 45% lower mortality rate during the follow-up period compared to non-fasters. However, it was unclear if other factors may have been involved.


The potential for intermittent fasting to positively affect cardiac health is promising. However, it’s unclear if IF is more effective than other methods for improving cardiovascular health, and more human studies are needed.

What to Eat While Intermittent Fasting

During Your Fast

The most important thing to remember with IF is to stay hydrated. You are trying to fast from calories and chemicals that your body would have to metabolize, not water.

There is some controversy over calorie-free drinks. Dr. Satchidananda Panda, a leading intermittent fasting researcher at the Saulk Institute, believes that it’s not just calories we should avoid. It’s also chemicals that non-water drinks contain. (17)

For example, coffee and tea contain caffeine, which would put the liver to work, starting metabolism processes.

For most effective fasting, it may be best only to drink plain water during each cycle.

When You’re Breaking Your Fast

Intermittent fasting is so popular because it doesn’t dictate what should be eaten during the non-fasting hours.

This keeps life simple and gives people the freedom to eat what they enjoy, which is not typical when dieting.

While eating foods you enjoy is important, restricting calories can also mean reducing some essential nutrients.

To get the most out of IF, it’s best to focus on nutrient-dense foods when you break your fasts.

This can be accomplished by combining intermittent fasting with a healthy eating plan, like a Mediterranean-style diet. A healthy eating plan should include:

  • A wide variety of colorful fruits and vegetables
  • Whole grains like oats, whole wheat, and brown rice
  • Healthy protein sources like nuts, seeds, legumes, fish, poultry, and lean cuts of meat
  • Low-fat dairy sources like cottage cheese and fat-free Greek yogurt
  • Cooking with healthier fats like avocado, olive, and canola oils


While the focus of intermittent fasting is when you eat and not what you eat, you’ll get the most out of your experience by choosing healthier food options during your eating cycles.

Side Effects and Safety Concerns

Fasting is not safe for everyone. Ask your doctor if fasting is right for you before beginning, especially if you have underlying conditions like diabetes.

Some people experience understandable side effects from fasting, including headaches, irritability, and low energy. (18)

There are very few long-term human studies on intermittent fasting. Therefore, there may be drawbacks for some people that haven’t been brought to light.

Some medications work best if they’re taken at certain times of the day or with foods. It’s best to discuss the safety of medications while fasting with your doctor.

Fasting is not appropriate for pregnant women or those who are trying to get pregnant. In addition, fasting may affect the milk supply of breastfeeding women.

I would not recommend this type of diet to anyone showing signs of disordered eating or any diagnosed eating disorder.

There’s a danger of low electrolytes or electrolyte imbalance if people heavily sweat or exercise during their fast.

These individuals should replenish electrolytes with food or electrolyte drinks like sports drinks.


Intermittent fasting may not be safe for certain people, including those who have diabetes, those who are pregnant, or those with a history of eating disorders.

Frequently Asked Questions About Intermittent Fasting

Which intermittent fasting method is the best for weight loss?

Choosing a weight loss plan is very personal and greatly depends on individual goals, underlying conditions, and preferences. 24-hour fasting is uncomfortable at best for most people and may be unsafe for some. Time-restricted eating is a milder, more sustainable method that your doctor or dietitian can tailor to help you reach your goals.

What weight loss results can I expect from intermittent fasting?

Most research shows that intermittent fasting is about as effective as many other calorie restriction diets. If the person can sustain IF and avoid excess calories during their non-fasting window, weight loss is possible.

How long is it safe to remain on an intermittent fasting diet?

There are not enough long-term studies to determine how long it’s safe to remain on any method of intermittent fasting. It’s important to discuss a long-term plan for any IF method with your doctor to be sure it’s safe for you.

What can I eat when I’m not fasting?

Many people are drawn to intermittent fasting because there are no strict rules about foods during the eating window. For optimal health, it’s best to follow The Dietary Guidelines for Americans and eat a wide variety of nutrient-rich foods when you’re not fasting.

Is drinking black coffee or tea okay during my fast?

The answer here is unclear. Because processing caffeine requires using the liver to metabolize the chemical, some possible benefits from fasting may be lost by drinking tea or coffee. For this reason, some dieters choose to avoid caffeine and drink only water during their fasting window.

What hours are best for practicing time-restricted feeding?

Some research involving time-restricted eating has a focus on keeping a regular circadian rhythm. Evidence shows that there may be health benefits to eating only during the day and letting the digestive system rest at night.

The Bottom Line

While intermittent fasting is currently trending in the United States, it isn’t a new concept.

The popularity of fasting has waxed and waned worldwide for as long as history has been recorded.

With a growing body of research to support its benefits, many experts no longer dismiss intermittent fasting as a fad diet.

IF may not be sustainable for people who are more sensitive to the side effects like discomfort from hunger.

Beginners considering intermittent fasting should keep in mind that some IF methods are more intense than others.

Time-restricted eating is gentler than other methods and simply extends the hours of fasting people normally manage at night while they’re sleeping.

Intermittent fasting isn’t safe for everyone. People considering a fasting diet should speak to their doctor to make sure it’s right for them, especially if they take medications or have underlying health conditions.

More long-term safety studies are needed to determine efficacy and safety.

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At WellnessVerge, we only use reputable sources, including peer-reviewed medical journals and well-respected academic institutions.

  1. Fasting:
  2. The Time and Place for Eating:
  3. Compensatory mechanisms activated with intermittent energy restriction: A randomized control trial:
  4. Time-Restricted Eating to Prevent and Manage Chronic Metabolic Diseases:
  5. Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting:
  6. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss:
  7. Intermittent fasting: Surprising update:
  8. Fasting, circadian rhythms, and time restricted feeding in healthy lifespan:
  9. Adult Obesity Facts:
  10. Intermittent fasting and weight loss:
  11. Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes:
  12. Effect of intermittent compared to continuous energy restriction on weight loss and weight maintenance after 12 months in healthy overweight or obese adults:
  13. The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial:
  14. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes:
  15. Intermittent Fasting: A Heart Healthy Dietary Pattern?:
  16. Abstract 11123: Intermittent Fasting Lifestyle and Human Longevity in Cardiac Catheterization Populations:
  17. Does drinking coffee break your fast?:
  18. 4 intermittent fasting side effects to watch out for: