What Is Intermittent Fasting? A Detailed Beginner's Guide

By now, you’ve no doubt heard of intermittent fasting (IF). Maybe your brother skipped out on brunch the last time you got together because it was too early for him to eat. Or maybe your friend couldn’t do a late dinner last time you saw her.

There are many reasons why you might try fasting, or specifically IF, from weight loss to wellness. Use this scientific guide to get the lowdown on IF specifically. You’ll also find tips for how to set yourself up for success if you decide to start.

What Is Intermittent Fasting?

Unlike some other diets, intermittent fasting doesn’t have a long list of rules. Instead, the approach is all about “entirely or partially restraining or abstaining from eating during a specific period of time,” says Heather Bauer, RDN, founder of Heather Bauer Nutrition in New York City.

In other words, IF involves pauses from eating. While some people find that they enjoy IF, this is not the right diet for everyone, she says.

You choose how you want to do IF by deciding which days of the week you will fast. On fasting days, you’ll likely follow a severe calorie-restricted diet or you may not eat at all. You can also fast for a certain time every day. Ultimately, this results in consuming fewer calories over the course of the week, and some experts, including Caroline Susie, RDN, a spokesperson for the Academy of Nutrition and Dietetics in Dallas, say that this calorie reduction is what sometimes leads to weight loss and then potentially additional metabolic benefits.

There is no one standard way to practice IF. “Intermittent fasting is an umbrella term for three different types of diets,” says Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago and a researcher on intermittent fasting. Here’s what you’re most likely to see, she says:

Alternate-Day Fasting

For the most common type of alternate-day fasting, you eat 500 calories every other day. On off days, you can eat what you want.

5:2 Diet

Popular in the United Kingdom, you consume 500 calories on two nonconsecutive days per week. On the other days, you eat whatever you like.

Time-Restricted Eating

You choose a window of time during which you can eat (feast); the rest of the day you don’t eat (fast). One popular setup is 16:8, which means you fast for 16 hours and you can eat during the other eight hours. For instance, you might set your eating window from 12 noon to 8 p.m. daily. (This could also be called skipping breakfast.)

Learn More About the Types of Intermittent Fasting

So proceed with caution. With that in mind, here’s how IF may benefit you:

Heart Disease Prevention

Treat Type 2 Diabetes

Fend Off Alzheimer’s Disease and Stroke

Improve Nonalcoholic Fatty Liver Disease

When on IF, you’re simply eating during fewer time periods, whether that be fewer hours in the day or fewer days of eating. “In our research, we’ve found that time-restricted eating naturally cuts out several hundred calories per day,” says Dr. Varady. That roughly results in losing about one to two pounds per week, she has observed in her research. It’s similar to doing a calorie-restricted diet every day, but a touted benefit is that you don’t have to count calories.

That said, more long-term data (based on following people for one to two years) is needed. The majority of Varady's published research has lasted a maximum of six months.

Before jumping into an entirely new way of eating, be aware that there are some side effects you may experience, says Susie:

• Headaches
• Fatigue
• Low energy
• Irritability
• Hunger

These can all happen because you’re not eating, says Susie. While they’re normal, they are uncomfortable, and it can affect your day-to-day sense of well-being. Some people will find that these are not good trade-offs and will choose to stop IF. This is completely okay. It’s not for everyone.

One note: Expect hunger to peak during the first 10 days, then decrease as your body adjusts to a new pattern of eating, says Varady.

Do not try intermittent fasting if you are pregnant or breastfeeding, says Bauer. Similarly, if you have a low (underweight) BMI or a history of an eating disorder, IF is not right for you. Varady also cautions adults older than 70 against trying fasting, as it can lead to muscle loss, which is critical to preserve in older age.

Before trying intermittent fasting, set yourself up for success by following these steps.

1. Decide on the Type

If your goal is weight loss, consider how much weight you want to lose. If it’s significant, it may make more sense to start with alternate-day fasting, though it’s more challenging, says Varady. This way, you can lose a good amount of weight in the first couple of months, which can keep motivation up. Then you may switch to time-restricted eating because it’s easier to stick with longer term, she says.

2. Set Your Window

If you’re going to try time-restricted eating, you’re going to have to decide on your eating window. This can be done by preference. Some clients tell Bauer that they simply don’t need to eat in the morning, so they’ll start their eating window with lunch, have a snack, and then eat dinner. Others will scrunch all three meals into the smaller eating window.

Not ready to go all the way? Try a 12-hour fast, which is the most natural pattern for people to fall into, says Bauer. It’s not as stringent, but it stops nighttime eating, which can help you lose weight and decrease heartburn or sleep problems caused by consuming food too close to bedtime, she says.

3. Plan Fasting Days Strategically

Fasting may trigger unpleasant emotions like “hanger” (anger caused by being hungry), as well as fatigue and headaches, Susie says. She recommends looking at your week and being mindful of the days that you need to perform especially well, like a day when you have a presentation for work. Those are not the days to plan a fast.

Similarly, if you have an important social function (a birthday party or another celebration), it can be really tough to fast on days when special foods are a big component. Looking ahead to fit IF around your lifestyle, not the other way around, is key to making this work for you long term.

4. Still Reach for Nutritious Foods

The belief that during feasting periods you can eat what you want is not quite true, especially if you want to do this healthfully. “Fasting is not a replacement for healthy eating,” says Bauer. To get the nutrients you need, focus on foods with lean protein, fiber, and low-GI carbohydrates, fruits, and vegetables, she says. Drink plenty of water throughout the day. This will ensure that you don’t get dehydrated, which only exacerbates side effects like headaches.

5. Slide Your Window

Again, if you’re following time-restricted eating, know that you don’t have to keep the same eating window every day. You can adjust it depending on your commitments. For example, if you have special brunch plans, then slide your window up so that you can participate — and feel happy and satisfied (not deprived) while intermittent fasting.

6. Consult a Healthcare Professional

You might have a lot of questions about if IF is right for you, what to eat, or how to make it work in your own life, especially if you have underlying health conditions. In that case, it’s best to reach out to a registered dietitian nutritionist for guidance on how to do this safely, says Susie. You can find one in your area using the nutrition expert finder tool from the Academy of Nutrition and Dietetics at EatRight.org.

Favorite Books

Fast. Feast. Repeat.

The New York Times bestseller is a go-to guide for a reason: It details how to craft your own IF schedule to fit your life, as well as how to get out of the dangerous dieting mindset that holds you back from your goals. How? With her “delay, don’t diet” approach — meaning you eat the delicious foods you like but delay them to a specific time frame, which may help increase satisfaction and eliminate feeling restricted.

The Everything Guide to Intermittent Fasting

When you’re not sure what to eat to maximize your nutrition — no matter if you’re doing 5:2, 16:8, or 24-hour fasts — this book delivers 100 delicious recipes (such as breakfast chia bowl and chicken breasts stuffed with spinach and feta) to keep you on plan and healthy.

The Every-Other-Day Diet: The Diet That Lets You Eat All You Want (Half the Time) and Keep the Weight Off

Count this book by IF researcher Varady as your how-to guide for successfully following an alternate-day fast. She takes knowledge gleaned from what worked for participants in her studies, outlines exactly how to follow an alternate-day fasting plan that aims to help you lose weight, and shows you how to keep it off.

Complete Guide to Fasting

An extremely popular read by Jason Fung, MD, a popular proponent of intermittent fasting and low-carb eating, this book dives deep into what you can expect when you start fasting — the good and bad — including how to do it, benefits, and how to track your progress.

Favorite E-Book

Intermittent Fasting: An Introduction

A team of nutrition scientists at Precision Nutrition, a private nutrition coaching and education company, compiled a very thorough (and free) e-book on everything you need to know about IF, including benefits, weight loss, choosing an IF schedule, and exactly what to do to put it into action.

Favorite Podcast

The Human Upgrade With Dave Asprey

Not sure where to begin? Science author and podcaster Dave Asprey has a list of nine must-listen episodes that cover IF in depth, making it a good roundup when you want to take a deeper dive. The episodes will walk you through some of the more scientific topics, such as how fasting improves metabolic flexibility, why fat is your friend, and how fasting affects women’s hormones.

Favorite Apps

EatWise — Meal Reminder

iOS, Android; free

Don’t get off track. This app allows you to set how many meals you’re eating, track the time between meals, and get reminders on when to eat. There’s also an area to track weight loss progress.

Fastic: Intermittent Fasting

iOS, Android; free, in-app purchases (memberships start at $11.99/month)

You’ve got everything you need to make the practical side of IF work for you: A fasting timer, water tracker, and nutrition plans, plus you can connect with other people doing IF to swap fasting hacks.