You must have already heard of IF. Perhaps your brother skipped the last brunch when you met up because it was too early to eat. Maybe your friend could not do the late dinner last time you saw her.
While IF has entered the mainstream dietary vocabulary in recent decades, fasting in general is not new. According to reports, Hippocrates was utilizing fasting as a treatment for disease in the fifth century B.C., and it plays a central role in several religious practices, including Islam.
There are numerous reasons why you may attempt fasting, or more precisely, intermittent fasting, ranging from weight loss to health. Utilize this scientific guide to learn the lowdown on IF in particular. You will also discover tips on how to prepare yourself for success if you choose to attempt it.
What Is Intermittent Fasting?
In contrast to some other diets, there isn’t a lengthy list of rules with intermittent fasting. Rather, the method is more about “completely or partially restricting or avoiding eating during a defined period of time,” says Heather Bauer, RDN, and founder of Heather Bauer Nutrition in New York City.
That is, IF includes fasting from food. Some individuals find that they like IF, but this is not the appropriate diet for everybody, she explains.
How Intermittent Fasting Works
You determine how you want to do IF by deciding which days of the week you’ll fast. On the days that you’re fasting, you’ll probably be on a strict calorie-restricted diet or you won’t be eating at all. You can also fast for a certain time every day. Ultimately, this results in consumingfewer calories over the course of the week, and some experts, including Caroline Susie, RDN, a spokesperson for the Academy of Nutrition and Dietetics who is based in Dallas, say that this calorie reduction is what sometimes leads to weight loss and then potentially additional metabolic benefits.
Types of Intermittent Fasting
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 whatyou’re most likely to see, she says:
Alternate-Day Fasting
For the most widely practiced form of alternate-day fasting, you consume 500 calories every other day. On days off, you can eat whatever you want.
5:2 Diet
In the United Kingdom, it’s popular to take in 500 calories on two nonconsecutive days of the week. You eat whatever you want on the other days.
Time-Restricted Eating
You select a period of time when you can eat (feast); the rest of the day, you don’t eat (fast). A common regimen is 16:8, which is fasting for 16 hours and eating in the remaining eight hours. For example, you could make your eating period 12 noon to 8 p.m. every day. (This might also be termed avoiding breakfast.)
Learn More About the Types of Intermittent Fasting
Potential Health Benefits of Intermittent Fasting
Overall, rigorous, long-term research on intermittent fasting is still limited, and many of the conclusions you’ll read online are based on animal studies, says Susie. It’s unclear whether any type of IF is truly safe or effective in the long run.[2]
So proceed with caution. With that in mind, here’s how IF may benefit you:
Heart Disease Prevention
Although additional research is required, according to one review, IF is potentially beneficial for cardiovascular health because it reduces cardiovascular risk factors, including obesity, high blood pressure, high cholesterol, and diabetes.[3] Currently, researchers indicate, it’s not known whether one form of IF is superior to the others for the heart.
Treat Type 2 Diabetes
IF can be an effective treatment for type 2 diabetes.[4] Fasting reduces weight, decreases insulin resistance, and benefits hormones secreted from fat cells that influence hunger and inflammation levels. Nevertheless, if you have diabetes, you shouldn’t try IF without consulting your physician first.
Fend Off Alzheimer’s Disease and Stroke
Although research continues, some studies indicate that intermittent fasting can reduce the risk of stroke and Alzheimer’s disease.[5] That being said, much of this knowledge is derived from animal studies, and it is not yet known when to begin IF throughout the lifespan to reduce the risk of neurological diseases. What’s more, despite what proponents say about IF improving cognitive abilities, such as focus, IF does not appear to be a short-term brain-booster.
Improve Metabolic Dysfunction-Associated Steatosis Liver Disease
In a meta-analysis and systematic review of six trials, individuals with metabolic dysfunction-associated steatosis liver disease (MASLD) — liver condition observed in obese, diabetic, and metabolic syndrome individuals — who were following intermittent fasting (IF) experienced normalization of liver tests compared to participants who did not fast. Previously known as nonalcoholic fatty liver disease, MASLD was given a new name in 2023 to be more descriptive of its causes and not use stigmatizing terminology to those living with the disease. The authors of the study paper concluded that liver function improvement was noted since IF results in weight loss.[6] Nevertheless, the researchers noted that large-scale, randomized studies are necessary before making any recommendations.
Weight Loss Effects of Intermittent Fasting
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 like doing a calorie-restricted diet daily, but one claimed advantage is that you don’t need to count calories.
One of her trials discovered that individuals who shortened their eating period to eight hours a day took in around 300 fewer calories and lost around 3 percent of their body weight within 12 weeks. Conversely, alternate-day fasting can assist an individual in cutting 25 to 35 percent of their daily calories (for the week) and 4 to 6 percent body weight loss for 12 weeks.
Another analysis of 11 meta-analyses determined that IF, specifically alternate-day fasting, was effective in reducing the body mass index (BMI), a weight measure, of overweight or obese adults more than a standard diet. “With alternate-day fasting, the weight comes off twice as fast,” says Varity. The problem is, relative to time-restricted eating, most people find alternate-day fasting harder to stick to and apply to everyday situations, she says. In Varady’s studies, 30 to 40 percent of individuals fell out of alternate-day fasting trials, she says. On the other hand, time-restricted eating has a dropout rate of only 5 percent in Varady’s studies.
That being said, more long-term information (based on tracking individuals for one to two years) is required. Most of Varady’s published studies have had a maximum duration of six months.
Intermittent Fasting Side Effects
Before diving into a completely new diet, know that there are some side effects you might encounter, says Susie:
Headache
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 exception: Hunger will be highest in the first 10 days, then will subside as your body adapts to a new cycle of eating, says Varady.
Health Risks of Intermittent Fasting
Do not attempt intermittent fasting if you are pregnant or lactating, advises Bauer. Likewise, if you have a low (underweight) BMI or a history of an eating disorder, IF is not appropriate for you. Varady also advises against attempting fasting in adults over the age of 70, as it can result in muscle loss, which is important to maintain in older age.
Also, if you have any form of diabetes, talk to your doctor first because IF might not be safe for you. Not eating a meal can result in low blood sugar, which causes dizziness, fainting, and falls.[9] If you are taking any medication that must be taken at a specific time and taken with food, you’ll need to talk to your doctor too.
6 Tips for Intermittent Fasting Beginners
Before attempting intermittent fasting, prepare yourself for success by doing the following steps.
- Choose the Type
If you’re trying to lose weight, think about how much weight you need to lose. If it’s a lot, it might be better to begin with alternate-day fasting, although it’s harder, according to Varady. This way, you can lose a significant amount of weight in the first few months, which can sustain motivation. Then you might transition to time-restricted eating because it’s easier to maintain long term, she says. - 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 quite ready to commit to the whole shebang? Experiment with a 12-hour fast, which is the most natural rhythm for humans to fall into, advises Bauer. It’s not as strict, but it eliminates nighttime snacking, which can cause weight loss and reduce heartburn or sleep issues brought on by eating too close to bedtime, she says.
- Strategically Plan Fasting Days
Fasting can make you feel uncomfortable emotions such as “hanger” (anger at being hungry), fatigue and headache, Susie advises. She suggests glancing at your week and being aware of the days that you must do particularly well, such as a day when you are giving a work presentation. Those are not the days to schedule a fast.
In the same way, if you have a significant social event (a birthday party or other celebration), it can be very difficult to fast on days when special foods are a significant part. Planning ahead to make IF work with your lifestyle, rather than trying to make your lifestyle work around IF, is the key to making this a long-term success for you.
- Still Reach for Healthy Foods
The idea that during times of feasting you should be able to eat anything is not necessarily accurate, particularly if you are going to do it healthfully. “Fasting is not an alternative to healthy eating,” says Bauer. To obtain the nutrients you require, emphasize lean protein, fiber, fruits and vegetables, and low-glycemic-index foods, she says. Drink a lot of water during the day. This will prevent you from getting dehydrated, which only worsens side effects such as headache — and it’s one of the reasons you will probably urinate more often during intermittent fasting. - Slide Your Window
Once more, if you’re doing time-restricted eating, remember that you don’t need to maintain the same eating window daily. You may modify it based on your schedule. Suppose you have special brunch reservations; then shift your window ahead so that you can enjoy it — and be content and pleased (not starved) while doing intermittent fasting. - Take Advice from a Healthcare Professional
You may have many questions regarding whether IF is appropriate for you, how to eat, or how to implement it in your life if you have any health issues. Then, it’s best to contact a registered dietitian nutritionist to ask them how to do this safely, advises Susie. You can locate one in your area by utilizing the Academy of Nutrition and Dietetics’ nutrition expert finder tool.
Resources We Trust for Intermittent Fasting
Favorite Books
Fast. Feast. Repeat.
The New York Times bestseller is a must-have guide because it explains how to create your own IF schedule to suit your lifestyle, as well as how to break out of the unhealthy dieting trap that keeps you from reaching your goals.
The Everything Guide to Intermittent Fasting
When you don’t know what to eat to optimize your nutrition — whether you’re fasting 5:2, 16:8, or 24 hours — this book provides 100 tasty recipes (like breakfast chia bowl and spinach and feta stuffed chicken breasts) to stay on track and healthy.
The Every-Other-Day Diet
Count this book by IF researcher Varady as your how-to book on successfully adapting an alternate-day fast. She applies knowledge that has been garnered from what best worked for research subjects in her experiments, specifically spells out step by step exactly how to execute an alternate-day plan of fasting with the hope that you lose weight, and instructs how to maintain.
A hugely sought-after read by Jason Fung, MD, a well-liked advocate for intermittent fasting and low-carb diets, this book goes in-depth about what you should prepare yourself for when you begin fasting — the good and the bad — including how to do it, advantages, and how to monitor your progress.
Favorite E-Book
Intermittent Fasting: An Introduction
A group of nutrition researchers at Precision Nutrition, a for-profit nutrition education and coaching business, put together an extremely comprehensive (and free) e-book on all you want to know about IF, including benefits, weight loss, selecting an IF schedule, and precisely what to do to implement it.
Favorite Podcast
The Human Upgrade With Dave Asprey
Not sure where to start? Science writer and podcaster Dave Asprey has a list of nine have-to-listen episodes that explore IF in depth, making it a solid roundup when you’d like to take a deeper dive. The episodes will guide you through some of the more science-y subjects, such as how fasting enhances metabolic flexibility, why fat is your friend, and how fasting impacts women’s hormones.
Favorite Apps
EatWise — Meal Reminder
iOS, Android; free
Don’t lose your way. This app lets you schedule how many meals you’re having, monitor the time between meals, and receive reminders on when to eat. There’s also a section to monitor weight loss progress.
Fastic: Intermittent Fasting
iOS, Android; free, with in-app purchases and paid memberships
You have all you need to get the practical application of IF working for you: A fasting timer, water tracker, and meal plans, and you can connect with other individuals on IF to exchange fasting hacks.