What to Expect When Intermittent Fasting

intermittent fasting

Westend61/Getty Images

In This Article

Intermittent fasting refers to a restrictive eating pattern that has many different variations. The common thread is that food consumption only occurs during specified periods of time. Those who practice intermittent fasting restrict food intake or eliminate it altogether during certain times of the day, week, or month for religious reasons, to gain health benefits, or to lose weight.

Studies have shown that intermittent fasting may improve certain health outcomes including body weight, longevity, and glucose control. More long-term studies are needed to confirm these benefits.

What to Eat

Intermittent fasting (IF) differs from other diets in that there are no recommended foods and there are no foods that are eliminated or restricted. In fact, for most variations of intermittent fasting on the days (or during the hours) when food consumption is not restricted, those following the eating plan consume an ad libitum diet.

"Ad libitum" is a Latin phrase that means "as much as necessary or desired." So during an eating phase of an intermittent fasting diet, you can eat whatever you want and as much as you want.

This ad libitum feature makes intermittent fasting appealing to some people who don't want to eliminate certain foods from their diets.

Intermittent fasting is appealing to many people who want to lose weight or improve their health without giving up foods that they enjoy. It is easier to restrict food intake sometimes if you know that you can eat whatever you want at other times.

It is important to note, however, that religious variations of intermittent fasting don't necessarily include ad libitum phases. For example, during Ramadan, healthy adult Muslims fast between sunrise and sunset. But during the suhoor (before sunrise) and iftar (after sunset) meals, they still eat halal (allowed) foods and avoid foods that are haram (forbidden).

Recommended Timing

Food timing is what makes intermittent fasting unique from other eating styles or dietary patterns. Each type of intermittent fasting comes with its own rules for when to eat and when to avoid or restrict food.

Types of Intermittent Fasting
Intermittent fasting with no calorie restriction Alternate between 24-hour periods of fasting and feeding with 200 percent of normal intake on fed days
Intermittent fasting with calorie restriction Alternate between 24-hour periods of fasting and feeding with 150 percent of normal intake on fed days
Time-restricted feeding Food intake is limited to specific time periods during the day, typically lasting between six and eight hours each day.
Alternate day fasting (complete) Consume no calories on fasting days and alternating fasting days with a day of unrestricted food intake, or a “feast” day
Alternate day fasting (modified) Consume less than 25 percent of baseline energy needs on “fasting” days, alternated with a day of unrestricted food intake, or a “feast” day
5:2 Diet Eating is unrestricted for five straight days each week, followed by 2 days of restricted caloric intake.
Periodic fasting Caloric intake is restricted for multiple consecutive days, such as five days in a row once a month, and unrestricted on all other days
Religious fasting A wide variety of fasting regimens are undertaken for religious or spiritual purposes.

Intermittent Fasting With No Caloric Restriction

This variation of intermittent fasting is one of the more popular styles simply because it is easy to follow. It is also appealing to some because it allows for indulgence. The basic premise is that on one day you fast and on the next, you are allowed to feast.

Consumers may find online sources (such as blogs or websites) devoted to this style of eating that offer guidelines or tips for following the program. Researchers who study intermittent fasting are investigating how fasting days impact food intake on the feast days. In current studies, during fasting days study participants are only permitted to drink water, herbal teas and black tea/coffee with no sugar.

Ongoing research will help scientists understand how this feeding style impacts overall health and physical activity levels.

Intermittent Fasting With Caloric Restriction

This variation of intermittent fasting allows for every-other-day feasting but limits the caloric intake on those days to 150 percent of your typical food intake. Fasting days still require a complete fast where only water, herbal teas, and black tea/coffee with no sugar is permitted.

This variation of IF may be less appealing to some because it requires that you monitor your calorie intake on days when you feast. In addition, some people prefer no limitations at all on days when they are allowed to eat.

Time-Restricted Feeding

Consumers who follow this variation of intermittent fasting restrict their food intake during certain hours of the day. For example, they might eat during a short window in the middle of the day generally lasting 6–8 hours. Complete fasting occurs during the other hours of the day.

During the eating window, food consumption may be modified or restricted in some way or it may be completely unrestricted (ad libitum). This eating pattern allows people to eat in accordance with normal daily circadian rhythms (usually daytime hours). Some researchers believe that this eating pattern that is necessary for optimal metabolic health.

Alternate Day Fasting

Alternate day fasting is really a variation of intermittent fasting with no caloric restriction. But consumers often see the words "Alternate Day Fasting" or "ADF" applied to this eating pattern in blogs or websites that promote the program. As with all IF programs, there are variations to the plan.

Some alternate day fasting (ADF) programs include eating 20 percent of your typical food intake on a fast day and then consume food ad libitum on the feed days. Others require you to determine your energy needs (recommended calorie intake) and consume 20 percent to 25 percent of that calorie number on the fast days and again eat whatever you want on the feed days.

5:2 Diet

This popular IF diet was developed by Dr. Michael Mosley, a UK-based journalist who did his original training as a doctor in London. Mosley wrote a book called The Fast Diet that outlines the program.

As the name implies, on this eating program you fast for two days of the week. On these days you reduce your calorie intake to 500 calories for women and 600 calories for men. The assumption is that by using these numbers men and women will consume 25 percent of their normal daily calorie needs.

Calorie intake on your non-fasting days does follow a calorie recommendation based on activity level. In short, you consume only the calories allowed according to your recommended total daily energy intake (TDEE). But Mosely explains that on these days you should be able to follow a pattern of "normal eating, with little thought to calorie control and a slice of pie for pudding if that’s what you want."

Periodic Fasting

The 5:2 diet is a type of periodic fasting. However, there are other variations. In short, this eating pattern involves fasting during occasional times of the week or month and then eating a typical diet during the rest of the time.

Periodic fasting usually does not include a "feasting" stage and is usually not associated with indulgent eating, but rather a modest or "normal" eating style on non-fasting days.

For example, some people take 2-3 days per month to fast in order to maintain their weight. Various detox diets (such as the 3-day diet and other detox plans) are variations of periodic fasting.

Religious Fasting

Some people fast for religious or spiritual reasons. For example, those who practice Islam might fast from dawn to sunset during the holy months of Ramadan. In addition, Latter-day Saints followers abstain from food and drink for extended periods of time and some Seventh-day Adventists consume their last of two daily meals in the afternoon, resulting in an extended nighttime fasting interval.

Resources and Tips

Common questions about intermittent fasting revolve around the best duration of the program, health benefits and/or the potential for weight loss, and what type of eating program should serve as a transition if you've used intermittent fasting for weight loss.

The best resource for the answers will be your healthcare provider. Since intermittent fasting includes periods of severe food restriction, you should get medical clearance before following the program. You might also get a referral to a registered dietitian (R.D.) who can provide personalized advice based on your medical profile.

There are, however, general guidelines that can provide some answers to the most common questions.

What Health Benefits Can I Expect from Intermittent Fasting?

Many studies investigating the health benefits of intermittent fasting have been performed on animals. For example, many studies have shown an extension of lifespan and reduced rates of several diseases, especially cancers. But since many of the studies have been performed on mice it may be premature to assume that all of the science promoting the benefits of IF is applicable to humans.

Human studies have suggested that intermittent fasting may improve body composition and body weight in overweight individuals. There is also mixed evidence suggesting that intermittent fasting may lead to reductions in insulin concentrations, improvements in lipid levels, or reductions in inflammatory factors.

How Long Should You Maintain Intermittent Fasting?

A common question asked by many consumers who consider intermittent fasting is about the longevity of the eating program. How long can one maintain an intermittent fasting lifestyle? Unfortunately, because there is no single eating style that defines intermittent fasting, there is no clear answer, but researchers have addressed the questions in published reports.

For example, some nutrition scientists have concerns about adherence to the eating style. While ad libitum eating is easy to maintain, fasting days can become tiresome.

In addition, some scientists have raised concerns about the safety of going on a long-term program that includes severe caloric restriction, stating that there is not enough evidence to know for sure if it is safe. Although current evidence suggests that intermittent fasting regimens are probably not physically or mentally in healthy, normal weight, overweight, or obese adults.

Some long-term observational studies have investigated the long-term health benefits of those that practice religious fasting protocols. In those studies, those who routinely fasted were less likely to have clogged arteries or coronary artery disease, according to the National Institutes of Aging.

However, the source notes that these studies did not control for other factors that could have affected the results, such as the kind of diet, quality of food consumed, or use of nutritional supplements.

How Do I Transition Off Intermittent Fasting?

Michael Mosely who developed and promoted the 5:2 diet, recommends that consumers shift to a 6:1 approach. That is, eat a normal diet six days each week and fast one day each week.

Others may adopt an approach similar to the 80/20 eating style promoted on many fitness and health websites, where you eat a healthy, nutritious diet 80 percent of the time and enjoy more indulgent meals 20 percent of the time.

Modifications

When you get clearance from your healthcare provider, he or she may provide modifications to the IF program you have chosen, especially to the caloric restriction on your fasting days. For example, if you are on a medication that you need to take with food, you may need to avoid programs that completely restrict food intake on certain days.

There are contraindications that have been suggested with regards to IF and you may want to address these with your healthcare provider, as well. It has been suggested that certain people should avoid intermittent fasting, including:

  • Those who are underweight or have an eating disorder
  • Anyone under 18 years old
  • People with diabetes (especially those on medication)
  • Pregnant women or women who are breastfeeding
  • People recovering from surgery

But even if you don't have a health concern that limits your participation in intermittent fasting, there are ways to make the program more healthy.

If you are a healthy individual who has chosen intermittent fasting, keep in mind that choosing nutritious foods on your feast days will help you to maintain good health.

Whether you follow intermittent fasting or a more typical eating pattern, your body still needs the important macro and micronutrients provided by fruits and vegetables, whole grains, healthy fats, and lean protein. If you consume these foods as part of your IF program, you'll be more likely to choose them and maintain a healthy body after you transition off the plan.

Was this page helpful?

Article Sources

  1. Ganesan K, Habboush Y, Sultan S. Intermittent Fasting: The Choice for a Healthier Lifestyle. Cureus. 2018;10(7):e2947. Published 2018 Jul 9. doi:10.7759/cureus.2947

  2. Sabry WM, Vohra A. Role of Islam in the management of Psychiatric disordersIndian J Psychiatry. 2013;55(Suppl 2):S205–S214. doi:10.4103/0019-5545.105534

  3. Templeman I, Thompson D, Gonzalez J, et al. Intermittent fasting, energy balance and associated health outcomes in adults: study protocol for a randomised controlled trialTrials. 2018;19(1). doi:10.1186/s13063-018-2451-8.

  4. Klempel MC, Kroeger CM, Bhutani S, Trepanowski JF, Varady KA. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese womenNutr J. 2012;11:98. Published 2012 Nov 21. doi:10.1186/1475-2891-11-98

  5. Patterson RE, Laughlin GA, LaCroix AZ, et al. Intermittent Fasting and Human Metabolic HealthJ Acad Nutr Diet. 2015;115(8):1203–1212. doi:10.1016/j.jand.2015.02.018

  6. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of FastingObesity. 2017;26(2):254-268. doi:10.1002/oby.22065.

  7. Calorie Restriction and Fasting Diets: What Do We Know? National Institute on Aging. August 14, 2018.

  8. Finnell JS, Saul BC, Goldhamer AC, Myers TR. Is fasting safe? A chart review of adverse events during medically supervised, water-only fastingBMC Complement Altern Med. 2018;18(1):67. Published 2018 Feb 20. doi:10.1186/s12906-018-2136-6

Additional Reading