Intermittent fasting is not a diet — it is a timed approach to eating. Unlike many other dietary plans, intermittent fasting does not specify which foods to eat or avoid. Intermittent fasting may be beneficial for health and weight management but is not suitable for everyone.
Intermittent fasting involves cycling between periods of eating and periods of fasting. At first, people may find it difficult to eat during a short window of time each day or to alternate between days of eating and not eating.
Intermittent fasting is a popular method that people use to:
- simplify their life
- lose weight
- improve their overall health and well-being, such as by minimizing the effects of aging
Though fasting is generally safe for most healthy, well-nourished people, it may not be appropriate for individuals who have any medical conditions. For those who may be able to safely fast according to their doctor, the following tips aim to help make the experience as easy and successful as possible.
1. Identify personal goals
Typically, a person who starts intermittent fasting has a goal in mind. It may be to lose weight, improve overall health, or improve metabolic health. A person’s ultimate goal will help them determine the most suitable fasting method and work out how many calories and nutrients they need to consume.
2. Pick the method
A person may try multiple potential methods when fasting for health reasons. They should pick a plan that suits their preferences and that they think they can stick to.
A few of the more popular fasting plans are:
- periodic fasting
- time-restricted eating
- alternate-day fasting
Typically, a person should stick with one fasting method for a month or longer to see whether it works for them before trying a different method. Anyone who has a medical condition should consult a healthcare professional before beginning any fasting method. Fasting is not a safe option for some people.
When deciding on a method, a person should remember that they do not need to eat a certain amount or type of food or avoid certain foods altogether. A person can eat what they want when following an intermittent fasting plan.
However, to reach health and weight management goals, it is a good idea to follow a balanced, high protein, high fiber, vegetable-rich diet during the eating periods.
Eating only foods that lack beneficial nutrients during eating periods can hinder health progress. It is also extremely important to drink lots of water or other no-calorie beverages throughout fasting periods.
This structure involves fasting within specific time periods, such as twice per week, as with the Eat Stop Eat plan and the 5:2 method.
Eat Stop Eat
Brad Pilon developed Eat Stop Eat, a fasting method that involves eating nothing for 24 hours twice a week. It does not matter on which days a person fasts or even when they begin. The only restriction is that fasting must last for 24 hours and occur on nonconsecutive days.
People who do not eat for 24 hours will likely become very hungry. Eat Stop Eat may not be the best method for people who are unfamiliar with fasting. A person should consult a doctor or registered dietitian before starting a fasting plan like this one.
A person on the 5:2 method eats 500–600 calories 2 days each week, on either consecutive or nonconsecutive days, depending on the specific plan.
Plans such as the Warrior Diet and the 16/8 or 14/10 method are considered time-restricted eating, in which a person consumes calories only within specific periods of time throughout the day.
Ori Hofmekler is the creator of the Warrior Diet, which entails eating very little for 20 hours each day. A person fasting in this way consumes all their typical food intake in the remaining 4 hours.
Eating a whole day’s worth of food in such a short time can make a person’s stomach quite uncomfortable. This is a more extreme fasting method. As with Eat Stop Eat, a person new to fasting may not want to start with this method and should consult a doctor before trying it.
Martin Berkhan created Leangains for weightlifters, but it has become popular among other people who are interested in fasting as well. Unlike Eat Stop Eat and the Warrior Diet, fasting for Leangains involves much shorter periods.
For example, males who choose the Leangains method fast for 16 hours and then eat what they want for the remaining 8 hours of the day. Females fast for 14 hours and eat what they want for the remaining 10 hours of the day.
During the fast, a person must avoid eating any food but can drink as many no-calorie beverages as they like.
16:8 method of intermittent fasting
16:8 intermittent fasting allows for a fast lasting 16 hours per day, with all foods eaten during the remaining 8 hours. The following time frames for eating are popular with this fasting plan, with nighttime hours included in the fasting time:
- 9 a.m. to 5 p.m.
- 10 a.m. to 6 p.m.
- noon to 8 p.m.
Some alternate-day fasting regimens add a third day of fasting each week. For the rest of the week, a person eats only the number of calories they burn during the day. Over time, this creates a calorie deficit that allows the person to lose weight if that is their goal.
3. Figure out calorie needs
There are no inherent dietary restrictions when intermittent fasting, but this does not mean that calories do not count.
People who are working with a doctor or dietitian to manage their weight need to create a calorie deficit, which means consuming fewer calories than they use. People who are looking to gain weight need to consume more calories than they use.
Many tools are available to help a person work out their calorie needs and determine how many calories they should consume each day to gain, lose, or maintain weight.
A person could also consult a healthcare professional or dietitian for guidance on how many calories they need. A professional can help a person determine the best foods for them and find an overall healthy way to lose weight.
4. Figure out a meal plan
A person interested in losing or gaining weight may find it helpful to plan what they are going to eat during the day or week.
Meal planning does not need to be overly restrictive. It considers calorie intake and incorporating proper nutrients into the diet. For example, the Centers for Disease Control and Prevention (CDC) recommends the MyPlatePlan, which focuses on showing a person food group targets for each day.
Meal planning offers many benefits, such as helping a person stick to their calorie count and ensuring they have the necessary ingredients on hand for recipes, quick meals, and snacks. As a potential bonus, meal planning could save money if it helps people waste less food.
5. Make the calories count
Not all calories are the same. Although these fasting methods do not limit the number of calories a person consumes during eating periods, it is essential to consider the nutritional value of the food.
In general, a person should aim to consume nutrient-dense foods, or foods with a high number of nutrients per calorie. They may not have to abandon less nutritious food entirely, but they should still practice moderation and focus on more nutritious options to gain the most benefits.
How effective is intermittent fasting?
Fasting has several effects on a person’s body:
- Fasting reduces levels of insulin, making it easier for the body to use stored fat.
- It lowers blood sugar, blood pressure, and inflammation levels.
- It may change the expression of certain genes, which can help the body protect itself from disease and promote longevity.
- It increases human growth hormone levels, which can help the body use body fat and grow muscle.
- According to a 2018 review of studies, calorie restriction and intermittent fasting can help the body activate a healing process called autophagy, which essentially means that the body digests or recycles old or damaged cell components.
Fasting dates back to ancient humans, who often went hours or days between meals because obtaining food was difficult. The human body adapted to this style of eating, allowing extended periods to pass between food intake times.
Intermittent fasting can be very effective for weight management. In fact, according to a 2020 review of studies, the practice may serve as a helpful tool in the treatment of obesity, though more long-term studies are needed.
This newer review backs up previous claims that fasting can help a person lose weight, though potentially not any more than other types of reduced calorie eating plans.
Research also suggests that fasting has a variety of other benefits.
It may be helpful for managing metabolic syndrome and diabetes. According to a 2010 study, it may help protect neuron function. Additionally, a small 2006 study indicates that fasting may be beneficial in treating digestive health conditions such as irritable bowel syndrome, although more research is needed. It may even extend the life spanTrusted Source, according to a study in mice.
For a healthy person, intermittent fasting has few potential side effects.
When a person first starts fasting, they may feel slightly physically and mentally sluggish as their body adjusts. After the adjustment, most people go back to functioning as they did before.
However, people with medical conditions should consult their doctor before beginning any fasting program. People who are particularly at risk of negative effects from fasting and who may require additional medical supervision include those who:
- are pregnant or nursing
- are trying to become pregnant
- are living with diabetes
- have difficulty regulating their blood sugar levels
- have low blood pressure
- take certain medications
- have eating disorders
- are underweight
The National Eating Disorders Association states that calorie restriction and fasting are risk factors for developing an eating disorder.
Also of note, the National Institute on Aging states that sufficient evidence is lacking to support the use of intermittent fasting, particularly for older adults. They recommend that a person always talk with their doctor before starting a diet plan that severely restricts calories.
Effects on exercise
For healthy individuals, intermittent fasting should not affect the ability to exercise, except during the period when their body is adjusting to the new eating schedule. After the adjustment period, a person should not experience any negative effects on their exercise routine as a result of fasting.
Those worried about losing muscle while fasting should be sure to consume enough protein during eating periods and engage in resistance training regularly. By keeping protein intake up, a person is less likely to lose muscle mass from fasting.
In an older 2016 study, researchers examined the effect on males who participated in both resistance training and intermittent fasting. They found that the participants lost overall body weight and maintained their muscle mass by following the 16:8 fasting plan.
However, more research is necessary to fully understand all the effects of intermittent fasting on the body.
Fasting is a natural part of the human life cycle. Most people have fasted unknowingly throughout their lifetimes by eating an early dinner and skipping breakfast the next day. More structured approaches may work well for some people.
It is important to keep in mind that although a person does not need to exclude certain foods from their diet while fasting, they should aim to eat a balanced diet rich in protein, fiber, fruits, and vegetables. It is also important to drink plenty of low calorie or no calorie fluids.
Though the average person will likely experience few or no side effects from fasting, people who have certain medical conditions or take certain medications should consult their doctor before trying a fasting plan.