But it can be confusing to figure out how much of the hype is backed by evidence.
This article discusses the different types of fasting, including intermittent fasting, along with some of their main benefits.
What Is Fasting?
Fasting is a period of time when little to no food is eaten.
People may choose to fast for health, religious or spiritual reasons.
The main types of fasting include:
- intermittent fasting (IF)
- water fasting
- liquid fasting
- juice fasting
- religious fasting
- fasting for a medical procedure.
There is some evidence that fasting was a natural part of life for our ancestors due to cycles of “feast or famine,” which may affect how our body stores and uses energy today (1).
Summary: Fasting involves eating little or no food for a set amount of time, and may be done for health, religious or spiritual reasons.
Types of Fasting
Each type of fasting has its own restrictions for when and how often you should eat.
Intermittent fasting (IF) is a pattern of eating that involves switching between periods of fasting and non-fasting.
IF is often a more manageable approach to fasting that offers many of its same benefits.
There are different types of IF including:
- The 5:2 Diet involves eating no more than 600 calories per day for two days a week, while eating normally the other five days.
- The 6:1 Diet (also called Eat Stop Eat) involves fasting for 24 hours once per week, without changing your diet on the remaining 6 days.
- Alternate Day Fasting usually involves alternating between fasting for 24 hours, then eating normally for the next 24 hours on repeat.
- The 16:8 Diet (also called the Leangains Method) involves eating within an 8-hour window everyday, and fasting for the other 16 hours. For example, many people choose to skip breakfast and eat between 12pm and 8pm.
Religious or Spiritual Fasting
One perceived benefit of fasting for those with spiritual beliefs is to “cleanse the soul.”
Most major religions promote periods of fasting, which often goes hand in hand with times of prayer.
Water fasting involves only consuming water, though some diets also allow black tea and coffee.
The length of water fasts can vary, but most last 24-72 hours. Some of the benefits of IF diets also apply to 24-hour water fasting since there’s an overlap with the 6:1 IF diet.
For example, one study found that a 24-hour water fast increased total cholesterol (including HDL, the “good” cholesterol) and reduced triglyceride levels and weight. However, the study was small and only lasted two days (2).
Medically-supervised water fasting has also shown to slightly reduce blood pressure (3).
However, while water fasting may have some potential benefits, it can also be an extreme and unhealthy way to diet.
In fact, one study found that water fasting can lead to fatigue, nausea, headaches, raised blood pressure, and even life-threatening cases of dehydration and low sodium levels (4).
Fasting for a Medical Procedure
Certain medical procedures require patients to fast beforehand.
This is to ensure that no unsafe regurgitation will occur under general anesthetic, and that images can be seen properly during a gastroscopy.
Summary: Intermittent fasting (IF) involves switching between periods of eating and fasting, while water fasting restricts all intake of food for a set period of time. Others choose to fast for religious or spiritual reasons. You may also be asked to fast before a medical procedure.
Health Benefits of Fasting
As you might expect, fasting can help you reduce your calorie intake because it limits the hours in a day that you can eat.
It can also help promote weight loss, which we’ll discuss more below.
The “thrifty genotype theory” suggests that we’ve evolved to benefit from periods of fasting. This means fasting may affect our biochemistry and metabolism and reduce the risk of conditions such as type 2 diabetes (7).
Fasting has also been linked with reducing inflammation and damage caused by free radicals.
Summary: Fasting diets can help reduce calorie intake, promote healthy aging, reduce inflammation and reduce risk of conditions like type 2 diabetes.
Benefits of Intermittent Fasting (IF)
IF has become increasingly popular as it’s a more manageable way to fast while still reaping many of its benefits.
Intermittent Fasting and Weight Loss
Studies have found specific weight loss benefits related to IF.
A trial from 2017 found that IF resulted in a similar amount of weight loss as traditional dieting (10).
Another study found that simply reducing the window of eating from 14 hours to 10-11 hours per day boosted weight loss (11).
Islamic fasting during Ramadan can also be described as a type of daily IF, since no food is eaten between sunrise and sunset everyday. This type of fasting amounts to an average intake of 1,220 calories per day and 1-2 kg weight loss, but most of this weight is usually regained within 2 weeks after Ramadan has ended (12, 13).
Similarly, studies among the Seventh-day Adventist population have found that having the largest meal of the day at breakfast time and eating fewer calories later in the day is associated with a healthier BMI level (14).
Intermittent Fasting May Reduce the Risk of Heart Disease, Diabetes and Depression
IF may also have a great effect on your physical and mental health.
Some studies have found that IF could help reduce the risk of heart disease since it’s been shown to improve cholesterol levels, reduce damage from free radicals, and decrease blood pressure (15, 16, 17, 18).
IF diets can also reduce the risk of diabetes. It’s been shown to improve insulin levels and insulin resistance to the same degree as traditional calorie restriction (19).
Practicing IF may also help people reduce symptoms of depression. One study found a significant reduction in anger, tension, confusion and low mood in a group of older men who were following an IF diet (22).
While many of these findings are promising, the evidence isn’t very strong. More research is needed to better verify these potential benefits.
Intermittent Fasting and Aging
Animal studies have found that IF may promote autophagy, a process our body uses to recycle damaged parts of cells that may potentially be harmful. This in itself could potentially slow down aging (25, 26).
This is a very interesting topic, but at the moment we need more research to look into whether IF definitely promotes healthy aging in humans.
Summary: Intermittent fasting (IF) involves regular cycles of fasting and non-fasting. IF can be an easier type of fasting to follow and may promote weight loss and healthy aging, reduce the risk of type 2 diabetes and heart disease, and improve mood.
What Are the Health Risks of Fasting?
Risks related to fasting can range from headaches and lack of energy to patterns of disordered eating and more serious health issues.
Who Should Avoid Fasting?
Fasting is not recommended for people who need a regular supply of nutrients, such as:
- growing children
- pregnant or breastfeeding women
- those who are malnourished
- some people with: type 1 diabetes, cancer, liver disease, kidney disease, reflux or heartburn (29, 30, 31, 32, 33).
Fasting May Lead to Disordered Eating
Fasting in an extreme or unhealthy way can be a symptom of an eating disorder.
Therefore, those who have an active or previous eating disorder should avoid fasting.
Females can be more susceptible to this. For example, a study of teenage girls found that those who fasted were at a higher risk of developing bulimia and binge-eating behavior as compared to those who dieted in a more traditional way (34).
Fasting Risks For Women
Some women report negative side effects related to fasting diets including:
- missed periods
- binge eating
- changes in metabolism
- early-onset menopause.
However, not much research has looked into why these things can happen, but it might be related to female hormones.
Other Possible Risks
Prolonged fasting has also been associated with:
- increased cholesterol
- pancreas damage
- worsened insulin function (which increases the risk of diabetes)
- irregular heartbeat, headaches and fainting
- slight reductions in athletic performance, exercise ability and muscle mass (35, 36, 37, 38, 39, 40).
There are even some reported cases of death caused by prolonged fasting as a weight loss method (41).
In general, more studies in humans need to be performed before we can say whether IF is a safe and healthy approach in the long-run (42).
Summary: Fasting is not recommended for those who need a regular supply of nutrients for their health, including pregnant or breastfeeding women, or people with certain medical conditions. Possible side effects related to fasting include malnutrition, dehydration, disordered eating and even death in some cases. But serious risks are rare and usually related to prolonged fasting, not IF.
Should You Try Fasting?
Fasting involves having little or no food for a period of time.
Intermittent fasting involves switching between periods of eating and fasting, while water fasting restricts all food intake for a set period of time. People may also fast for religious or spiritual reasons, or in preparation for a medical procedure.
There is some evidence that fasting may help with weight loss and reducing the risk of type 2 diabetes and heart disease. This is mostly because fasting typically helps people reduce their calorie intake.
The benefits of fasting may also include a reduction in inflammation and damage caused by free radicals, improved mood and even promoting healthy aging.
However, more research is needed to confirm that fasting offers such benefits, and in a variety of populations.
This is because fasting doesn’t seem to be suitable for everyone, including those who need a regular supply of nutrients for their health, like pregnant or breastfeeding women and people with certain medical conditions.
Fasting must be done with caution, as it can come with some serious health risks like malnutrition, dehydration or a development of disordered eating.
In general, if you’re fasting to lose weight or improve your health, you may want to consider less risky—and way more pleasurable—ways of eating to boost your health.
You may wish to discover the best diet for your tastes and goals by playing this short game.
Shared with permission from our friends at Diet vs Disease. This article was written by Maeve Hanan, a UK Registered Dietitian. Maeve graduated with a First Class Honours Degree in Dietetics from the University of Ulster Coleraine (Northern Ireland) in 2013. As a UK Registered Dietitian, Maeve worked in the NHS for over four years to gain a variety of clinical experience. This includes: infant and childhood nutrition, nutrition for older people, food allergies, eating disorders, weight management, diabetes, stroke, and gastrointestinal conditions.