sveta_zarzamora / iStock / Getty Images Plus

Health Benefits of Fasting: Splitting Fact From Fiction

By Maeve Hanan

Fasting is becoming increasingly popular as a way to boost health.

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

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.

For example, some Christians choose to fast during Lent, while people of Islamic faith fast from sunrise to sunset during Ramadan.

Water Fasting

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.

For example, before having a general anesthetic or gastroscopy you will usually be advised to avoid food for 6-8 hours and fluids for 2 hours.

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.

While still meeting nutritional requirements, restricting calorie intake promotes healthy aging and reduces the risk of heart disease, type 2 diabetes and cancer (5, 6).

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.

Resting the digestive tract may contribute to reducing inflammation as well, but there is not much evidence to support this (8, 9).

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).

IF may even help suppress the growth of cancer cells, at least in animal studies. However, there isn't good evidence that this occurs in humans as well (20, 21).

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

There is some evidence that IF may promote healthy aging by improving cellular repair and regeneration, reducing inflammation and free radical damage (23, 24).

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).

Autophagy may also help reduce the risk of Alzheimer's disease and the growth of certain tumors, but more studies are needed to investigate this further (27, 28).

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.

Reposted with permission from our media associate Diet vs Disease.

Show Comments ()

EcoWatch Daily Newsletter

In 2018, the Arctic region had the second-lowest overall sea-ice coverage on record. NOAAPMEL / YouTube

The Past 5 Years Were the Arctic's Warmest on Record

The Arctic is still warming at twice the rate of anywhere else on Earth, and the region's air temperatures in the past five years between 2014-2018 have exceeded all previous records since 1900, according to a peer-reviewed report released by the U.S. National Oceanographic and Atmospheric Administration (NOAA) on Tuesday.

The agency's 13th annual Arctic Report Card also concluded that 2018 was second only to 2016 in terms of the region's overall warmth.

Keep reading... Show less
Partial solar eclipse. ndersbknudsen, CC BY 2.0

3 Key Dangers of Solar Geoengineering and Why Some Critics Urge a Global Ban

By Justin Mikulka

A Harvard research team recently announced plans to perform early tests to shoot sunlight-reflecting particles into the high atmosphere to slow or reverse global warming.

These research efforts, which could take shape as soon as the first half of 2019, fall under the banner of a geoengineering technology known as solar radiation management, which is sometimes called "sun dimming."

Keep reading... Show less
Even an increase of 2°C would cause significant sea level rise. pxhere

Report: Current Climate Policies Will Warm the World by 3.3˚C

This past October, a widely disseminated United Nations report warned that far-reaching and significant climate impacts will already occur at 1.5˚C of warming by 2100.

But in a study released Tuesday, researchers determined that the current climate polices of governments around the world will push Earth towards 3.3˚C of warming. That's more than two times the aspirational 1.5˚C target adopted by nearly 200 nations under the 2015 Paris agreement.

Keep reading... Show less
garett_mosher / iStock / Getty Images

McDonald's to Reduce Antibiotics Use in Beef

In a significant win in the fight to save antibiotics, McDonald's—the largest and most iconic burger chain on the planet—announced Tuesday that it will address the use of antibiotics in its international supply chain for beef by 2021.

Keep reading... Show less
Protesters clashes with riot police on Foch avenue next to the Place de l'Etoile, setting cars ablaze during a Yellow Vest protest on Dec. 1 in Paris. Etienne De Malglaive / Getty Images

The Lesson From a Burning Paris: We Can’t Tax Our Way Out of the Climate Crisis

By Wenonah Hauter

The images from the streets of Paris over the past weeks are stark and poignant: thousands of angry protesters, largely representing the struggling French working class, resorting to mass civil unrest to express fear and frustration over a proposed new gas tax. For the moment, the protests have been successful. French President Emmanuel Macron backed off the new tax proposal, at least for six months. The popular uprising won, seemingly at the expense of the global fight against climate change and the future wellbeing of our planet.

Keep reading... Show less
Rainbow Mountains in Vinicuna, Perú. Megan Lough / UI International Programs / CC BY-ND 2.0

7 Reasons Why #Mountains Matter

December 11 is International Mountain Day, an annual occasion designated by the United Nations to celebrate Earth's precious mountains.

Mountains aren't just a sight to behold—they cover 22 percent of the planet's land surface and provide habitat for plants, animals and about 1 billion human beings. The vital landforms also supply critical resources such as fresh water, food and even renewable energy.

Keep reading... Show less
Tetra Images / DigitalVision / Getty Images

Don’t Stress About What Kind of Christmas Tree to Buy, but Reuse Artificial Trees and Compost Natural Ones

By Bert Cregg

Environmentally conscious consumers often ask me whether a real Christmas tree or an artificial one is the more sustainable choice. As a horticulture and forestry researcher, I know this question is also a concern for the Christmas tree industry, which is wary of losing market share to artificial trees.

Keep reading... Show less
The Woolsey Fire seen from Topanga, California on Nov. 9. Peter Buschmann / Forest Service, USDA

Hotter Planet Makes Extreme Weather Deadlier, New Study Finds

By Jake Johnson

With people across the globe mobilizing, putting their bodies on the line, and getting arrested en masse as part of a broad effort to force the political establishment to immediately pursue ambitious solutions to the climate crisis, new research published on Monday provided a grim look at what the future will bring if transformative change is not achieved: colossal flooding, bigger fires, stronger hurricanes and much more.

Keep reading... Show less


The best of EcoWatch, right in your inbox. Sign up for our email newsletter!