Updated 10th October 2022

What does science say about fasting?

Fasting can mean different things to different people. From a scientific point of view, it means taking in no calories. 

People fast for a variety of reasons. It might be a religious practice, or an effort to build self discipline or benefit health. 

In this article, we’ll delve into how your body responds to fasting and whether it really has health benefits.

Types of fasting

Fasting can be intermittent or prolonged.

“Intermittent” means that you fast for less than a couple of days or you switch between eating and fasting. “Prolonged” means fasting from 2 days to several weeks.

A popular form of intermittent fasting is time-restricted eating. This involves only eating within a certain time period each day.

Some of the most common methods include:

  • 16/8: eating in an 8-hour window (say from 10 a.m. to 6 p.m.) and fasting for the rest of the day

  • 18/6: eating in a 6-hour window and fasting for the remaining 18 hours

  • 20/4: eating in a 4-hour window and fasting the rest of the day

Other versions of intermittent fasting include alternate day fasting and the 5:2 diet. This involves only eating on 5 days and fasting for the other 2 days of the week.

But in both of these versions, people often eat small amounts on their “fasting” days.

Prolonged, or periodic, fasts last for several days or weeks. Before starting this, talk to a healthcare professional to make sure it’s appropriate for you.  

Regardless of the type of fast, your body responds to the lack of fuel. There may be potential health benefits, but there are also some risks. Fasting may not be appropriate for everyone.

And no matter your eating schedule, the foods you choose are important. 

With the ZOE at-home test, we analyze your blood sugar and blood fat responses, as well as your gut microbiome. Using this information, we’ll give you personalized nutrition advice so you find the best way to eat for your body.

To get started, take our free quiz.

What happens when you’re fasting?

Your body has an incredible ability to adapt. Anytime you go without fuel, your body makes several changes in response. Here’s what happens to your body when you fast.

Change in energy source

Your body uses compounds in food for energy. Typically, it breaks down carbohydrates into simple sugar molecules. 

This is your body’s main source of energy. Whatever isn’t used gets stored in your liver as glycogen and as fatty acids in your fat cells.  

For the first few hours of not eating, your body pulls from its glycogen stores for energy. But as these stores eventually run out, your body finds a different fuel source. This is known as the metabolic switch

When glucose is no longer available, your body begins to break down body fat into ketones. Next, these are transported to your cells and used as energy. When you have an increased amount of ketones in your blood, you’re said to be in “ketosis.” 

Depending on how much energy you use, what your last meal was, and the amount of stored glycogen in your liver, this switch can take place after going 12–36 hours without food.

Stress resistance

As you repeatedly go without food, your cells undergo a coordinated response to the stress that fasting exposes them to. 

Scientists believe that this adaptive response allows your cells to improve their resistance to stress and disease — though most of the relevant research stems from animal and cell studies.

Autophagy

Autophagy means “self-eating.” It's the process your cells use to clean out and recycle old, damaged, or abnormal proteins and cell components.

Autophagy is one way that researchers think your cells become more resilient when faced with stress.

This process is an important part of maintaining healthy cells. Autophagy likely plays a key role in preventing diseases, including heart disease, diabetes, cancer, and infections.

Autophagy happens all the time in our cells. If it doesn’t work properly, it’s harmful to cells in a way that’s linked with health conditions. 

Fasting may be able to enhance autophagy, and it could be a simple and safe way to do this.

However, most of our evidence currently comes from animal studies. The evidence in humans is limited. 

Gut health 

In your gut, there are trillions of bacteria and other microorganisms, collectively known as your gut microbiome. The gut microbiome affects your overall health in many ways.

Evidence suggests that some forms of fasting may benefit your gut microbiome.

Studies involving men who fasted for 16 hours a day found that the participants had more beneficial bacteria in their gut microbiomes. These bacteria have been linked with a variety of health benefits, including better metabolic health, improved heart health, and a lower risk of obesity, cancer, and inflammatory bowel disease.

In one study, however, when the men returned to their normal eating patterns, the makeup of their microbiomes went back to how it was before they fasted.

The natural fasting period during sleep might suggest how a longer fast may improve the health of your gut barrier. 

During sleep, the activity in your gut slows down, which allows the cells lining your gut to be repaired. 

Studies suggest that increasing this fasting window could strengthen your gut barrier, which may reduce chronic inflammation.

Research into the link between fasting and gut health is still ongoing, but these effects could help explain the reported benefits. Still, fully understanding this topic will require more research. 

Long-term benefits

Most research on fasting and long-term health focuses on intermittent fasting.

This area of study is in its early stages. Overall, understanding the effects of fasting over months or years requires more long-term studies. Recent evidence suggests that some forms of fasting may benefit your health over time. Here’s what the current research says.

Better blood sugar control

Some forms of intermittent fasting may help you control your blood sugar. 

Multiple studies suggest that the alternate-day method of intermittent fasting may help improve insulin sensitivity and blood sugar control.

Still, fully understanding the relationship between fasting and blood sugar control in the long term requires more research. 

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Heart health

Intermittent fasting may also support heart health over time. 

Evidence from multiple studies suggests that intermittent fasting may help improve heart health by:

  • reducing blood pressure

  • lowering inflammation

  • reducing levels of low-density lipoprotein (also called LDL or “bad”) cholesterol and triglycerides

  • raising levels of high-density lipoprotein, or HDL, cholesterol — the type that promotes heart health

Limited research has associated these effects with alternate day, time-restricted, and modified methods of intermittent fasting.

Possible weight loss

Short-term fasting, such as intermittent fasting, may help some people lose weight. But growing evidence suggests that this may not be more effective than traditional calorie-restricted diets. 

There’s less research about prolonged fasting. While it may also help people lose weight — evidence shows that any lost weight may not come from body fat. 

In one small study, men who fasted for 10 days experienced, on average, a 7% decrease in body weight. However, 60% of the weight loss was from a decrease in water, protein, and other lean tissues.

Currently, the research doesn’t indicate that fasting is particularly effective for weight loss, but more research is needed. 

The risks

Different types of fasting may help improve your health, but there are risks to consider.

These include: 

  • Fatigue and mood changes: If you’re not eating for a stretch of time, you may feel more lethargic. Low energy, irritability, and difficulty concentrating are common when people start fasting, though these symptoms often improve over time.

  • Migraine headaches: Skipping meals is a common migraine trigger. Interestingly, one study involving almost 300 people with migraine found that after consistent fasts, such as during the month of Ramadan, these headaches may actually subside over time.

  • Disordered eating: Fasting may promote a negative relationship with food for those at risk of disordered eating. Intermittent fasting, in particular, may be related to an increased risk of binge eating. 

Who shouldn’t fast?

Fasting may not be appropriate for everyone. Those who should avoid it include: 

  • pregnant women

  • people with a history of disordered eating 

  • older adults

  • people with type 1 diabetes

  • anyone who takes medication with food

Summary

Fasting can mean going without foods and drinks that contain calories. There are different types, depending on how long you fast. 

Fasting may have positive effects on your health, such as promoting heart health, better blood sugar control, and weight loss. 

However, there are risks. These include feeling tired and irritable, particularly when you start, and having migraine headaches. Also, people with certain health factors should avoid fasting.

If you’re considering fasting, particularly prolonged fasting, talk to a healthcare professional to see if it’s right for you. 

With ZOE’s personalized nutrition program, you can discover how to eat for your body and your long-term health goals. 

You can take our free quiz to get started.

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