Updated 18th April 2024

How to fast: Your guide to fasting safely

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In scientific terms, fasting means taking in no calories for a set period. This might be for a few hours every day or sometimes for entire days. 

People fast for a variety of reasons. For instance, some people do it as part of a religious practice, while others are looking for health benefits

And there’s evidence that certain forms of fasting might help you improve your heart health, blood sugar control, and other aspects of health.

In this article, we’ll outline some of the many forms of fasting and the health benefits that they might provide.

We’ll also offer some tips for fasting safely if you're concerned or just thinking of trying it out. 

Types of fasting

There are many different ways to fast. One of the most common is intermittent fasting, which involves switching between periods of fasting and eating.

Time-restricted eating (TRE) is a common form of intermittent fasting. It means that you eat within a set window of time every day.

Some approaches to TRE are:

  • 16/8: You fast for 16 hours, then eat within an 8-hour window. So, you might eat breakfast at 10 a.m. and stop eating at 6 p.m.

  • 18/6: You fast for 18 hours and eat within a 6-hour window. So, you could eat between noon and 6 p.m.

  • 20/4: You fast for 20 hours and eat for 4 hours. For example, you might only eat between midday and 4 p.m. 

Other versions of fasting involve severely limiting your calorie intake on certain days. 

For instance, on the 5:2 diet, you eat as you normally would for 5 days of the week and restrict your energy intake to around 500–800 calories every day for the other 2 days.

Another form of intermittent fasting involves fasting for 24 hours once or twice a week.

Some people fast for several days at a time. But there’s no good evidence that long fasts benefit your health or support weight loss. Plus, there are risks.

It’s a good idea to consult your doctor before starting this type of fast. 

Are there any health benefits?

A recent review and meta-analysis found several benefits of intermittent fasting. We explore some of these below.

Weight loss

Since intermittent fasting doesn’t require calorie counting or cutting out specific food groups, it’s popular among people who are trying to lose weight

Several studies show that this approach may be associated with weight loss, especially in people with obesity. 

Still, recent evidence suggests that intermittent fasting might not be any more effective than calorie counting

Blood sugar control 

Some clinical trials have found an association between fasting and improvements in insulin sensitivity and blood sugar control.

Improving your insulin sensitivity can help reduce the risk of developing type 2 diabetes. 

Heart health

A number of studies have found that intermittent fasting may improve heart health by reducing certain risk factors for cardiovascular disease.

For instance, intermittent fasting is linked to reduced levels of triglycerides, total cholesterol, and low-density lipoprotein cholesterol in your blood. This last type is also known as LDL or “bad” cholesterol.

Plus, intermittent fasting may lower blood pressure and limit inflammation, which can also reduce the risk of cardiovascular disease.

Improved gut health

There’s some evidence that TRE can alter your gut microbiome — the trillions of bacteria that live in your gut. 

This approach may increase the number of beneficial bacteria in your gut.  

Plus, TRE may improve the function of your gut barrier, as Prof. Tim Spector, ZOE’s scientific co-founder, explained in an episode of the ZOE Science & Nutrition podcast.  

Psychological changes

In 2022, ZOE launched the largest-ever community-based study of intermittent fasting, which is sometimes called "IF."

More than 100,000 participants joined The Big IF Study

Early results show that intermittent fasting was associated with a 9% increase in mood and an 18% increase in energy, compared with participants’ baselines before they started these fasts.

Participants also reported feeling less nervous and hungry. 

At ZOE, we know that everyone responds differently to food — even identical twins. 

If you’d like to know how your blood sugar and blood fat levels respond to food, you can start by taking our free quiz

When you join ZOE, we’ll also analyze your gut microbiome and provide ongoing nutrition support to help you move toward your long-term health goals.

Is intermittent fasting safe?

When done correctly, intermittent fasting is safe for most people. But it may be uncomfortable to begin with

Overall, fasting does have risks to consider. For instance, some people may have migraine episodes, headaches, fatigue, and mood changes.

There’s also a risk that it can lead to disordered eating.

Plus, fasting may make you feel mentally sluggish. And it can affect your ability to do physical activity.

People who are pregnant or breastfeeding, people with type 1 diabetes, and anyone with a history of disordered eating shouldn’t try intermittent fasting. 

Also, older adults and people who need to take medication with food should speak with a doctor before trying any type of fast.  

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Tips for fasting safely

If you’re thinking of trying intermittent fasting, here are some tips to help you stay safe and get the most out of it.

If you’re new to fasting, take it slow 

Start with a time-restricted fast, such as the 16:8 approach that we describe above. Or, you might even start with a 12:12 fast to get used to eating within a specific time frame. 

Gradually, you can reduce your eating window. Taking it slowly may make it easier to keep it up in the long run.

Avoid extended fasts

Experts don’t recommend fasting for days at a time. It’s not a sustainable way to manage your weight, and the side effects might interfere with your daily life. 

So, it’s a good idea to limit your fasting to small, realistic windows. 

If fasting makes you feel bad, stop 

If you have severe headaches, lethargy, or hunger, stop fasting. If you feel really unwell in any way, don’t continue. 

Stay hydrated

The body can survive without food for some time, but it can’t function without water. So, continue to drink plenty of water while you’re fasting. 

Aside from plain water, you can also drink black coffee and black tea during a fast. These also help keep you hydrated.

It might help to keep an eye on the toilet. Your urine should be pale yellow. This shows that you’re properly hydrated. 

Eat well

When you’re not fasting, be sure to have a healthy, diverse diet with plenty of plant foods. 

This is always a good plan, but if you’re just starting fasting, it’s even more important to get all the nutrients that your body needs to thrive.

Avoid extreme physical activity while fasting

Physical activity is safe, but if you’re just starting to fast, keep it relatively light.

The body uses glucose for energy during exercise, so working out may make you feel fatigued and limit your performance.

Talk to a healthcare provider before you start

If you have a medical condition or any medical concerns, check with a doctor before you try intermittent fasting. 


There are several types of fast, and people decide to try them for a variety of reasons.

Research suggests that intermittent fasting may have health benefits, such as improvements in blood sugar control, heart health, and mood. 

Although intermittent fasting is safe for most people, some of us should avoid it.

These groups include pregnant and breastfeeding people, anyone with a history of disordered eating, and people with type 1 diabetes.

And if you have a chronic health conditions or need to take any medication with food, it’s especially important to check with a healthcare provider before you try fasting. 

To do it safely, start gradually — avoid fasting for long periods. Listen to your body, stay hydrated, and don’t do intense physical activity. Be sure to discuss any concerns with a healthcare provider. 


A meta-analysis comparing the effectiveness of alternate day fasting, the 5:2 diet, and time-restricted eating for weight loss. Obesity. (2022). https://onlinelibrary.wiley.com/doi/10.1002/oby.23568

Effectiveness of intermittent fasting and time-restricted feeding compared to continuous energy restriction for weight loss. Nutrients. (2019). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6836017/

Effect of time-restricted feeding on metabolic risk and circadian rhythm associated with gut microbiome in healthy males. British Journal of Nutrition. (2020). https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/effect-of-timerestricted-feeding-on-metabolic-risk-and-circadian-rhythm-associated-with-gut-microbiome-in-healthy-males/A8C3BF83CBE5BF9CAC65ED783FA0FFD2 

Health effects of the time-restricted eating in adults with obesity: A systematic review and meta-analysis. Frontiers in Nutrition. (2023). https://www.frontiersin.org/articles/10.3389/fnut.2023.1079250/full

Intermittent fasting and metabolic health. Nutrients. (2022). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8839325/

Intermittent fasting and weight loss. Canadian Family Physician. (2020). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/ 

Intermittent fasting in cardiovascular disorders — an overview. Nutrients. (2019). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6471315/

Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: A systematic review and meta-analysis of randomized controlled trials. Journal of Translational Medicine. (2018). https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-018-1748-4 

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