How to fast: tips for how to fast safely

You’ve probably heard about fasting, and you may be wondering how to fast safely to capture the practice’s potential health benefits.

First, you need to understand that there are many different approaches to fasting. Some forms involve cutting out all food—or even water (“dry fasting”)—for days at a time. Other types simply compress the amount of food you eat each day into a smaller “fasting window.”

Research has yet to figure out which fasting approach is optimal. But studies do tie some of these approaches to real health improvements.

You’re probably used to eating two or three meals a day, plus snacks. But this habit of eating steadily throughout the day is relatively new. For example, the Ancient Romans ate only one large meal a day, usually late in the afternoon (Paoli, 2019).

The term “fasting” generally means to restrict your food intake for a longer-than-usual period of time (Sen, 2018). This is also sometimes called calorie restriction. 

Technically, you’re fasting whenever you’re not eating. But these days, when you hear about fasting, that usually means someone is going without food or energy-containing drinks (such as smoothies, soft drinks, juice, etc.) for anywhere from 12 hours to several days (Patterson, 2017).

Some people fast for religious purposes. Many Muslims, for example, do not eat or drink between sunrise and sunset during Ramadan—an annual religious observance (Sadeghirad, 2012). But recently, there has also been a rise in the popularity of fasting among those trying to lose weight or improve their health (Sen, 2018).  

There are some different fasting approaches that may be beneficial. Most of the research on fasting has looked at intermittent fasting (IF) plans. This is a broad term for a number of approaches that mix periods of low food or energy intake with periods of normal eating (Mattson, 2018).  

A lot of different fasting diets fall into this IF category. These diets include (Paterson, 2017; Mattson, 2018):

• Complete alternate-day fasting: You eat as usual one day of the week, and the next day you cut out all calorie-containing foods or drinks. Repeat indefinitely.  

• Modified alternate-day fasting: The same alternate-day approach, except on your fasting days, you eat roughly 30% of a typical day’s number of calories—rather than going totally without calories.

• 5:2 diet: You eat roughly 25% of a normal day’s calories on two consecutive days each week. The rest of the week, you eat as you usually would (Patterson, 2017).  

• Time-restricted feeding: You squeeze all of your food and energy intake into a daily 6–8 hour eating window. Then you fast for the remaining 16–18 hours of the day.

• Fasting-mimicking diet (FMD): You eat a specialized low-calorie, plant-based diet that “mimics” the physical effects of fasting. Research suggests your body responds as though you’re going without food, but you still get to eat something during the day. This diet also includes supplements to ensure you’re avoiding nutritional deficiencies (Wei, 2019). 

All of these IF fasting plans are essentially short-term fasting diets. Even if you stick with them for months or years, none of them severely restricts your calorie intake for more than two days at a time.

Meanwhile, long-term fasting plans restrict your calories for three days or longer. For example, periodic fasting diets usually last three (or more days), and people may perform them once every few weeks (Paoli, 2019). 

Finally, there are some fasting approaches—water fasting and dry fasting—that you may hear about from time to time. A water-only fast means that you’re eating nothing and drinking only water for an extended period (Finnell, 2018). A dry fast involves cutting out all food and liquids, which means you don’t even get to drink water (Papagiannopoulos-Vatopaidino, 2020). 

It’s worth noting that a lot of the fasting terms and definitions are slightly up for debate. Even experts sometimes disagree about how to use them.

While you should reach out to your healthcare provider before engaging in a new approach to eating, there are a few general safety tips that some experts recommend to people wanting to pursue a fasting regimen (Attiná, 2021):

• Avoid eating a processed or high-calorie meal right before the start of your fast.

• Avoid soft drinks or alcoholic beverages right before the start of your fast.

• If you plan on using caffeine during your fast, consume the Recommended Dietary Allowance of 200 mg/day.

• To best prepare your body for a fast, eating a Mediterranean-style diet can help provide your body with the micronutrients, fibers, and flavonoids it needs.

• Make sure you hydrate before and during your fast.

• Avoid strenuous activities like hard weightlifting workouts; instead, engage in a lighter activity like yoga.

• If you work in manual labor, it is best to plan your fasting days on your days off of work.

Your healthcare provider may have additional safety tips that they want you to follow before fasting or tips that differ from those above. 

Fasting causes several different things to happen inside your body, including shifting the way your body makes and uses energy and changing the activity of your hormones, cells, immune system, and metabolism (Finnell, 2018). 

Much of the research on fasting looks at its effects in animals, resulting in a long list of possible—though unproven—health benefits. Some of those include a lower risk for cancer, a longer lifespan, and a lower risk for brain diseases (Sen, 2019). 

Meanwhile, the results of clinical trials in people reveal numerous benefits. Some of these are (Finnell, 2018; Sen, 2019):

• Weight loss or reduced risk of obesity

• Improvement of blood pressure

• Cholesterol improvements

• Improvement of arthritis symptoms

• Lower risk for cardiovascular disease

• Lower risk for metabolic syndrome

• Reduced chronic pain symptoms

• Improved energy and quality of life

• Improved skin 

• Reduction in symptoms of chronic inflammatory disorders

While those findings are promising, experts are still sorting out many of the details. A lot of the long-term effects of specific fasting plans aren’t known. There’s some evidence that the effects of fasting change—for better, but maybe also for worse—if people practice these diets when they’re older, instead of early in life (Sen, 2018).

The big takeaway here is that the research on fasting’s benefits is encouraging. But there’s a lot we still don’t know.

Here again, the research is all over the place. Some of the risks associated with fasting may be short-lived and minor, while others may be more serious or likely to emerge only during longer fasts.

Some of the common short-term risks and side-effects include (Finnell, 2018; Willhelmi de Toledo, 2013):

• Fatigue

• Nausea

• Food cravings

• Insomnia

• Lightheadedness

• Blurry vision

• Headaches

• Upset stomach

• Back pain

• Muscle cramps

• Heart arrhythmias

While less common, some serious health complications are possible. Dehydration and sodium deficiency—both of which can lead to hospitalization—can occur in people who are fasting. Nutrient deficiencies are also a risk (Finnell, 2018).

Some of the possible risks may change depending on your age, health status, weight status, and other factors. For example, research on mice shows that intermittent fasting lengthens life among young mice, but it has the opposite effect among older mice (Sen, 2018). 

These are the sorts of nuances that experts are still sorting out regarding fasting diets and their effects on humans. 

Research on intermittent fasting methods, such as alternate-day fasting, has turned up a few serious short-term safety concerns (Hoddy, 2015). However, some researchers have argued that these fasting plans are understudied in people with chronic health issues, such as type-2 diabetes. Also, the long-term effects of these diets aren’t well-understood (Horne, 2020).  

More work has found that, for people who are unwell and turning to longer fasts as a form of therapy, these diets may be risky. People who have had an eating disorder, thyroid disease, organ disease, muscle weakness or wasting should be cautious. The same is true if you’re pregnant, depressed, on medication, underweight, or if you have type-1 or type-2 diabetes, heart disease, or other medical conditions (Willhelmi de Toledo, 2013). 

Because fasting may be risky, you should seek medical advice from a licensed dietitian or some other medical provider first if you’re considering any one of these diets (Willhelmi de Toledo, 2013). 

While there are some risks associated with fasting, most serious problems have turned up among those practicing more extreme approaches—such as diets that restrict your food or energy intake for three days or longer or those that cut out water and other forms of hydration. 

When it comes to intermittent fasting plans (alternate-day fasting, time-restricted eating, etc.), most studies show few, if any, significant health complications or problems (Welton, 2020). 

Additionally, most of the research on fasting’s benefits examines intermittent fasting, not long-term fasts. It’s not at all clear that fasting for more extended periods is somehow better or healthier than intermittent fasting (Ganesan, 2018). 

According to resources from Johns Hopkins University, intermittent fasting is safe for anyone 18 or older who is not pregnant or breastfeeding and who has no history of an eating disorder or blood-sugar problems (such as diabetes). Again, talk to your healthcare provider before starting a new fasting diet—or if you begin to experience anxiety, headaches, nausea, or other worrying symptoms once you begin fasting (JHU, n.d.). 

Right now, no one can tell you for sure which type of fasting diet is best. Also, many of the benefits and risks remain scant. But what we know today is that intermittent fasting approaches seem to be relatively safe and may help treat or prevent some health problems.

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Current version

October 19, 2021

Written by

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Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.