Fasting is abstaining from eating and drinking for a certain period of time. Some people fast during specific times of the year as a religious practice, but fasting is also commonly used to manage weight, improve blood sugar, and support better overall health.¹

Always consult your healthcare provider before fasting—it may be unsafe if you have low blood pressure or underlying health conditions that may worsen while fasting for weeks.¹

Types of Fasting

There are several types of fasting. Intermittent fasting (IF) is the most popular type for weight management, reducing inflammation, and preventing symptoms of chronic health conditions.¹

Prolonged Fasting

Prolonged fasting involves reducing your calorie intake of food for more than two days. Typically, prolonged fasting lasts seven days or longer. Drinking a good amount of water during prolonged fasting keeps you from being dehydrated.¹

Fasts lasting no longer than 20 days are generally safe but have some risks. Not eating enough calories can cause a significant loss of muscle mass and electrolytes (electrically charged minerals found in blood, sweat, and urine that help regulate the nervous system, hydration, muscle function, and blood pH).²

People with hypotension (low blood pressure), heart conditions, or who are underweight are at greater risk of health complications when fasting for prolonged periods. While fasting, your body readjusts to your new calorie intake schedule, which may cause changes in your hormones and how quickly your body processes food.²

Intermittent Fasting (IF) 

Intermittent fasting (IF) involves alternating periods of fasting. Intermittent fasting includes time-restricted feeding (TRF), alternate-day fasting (ADF), modified alternate-day fasting (MADF), and periodic fasting.³

• Time-restricted feeding: Eating during the same window of time every day. The time frame you choose to eat during can be between 12-20 hours of the day. Some people who practice TRF use the 16:8 method—fasting for 16 hours and eating within an eight-hour window.³
• Alternate-day fasting: Choosing to fast every other day for 24 hours
• Modified alternate-day fasting: A less restrictive version of ADF. During modified ADF, you can have a small meal of around 500 calories on a fasting day.⁴
• Periodic fasting: Occurs once or twice a week every few months. Most periodic fasts last 24 hours and do not require calorie intake, while modified versions allow for a small amount of calories during fasting.⁵

No rules exist regarding what to eat when you break your fast. However, foods high in protein, fiber, and healthy fats support weight management and improve metabolic (blood sugar and lipid levels) health.³

The research cited in this article claims that fasting can lead to weight loss. An individual’s weight is affected by a variety of biological, environmental, and social factors. Health.com does not promote or condone weight loss that’s not under the care of a healthcare provider. Please contact your healthcare provider if you’re concerned about losing weight responsibly and healthfully.

Religious Fasting

Fasting is a common religious practice to show devotion and faith. For example, Ramadan is a holy month in the Islamic tradition, during which people refrain from eating and drinking from sunrise to sunset. During this time, people wake up before dawn to eat and break their fast in the evening after sunset for the month.⁶

Some religious fasts involve restricting certain foods rather than having a particular window of time to eat and drink. This may include only eating plant-based foods during certain times of the year. Other fasts allow proteins like fish but restrict other foods during a fast.¹

Potential Health Benefits

Fasting can have many health benefits, but it is important to check with your healthcare provider before changing your dietary routine.¹

Many people try intermittent fasting for weight management. According to research, IF helps lose visceral (around your abdominal) fat. In one study, participants who ate within an 8-hour window experienced an 11.1% average reduction in visceral fat.⁷

However, more research is needed to determine if fasting is the most effective option for losing visceral fat. Some research suggests that fasting may benefit low-density lipoprotein (LDL) cholesterol, triglyceride (the most common fat in your body) levels, and cholesterol more than diets.⁸⁹

Intermittent fasting may also help your body preserve muscle mass while losing weight and maintaining a healthy metabolism. Fasting can help with insulin resistance by regulating your blood sugar levels.¹⁰

Some research in this article utilizes BMI. Body Mass Index, or BMI, is a biased and outdated metric that uses your weight and height to make assumptions about body fat and, by extension, your health. This metric is flawed in many ways and does not factor in your body composition, ethnicity, sex, race, and age. Despite its flaws, the medical community still uses BMI because it’s an inexpensive and quick way to analyze health data.

Potential Risks

It’s important to recognize potential risks associated with fasting and their symptoms.

Extended periods without food can lead to nutritional deficiencies, decreased energy levels, and impaired cognitive function (ability to think, reason, and remember). Prolonged fasts have been associated with low sodium and chloride levels as well.¹¹²

For people with preexisting health conditions, such as diabetes, fasting can worsen symptoms of these conditions and contribute to potential disease complications.¹²

Improper fasting methods can disrupt your regular eating patterns, potentially leading to an eating disorder such as binge eating disorder (BED) or anorexia nervosa (AN).¹³

If you are considering fasting, consult your healthcare provider or a registered dietitian or nutritionist to ensure a balanced nutritional intake. They can help you adjust your diet or the parameters of your fasting.

How To Fast Safely

Start slowly if you’re interested in fasting to improve your health. For example, the 16:8 method allows for a relatively normal dietary pattern before gradually decreasing the window of time you can eat. Eating healthy while fasting can also keep your energy up during times of the day when you are not taking in any nutrients.²

Eating a healthy diet of high protein, fiber, and healthy fats also helps your body not lose any nutrients it needs. Although there are no dietary restrictions associated with fasting, it is recommended that you avoid alcoholic and caffeinated beverages.²

Some researchers suggest mixing an electrolyte powder into drinks to prevent sodium and chloride deficiencies. However, most religious fasts only allow water as a beverage during the day.²

Stop fasting immediately if you experience nausea, low blood pressure, or fatigue. Consult your healthcare provider on options that may be safer for your body. They can help you determine whether fasting affects other parts of your health.¹⁴

There are many options for weight management and metabolic health other than fasting that your healthcare provider can recommend. If fasting for religious purposes, your healthcare provider may be able to help you find different ways to fast that keep you healthy without disrupting your practice.¹⁵

Who Should Avoid Fasting?

Fasting is generally not advised for people who are pregnant, breastfeeding, or taking certain diabetic or heart medications. Fasting is more likely to cause very low blood sugar in people with type 1 diabetes and type 2 diabetes who are dependent on insulin.¹⁶

You may experience more side effects with prolonged fasts. Some people experience fatigue, nausea, and mood swings from the drastic change in eating patterns. If you have low blood pressure or are prone to fainting or periods of lightheadedness, it’s best to avoid fasting methods that involve long periods of calorie restriction.¹⁴

People who have a history of eating disorders should consult a healthcare provider before fasting.¹⁵

A Quick Review

There are various reasons someone may go on a fast. Fasting for religious purposes is a common practice not tied to potential health benefits. Some people may also fast for personal goals, such as decreasing visceral fat or setting new dietary patterns.

Fasting isn’t the best choice for everyone. If you are living with diabetes, an eating disorder, or low blood pressure, fasting may have adverse effects on your health. Consult your healthcare provider before starting a fast to discuss your options and whether fasting overall is safe for you.

Frequently Asked Questions

• Can I drink water during a fast?

Most fasts allow for water consumption during the fasting window. It’s recommended to stay hydrated during fasting, especially if you’re partaking in a longer-term fast, to reduce the risk of side effects like headaches and low blood pressure.¹

• Can I drink coffee during a fast?

Many fasts, such as intermittent fasting, allow zero-calorie beverages, like black coffee. However, some researchers recommend avoiding caffeinated drinks during your fast.²

• Can I exercise while fasting?

Whether you exercise while fasting depends on your energy levels, health goals, and how your body responds. There are benefits to exercising while fasting, such as increased fat breakdown. However, it’s important to listen to your body and avoid high-intensity exercises if you have low energy or are feeling fatigued while fasting.¹⁷

Sources:

1. Attinà A, Leggeri C, Paroni R, et al. Fasting: how to guide. Nutrients. 2021;13(5):1570. doi:10.3390/nu13051570
2. Dai Z, Zhang H, Wu F, et al. Effects of 10-day complete fasting on physiological homeostasis, nutrition and health markers in male adults. Nutrients. 2022;14(18):3860. doi:10.3390/nu14183860
3. Przulj D, Ladmore D, Smith KM, Phillips-Waller A, Hajek P. Time restricted eating as a weight loss intervention in adults with obesity. PLoS One. 2021;16(1):e0246186. doi:10.1371/journal.pone.0246186
4. Vasim I, Majeed CN, DeBoer MD. Intermittent fasting and metabolic health. Nutrients. 2022;14(3):631. doi:10.3390/nu14030631
5. Wang R, Lv X, Xu W, et al. Effects of the periodic fasting-mimicking diet on health, lifespan, and multiple diseases: a narrative review and clinical implications. Nutr Rev. Published online January 29, 2024:nuae003. doi: 10.1093/nutrit/nuae003
6. Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent fasting and metabolic health: from religious fast to time-restricted feeding. Obesity (Silver Spring, Md). 2020;28(Suppl 1):S29. doi: 10.1002/oby.22829
7. Chow LS, Manoogian EN, Alvear A, et al. Time restricted eating effects on body composition and metabolic measures in humans who are overweight: a feasibility study. Obesity (Silver Spring). 2020;28(5):860-869. doi:10.1002/oby.22756
8. Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med. 2022;386(16):1495-1504. doi:10.1056/NEJMoa2114833
9. Yuan X, Wang J, Yang S, et al. Effect of intermittent fasting diet on glucose and lipid metabolism and insulin resistance in patients with impaired glucose and lipid metabolism: a systematic review and meta-analysis. Int J Endocrinol. 2022;2022:6999907. doi:10.1155/2022/6999907
10. Gu L, Fu R, Hong J, Ni H, Yu K, Lou H. Effects of intermittent fasting in human compared to a non-intervention diet and caloric restriction: a meta-analysis of randomized controlled trials. Front Nutr. 2022;9:871682. doi:10.3389/fnut.2022.871682
11. Gudden J, Arias Vasquez A, Bloemendaal M. The Effects of Intermittent Fasting on Brain and Cognitive Function. Nutrients. 2021;13(9):3166. doi:10.3390/nu13093166
12. National Institute of Diabetes and Digestive and Kidney Diseases. Fasting Safely with Diabetes.
13. Blumberg J, Hahn SL, Bakke J. Intermittent fasting: consider the risks of disordered eating for your patient. Clin Diabetes Endocrinol. 2023;9(1):4. doi:10.1186/s40842-023-00152-7
14. Phillips MCL. Fasting as a therapy in neurological disease. Nutrients. 2019;11(10):2501. doi:10.3390/nu11102501
15. Soliman GA. Intermittent fasting and time-restricted eating role in dietary interventions and precision nutrition. Front Public Health. 2022;10:1017254. doi:10.3389/fpubh.2022.1017254
16. Herz D, Haupt S, Zimmer RT, et al. Efficacy of fasting in type 1 and type 2 diabetes mellitus: a narrative review. Nutrients. 2023;15(16):3525. doi:10.3390/nu15163525
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Important Notice: This article was also published at www.health.com by Jillian Kubala, RD where all credits are due. Medically reviewed Simone Harounian, MS.