What are the health benefits of fasting?
There are numerous health benefits to fasting. Many of us have been fasting our entire lives from the time we go to sleep until the time we eat the next day. The concept of break-fast, describes the time that you consume food or drink after having gone without food while sleeping. You can think of fasting as a period when you provide your body with a break from food and do not eat or eat an extremely low number of calories,1 essentially reducing the amount of time in a day that you spend eating.
Fasting is a technique to help your body shift from fat storage to use. Once your cells absorb glucose and you use your glycogen stores, your body begins to turn fat into free fatty acids and fatty-acid derived ketones, allowing your body to burn fat, and use ketones for energy and improve cell and organ function. Even after the fasting period, referred to as the refeeding period, the body continues to show signs of improved glucose absorption, known as metabolic flexibility.234 When our blood glucose levels are able to return to normal after eating, our insulin response remains sensitive, preventing insulin resistance, a precursor of diabetes caused by excess glucose in the bloodstream.
Overall health benefits of fasting:
- Improvement in glucose regulation and insulin sensitivity and reduced risk of insulin resistance, important in the prevention of obesity and type 2 diabetes5
- Reduction of abdominal fat while maintaining muscle mass678
- Reduced blood pressure and heart rate and increased heart rate variability
- Enhanced brain health, learning and memory, and motor function
- Protection of brain neurons against dysfunction and degeneration in animal models of Alzheimer’s disease, Parkinson’s disease, stroke, and Huntington’s disease
- Reduced risk of dyslipidemia, hypertension, inflammation and cardiometabolic diseases9
What happens to your body during fasting?
Typically, after eating, your blood glucose levels rise and your pancreas releases insulin, allowing your cells to absorb the glucose and convert it into fuel for immediate use.9 The excess glucose then either gets stored as glycogen in your liver and muscles or as fat and can be converted into ketones for future energy use. The more excess glucose we have in our bloodstream, the more fat our body will store in the form of body fat.
When we fast, we give our body the time it needs to absorb the glucose in our bloodstream so that it does not need to continuously pump out insulin. This allows our insulin levels to drop and our body to convert stored glycogen and fat into energy. After approximately half a day of fasting,10 the metabolic benefits of using fat for energy begin to occur. For most people, it takes at least 12 hours for your body to use the glucose in your bloodstream and the glycogen stored in the liver, both which need to occur before the lipids in your fat cells get converted into free fatty acids and can be metabolized into ketones.
What are different types of Fasting?
There are numerous fasting options, so you can choose the approach that works best for your body and your goals. Each option has modified versions ranging from only drinking water to also drinking bone broth, to regimens that allow you to eat during the fasting period, but only very few calories (e.g., £500 calories/day). The restricted calorie fasts are sometimes referred to as fast-mimicking diets. It is preferable to begin fasting a few hours before going to sleep.
There are circumstances in which a clinician should be consulted or under which fasting is not advised, including during pregnancy, when breastfeeding, if experiencing hormonal imbalances, if having or at risk of an eating disorder or if underweight.
Common fasting options
- Extended fasting: One of the most restrictive fasting models involves restraining from eating, or consuming an extremely low-calorie diet, for two or more days. There is a growing body of evidence of the benefit of occasionally allowing your body to undergo an extended fast of up to five days.11 Before engaging in an extended fast, consultation with a clinician is advised.
- Intermittent fasting/Time-restricted feeding: Intermittent-fasting diets have increased in popularity over the past decade, particularly with respect to clinical intervention studies.12 This practice involves restricting the time window in which you can eat each day. This approach allows you to determine which period you wish to give your body a break from eating. Ideally, you provide your body with a minimum of a 12-hour break, referred to as 12/12 (12-hour break/12 hours eating period). You can then work your way up to a longer break, ideally up to 16/8. Skipping breakfast or eating a late breakfast is a common way to set up your fast. For example, you would stop eating after dinner and then wait to eat until the next morning (e.g., fast from 6 p.m. until 10 a.m.) or afternoon (e.g., 12 p.m.).
- Alternate-day fasting: A model that allows you to eat one day and then restrain from eating on the next day, involves a “fast day,” when individuals consume ≤25% of baseline energy needs during a 24-hour period, alternated with a “feast day,” when ad libitum food consumption is permitted for 24 hours.
- Periodic fasting: A model that allows you to select occasional days each week or month to fast. For example, you may choose one to two days each week to be your fasting days and the rest of the week to eat freely. You may hear people refer to this common approach as the 5:2 approach, 5 days of eating and 2 days of fasting.
- 24-hour fasting: This is an example of more extreme intermittent fasting, in which you eat one meal a day, around the same time each day. For example, you may fast from 12 p.m. one day until 11 a.m. the next day (23/1). In this model, you may only eat lunch or dinner.
Do you allow your body to fast for at least 12 hours each day? If not, see if fasting works for you. You can try it out for a month and enjoy the health benefits. If you already fast for 12 hours a day, try extending your fast and enjoy the health benefits of perhaps skipping breakfast. You can still drink water, and black tea and coffee while fasting.
- Sainsbury A, Wood R, Seimon R, et al. Rationale for novel intermittent dieting strategies to attenuate adaptive responses to energy restriction. Obes Rev. 2018;19:47-60.
- Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes. 2011;35(5):714-727.
- Hoddy KK, Marlatt KL, Çetinkaya H, Ravussin E. Intermittent fasting and metabolic health: from religious fast to time‐restricted feeding. Obesity. 2020;28:S29-S37.
- Smith RL, Soeters MR, Wüst RCI, Houtkooper RH. Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease. Endocr Rev. 2018;39(4):489-517. doi:10.1210/er.2017-00211
- Patterson RE, Laughlin GA, Sears DD, et al. Intermittent fasting and human metabolic health. J Acad Nutr Diet. 2015;115(8):1203.
- Harris L, Hamilton S, Azevedo LB, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Evid Synth. 2018;16(2). https://journals.lww.com/jbisrir/Fulltext/2018/02000/Intermittent_fasting_interventions_for_treatment.16.aspx
- Tinsley GM, La Bounty PM. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutr Rev. 2015;73(10):661-674. doi:10.1093/nutrit/nuv041
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi:10.1001/jamainternmed.2017.0936
- De Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551.
- Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obes Silver Spring Md. 2018;26(2):254-268. doi:10.1002/oby.22065
- Wei M, Brandhorst S, Shelehchi M, et al. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Sci Transl Med. 2017;9(377):eaai8700.
- Cioffi I, Evangelista A, Ponzo V, et al. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018;16:371. doi:10.1186/s12967-018-1748-4