Intermittent fasting (intermittent energy restriction or intermittent calorie restriction) is an umbrella term for various eating protocols that cycle between a period of fasting and non-fasting over a defined period. Intermittent fasting is under preliminary research to assess if it can produce weight loss comparable to long-term calorie restriction.
Practice and variants
Intermittent fasting protocols can be grouped into two categories: whole-day fasting and time-restricted feeding.
- Whole-day fasting involves regular one-day fasts. The strictest form would be alternate day fasting (ADF). This involves a 24-hour fast followed by a 24-hour non-fasting period. The alternate day modified fasting (ADMF) and 5:2 diets—the latter defined as five days per week not fasting and two days per week either total fasting or modified fasting—both allow the consumption of approximately 500–600 calories on fasting days.
- Time-restricted feeding (TRF) involves eating only during a certain number of hours each day. A common form of TRF involves fasting for 16 hours each day and only eating during the remaining 8 hours, typically on the same schedule each day. A more liberal practice would be 12 hours of fasting and a 12 hour eating window, or a stricter form would be to eat one meal per day, which would involve around 23 hours of fasting per day.[medical citation needed]
Variants include modified fasting, such as ADMF, with limited caloric intake (20% of normal) during fasting periods rather than none at all. Intermittent fasting has a different duration (up to 48 hours) than periodic fasting (more than 48 hours).
Forms of intermittent fasting exist in religious practices in various groups across the world. Religious fasting regimens include, but are not limited to, Vrata in Hinduism, Ramadan fasting (Islam), Yom Kippur fasting (Judaism), Orthodox Christian fasting, Fast Sunday (The Church of Jesus Christ of Latter-day Saints), and Buddhist fasting. Certain religious fasting practices, like Buddhist fasting, only require abstinence from certain foods, while others, like the Jewish fast on Yom Kippur, last for a short period of time and would cause negligible effects on the body. Islam is the only major religion that engages in a fasting practice reflective of intermittent fasting in terms of both food consumption and diet consistency. The duration of the Ramadan fast is between 28 and 30 days, depending on the year, and consists of not eating or drinking from sunrise until sunset. During the holiday, Muslims eat twice per day: once in the morning before dawn and once in the evening after dusk. A meta-analysis on the health of Muslims during Ramadan shows significant weight loss during the fasting period of up to 1.51 kilograms (3.3 lb), but this weight was regained within about two weeks of Ramadan ending. The analysis concluded that "Ramadan provides an opportunity to lose weight, but structured and consistent lifestyle modifications are necessary to achieve lasting weight loss." Negative effects of Ramadan fasting include increased risk of hypoglycemia in diabetics as well as inadequate levels of certain nutrients.
Intermittent fasting (specifically the 5:2 diet) became popular in the UK in 2012 after the BBC2 television Horizon documentary Eat, Fast and Live Longer. Via sales of best-selling books, it became widely practiced. In the United States, intermittent fasting has become a trend among Silicon Valley companies. According to NHS Choices as of 2012, people considering the 5:2 diet should first consult a physician, as fasting can sometimes be unsafe. A news item in the Canadian Medical Association Journal expressed concern that promotional material for the diet showed people eating high-calorie food, such as hamburgers and chips, and that this could encourage binge eating since the implication was that "if you fast two days a week, you can devour as much junk as your gullet can swallow during the remaining five days".
As of 2018, interest in intermittent fasting led some companies to commercialize diet coaching, dietary supplements, and full meal packages. These companies have been criticized for offering products or services that are expensive and not backed by science.
A 2018 review of intermittent fasting in obese people showed that reducing calorie intake one to six days per week over at least 12 weeks was effective for reducing body weight on an average of 7 kilograms (15 lb); the results were not different from a simple calorie restricted diet, and the clinical trials reviewed were run mostly on middle-aged women from the US and the UK, limiting interpretation of the results. Intermittent fasting has not been studied in children, the elderly, or underweight people, and could be harmful in these populations.
Intermittent fasting is under preliminary research for how it may affect cardiovascular and metabolic biomarkers, such as body fat, LDL cholesterol, triglycerides, insulin sensitivity, blood pressure. Alternate day fasting does not appear to affect lean body mass.
Understanding the potential adverse effects of intermittent fasting is limited by an inadequate number of rigorous clinical trials. One 2015 review of low-quality clinical studies found that short-term intermittent fasting may produce minor adverse effects, such as continuous feelings of weakness and hunger, headaches, fainting, or dehydration. Long-term fasting may cause eating disorders or malnutrition, with increased susceptibility to infectious diseases.
- Patterson, Ruth E.; Sears, Dorothy D. (21 August 2017). "Metabolic Effects of Intermittent Fasting". Annual Review of Nutrition. 37 (1): 371–393. doi:10.1146/annurev-nutr-071816-064634. PMID 28715993.
- Davis, C S; Clarke, R E; Coulter, S N; Rounsefell, K N; Walker, R E; Rauch, C E; Huggins, C E; Ryan, L (25 November 2015). "Intermittent energy restriction and weight loss: a systematic review". European Journal of Clinical Nutrition. 70 (3): 292–299. doi:10.1038/ejcn.2015.195. PMID 26603882.
- Johnstone, A (26 December 2014). "Fasting for weight loss: an effective strategy or latest dieting trend?". International Journal of Obesity. 39 (5): 727–733. doi:10.1038/ijo.2014.214. PMID 25540982.
- Tinsley, GM; La Bounty, PM (1 October 2015). "Effects of intermittent fasting on body composition and clinical health markers in humans". Nutrition Reviews. 73 (10): 661–74. doi:10.1093/nutrit/nuv041. PMID 26374764.
- Harvie, Michelle N; Howell, Tony (July 2016). "Could Intermittent Energy Restriction and Intermittent Fasting Reduce Rates of Cancer in Obese, Overweight, and Normal-Weight Subjects? A Summary of Evidence". Advances in Nutrition. 7 (4): 690–705. doi:10.3945/an.115.011767. ISSN 2156-5376. PMC 4942870. PMID 27422504.
- Rothschild, Jeff; Hoddy, Kristin K; Jambazian, Pera; Varady, Krista A (2014). "Time-restricted feeding and risk of metabolic disease: A review of human and animal studies". Nutrition Reviews. 72 (5): 308–18. doi:10.1111/nure.12104. PMID 24739093.
- Mattson, M. P; Longo, V. D; Harvie, M (2017). "Impact of intermittent fasting on health and disease processes". Ageing Research Reviews. 39: 46–58. doi:10.1016/j.arr.2016.10.005. PMC 5411330. PMID 27810402.
- Persynaki, A; Karras, S; Pichard, C (1 March 2017). "Unraveling the metabolic health benefits of fasting related to religious beliefs: A narrative review". Nutrition. 35: 14–20. doi:10.1016/j.nut.2016.10.005. PMID 28241983.
- Sadeghirad, B; Motaghipisheh, S; Kolahdooz, F; Zahedi, MJ; Haghdoost, AA (1 February 2014). "Islamic fasting and weight loss: a systematic review and meta-analysis". Public Health Nutrition. 17 (2): 396–406. doi:10.1017/S1368980012005046. PMID 23182306.
- "How to diet". Live Well - NHS Choices. UK National Health Service. 9 December 2011. Retrieved 10 February 2014.
- Trueland, Jennifer (2013). "Fast and effective?". Nursing Standard. 28 (16): 26–7. doi:10.7748/ns2013.12.28.16.26.s28. PMID 24345130.
- Healy A (11 June 2013). "Dietitians warn against fad diets". Irish Times.
- Mosley, Michael (5 September 2012). "Eat, Fast & Live Longer". Horizon. Episode 49x03. BBC. 2. Retrieved 10 February 2014.
- Fleming, Amy (27 January 2015). "Fasting facts: is the 5:2 diet too good to be true?". The Guardian. Retrieved 18 January 2018.
- "The UK's Hot New 5:2 Diet Craze Hits The U.S. - Weight Loss Miracle?". Forbes. 17 May 2013. Retrieved 10 February 2014.
- Solon, Olivia (2017-09-04). "The Silicon Valley execs who don't eat for days: 'It's not dieting, it's biohacking'". The Guardian. Retrieved 2018-11-05.
- "News analysis: Does the 5:2 intermittent fasting diet work?". Health News. UK National Health Service - NHS Choices. May 2013. Retrieved 23 November 2016.
- Collier, R (2013). "Intermittent fasting: The science of going without". Canadian Medical Association Journal. 185 (9): E363–4. doi:10.1503/cmaj.109-4451. PMC 3680567. PMID 23569168.
- Giles, Tom (24 April 2018). "Silicon Valley wants to cash in on fasting". Bloomberg News. Retrieved 8 November 2018.
- "Not so fast: Pros and cons of the newest diet trend - Harvard Health". Harvard Heart Letter. 1 October 2017.
- Harris, L; Hamilton, S; Azevedo, LB; Olajide, J; De Brún, C; Waller, G; Whittaker, V; Sharp, T; Lean, M; Hankey, C; Ells, L (February 2018). "Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis". JBI Database of Systematic Reviews and Implementation Reports. 16 (2): 507–547. doi:10.11124/JBISRIR-2016-003248. PMID 29419624.
- Longo, Valter D; Mattson, Mark P (2014). "Fasting: Molecular Mechanisms and Clinical Applications". Cell Metabolism. 19 (2): 181–92. doi:10.1016/j.cmet.2013.12.008. PMC 3946160. PMID 24440038.
- Anton, SD; Moehl, K; Donahoo, WT; Marosi, K; Lee, SA; Mainous AG, 3rd; Leeuwenburgh, C; Mattson, MP (1 February 2018). "Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting". Obesity. 26 (2): 254–268. doi:10.1002/oby.22065. PMC 5783752. PMID 29086496.
- Cherif, Anissa; Roelands, Bart; Meeusen, Romain; Chamari, Karim (5 October 2015). "Effects of Intermittent Fasting, Caloric Restriction, and Ramadan Intermittent Fasting on Cognitive Performance at Rest and During Exercise in Adults". Sports Medicine. 46 (1): 35–47. doi:10.1007/s40279-015-0408-6. PMID 26438184.
- "Réseau NACRe - Réseau National Alimentation Cancer Recherche - Rapport NACRe jeûne regimes restrictifs cancer 2017". www6.inra.fr (in French). November 2017. Retrieved 19 September 2018.
- Horne, B. D.; Muhlestein, J. B.; Anderson, J. L. (2015). "Health effects of intermittent fasting: Hormesis or harm? A systematic review". The American Journal of Clinical Nutrition. 102 (2): 464–70. doi:10.3945/ajcn.115.109553. PMID 26135345.