In some contexts, fasting allows the consumption of a limited amount of low-calorie beverages such as coffee or tea.
One form of intermittent fasting, alternate day fasting (ADF), involves a 24-hour fast followed by a 24-hour non-fasting period. This is sometimes referred to as every other day fasting or every other day feeding. Alternate-day calorie restriction may prolong life span.
Modified fasting involves limiting caloric intake (e.g., 20% of normal) on fasting days rather than none at all. A study suggests that this regimen may retain most of the benefits of intermittent fasting. The scientific literature for intermittent fasting, in its various forms, was extensively reviewed in 2014.
Another form involves eating only one meal per day. In cases that do not restrain consumption, overall calorie intake may increase which worsens some cardiovascular disease risk factors.
More generally, forms may choose to specify various ratios of fasting to non-fasting periods. The BBC2 Horizon documentary Eat, Fast and Live Longer  covered people who committed to fasting two non-consecutive days per week. Known as the 5:2 diet, people consumed 400–500 calories (women) or 500–600 calories (men) during the days of fasting. During feed days, the diet was regular.
A 2014 study done by Longo and Mattson shed light on intermittent fasting's role in adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular production. The study showed how, in lower eukaryotes, chronic fasting extends longevity, in part, by reprogramming metabolic and stress resistance pathways. In rodents, intermittent fasting was shown to protect against diabetes, cancers, heart disease and neurodegeneration, while in humans it helps reduce obesity, hypertension, asthma, and rheumatoid arthritis.
Studies in mice and rats
A scientific study of intermittent fasting in rats conducted in 1943 found that fasting rats one day out of two, three or four days prolonged the life span of rats (by 15-20% in the case of one day out of three), compared to rats that were allowed to eat whenever they wish. None of the intermittent fasting in the study had detrimental effects on growth.
In one study, intermittent fasting has been shown to extend lifespan and increase resistance to age-related diseases in rodents and monkeys, and improve the health of overweight humans. The study suggests that intermittent fasting may have benefits that are similar to the effects of caloric restriction (CR). Specifically, it has been proposed that intermittent fasting improves the cardiovascular and neurological systems.
One study on mice suggests that benefits from intermittent fasting seems to be unrelated to an overall reduction in caloric intake. Another study on rats dealt with the benefits of dietary restriction, including intermittent fasting.
A 2007 review of alternate day fasting said, "the findings in animals suggest that ADF may effectively modulate several risk factors, thereby preventing chronic disease, and that ADF may modulate disease risk to an extent similar to that of CR. More research is required to establish definitively the consequences of ADF."
A study on hypercholesterolaemic mice showed that food restriction by intermittent fasting induces diabetes and obesity and aggravates spontaneous atherosclerosis development. Non-hypercholesterolaemic (normal, wild) control mice lost fat and lowered cholesterol as expected.
Studies on humans suggest possible benefits:
- Intermittent fasting may function as a form of nutritional hormesis.
- Alternate-day fasting may encourage fat oxidation.
- Alternate-day fasting may reduce body weight, LDL, and triglyceride levels to the same degree regardless of maintenance of low fat or high fat diet on the feeding day.
- "An Introduction to Intermittent Fasting". Retrieved 2012-11-26.
- Johnson, James B.; Laub, Donald R.; John, Sujit (2006). "The effect on health of alternate day calorie restriction: Eating less and more than needed on alternate days prolongs life". Medical Hypotheses 67 (2): 209–11. doi:10.1016/j.mehy.2006.01.030. PMID 16529878.
- Antoni, Rona; Johnston, Kelly L.; Collins, Adam L.; Robertson, M. Denise (2014). "The Effects of Intermittent Energy Restriction on Indices of Cardiometabolic Health" 2014.
- Stote, KS; Baer, DJ; Spears, K; Paul, DR; Harris, GK; Rumpler, WV; Strycula, P; Najjar, SS et al. (2007). "A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults". The American journal of clinical nutrition 85 (4): 981–8. PMC 2645638. PMID 17413096.
- Mosley, Michael. "Eat, Fast and Live Longer". BBC. Retrieved 27 October 2013.
- "Eat, Fast and Live Longer with Michael Mosley". PBS. 3 April 2013.
- Mattson, MP (February 4, 2014). "Fasting: molecular mechanisms and clinical applications". National Center for Biotechnology Information 19 (1932-7420): 181–92. doi:10.1016/j.cmet.2013.12.008. PMID 24440038. Retrieved 11 August 2014.
- Carlson, AJ; Hoelzel, F (1946). "Apparent prolongation of the life span of rats by intermittent fasting". The Journal of nutrition 31: 363–75. PMID 21021020.
- Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems by Mark Mattson and Ruigian Wan
- Anson, R. Michael; Guo, Zhihong; de Cabo, Rafael; Iyun, Titilola; Rios, Michelle; Hagepanos, Adrienne; Ingram, Donald K.; Lane, Mark A.; Mattson, Mark P. (2003). "Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake". Proceedings of the National Academy of Sciences 100 (10): 6216–20. Bibcode:2003PNAS..100.6216A. doi:10.1073/pnas.1035720100. JSTOR 3147568. PMC 156352. PMID 12724520.
- Wan, R; Camandola, S; Mattson, MP (2003). "Intermittent food deprivation improves cardiovascular and neuroendocrine responses to stress in rats". The Journal of nutrition 133 (6): 1921–9. PMID 12771340.
- Varady, KA; Hellerstein, MK (2007). "Alternate-day fasting and chronic disease prevention: A review of human and animal trials". The American journal of clinical nutrition 86 (1): 7–13. PMID 17616757.
- Mattson, Mark P. (2008). "Dietary factors, hormesis and health". Ageing Research Reviews 7 (1): 43–8. doi:10.1016/j.arr.2007.08.004. PMC 2253665. PMID 17913594.
- Heilbronn, Leonie K; Smith, Steven R; Martin, Corby K; Anton, Stephen D; Ravussin, Eric (2005). "Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolism". The American Journal of Clinical Nutrition 81 (1): 69–73. PMID 15640462.
- Klempel, Monica C.; Kroeger, Cynthia M.; Varady, Krista A. (2013). "Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet". Metabolism 62 (1): 137–43. doi:10.1016/j.metabol.2012.07.002. PMID 22889512.
- Bowerman, Susan (10 December 2007). "Feast, fast and reduce risks". Los Angeles Times. Retrieved 21 February 2010.[dead link]
- Silver, Cheryl S. (9 July 2004). "Eat Less, Live Longer?". Genome News Network. Retrieved 21 February 2010.
- Yang, Sarah (14 March 2005). "Fasting every other day, while cutting few calories, may reduce cancer risk". UC Newsroom. Retrieved 21 February 2010.
- Dobson, Roger (13 February 2010). "How fasting could help you slow down the ageing process". Daily Mail. Retrieved 21 February 2010.
- Mosley, Michael (5 August 2012). "The power of intermittent fasting". BBC News. Retrieved 9 August 2012.
- Kumar, Sushil; Kaur, Gurcharan (2013). Mezey, Eva, ed. "Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis". PLoS ONE 8 (1): e52416. Bibcode:2013PLoSO...852416K. doi:10.1371/journal.pone.0052416. PMC 3558496. PMID 23382817.
- Antoni, R; Johnston, KL; Collins, AL; Robertson, MD (2014). "The Effects of Intermittent Energy Restriction on Indices of Cardiometabolic Health". Research in Endocrinology. 2014 Article ID=459119. doi:10.5171/2014.459119.
- Collier, R. (2013). "Intermittent fasting: The science of going without". Canadian Medical Association Journal 185 (9): E363. doi:10.1503/cmaj.109-4451. PMID 23569168.
- Collier, R. (2013). "Intermittent fasting: The next big weight loss fad". Canadian Medical Association Journal 185 (8): E321. doi:10.1503/cmaj.109-4437. PMID 23529969.
- Chaouachi, Anis; Leiper, John B.; Chtourou, Hamdi; Aziz, Abdul Rashid; Chamari, Karim (2012). "The effects of Ramadan intermittent fasting on athletic performance: Recommendations for the maintenance of physical fitness". Journal of Sports Sciences 30: S53–73. doi:10.1080/02640414.2012.698297. PMID 22738880.
- Chaouachi, A; Leiper, JB; Souissi, N; Coutts, AJ; Chamari, K (2009). "Effects of Ramadan intermittent fasting on sports performance and training: A review". International journal of sports physiology and performance 4 (4): 419–34. PMID 20029094.
- Roky, Rachida; Houti, Imad; Moussamih, Samya; Qotbi, Souad; Aadil, Nadia (2004). "Physiological and Chronobiological Changes during Ramadan Intermittent Fasting". Annals of Nutrition and Metabolism 48 (4): 296–303. doi:10.1159/000081076. PMID 15452402.
- Johnston, Robert Dave (21 May 2013). "Intermittent Fasting for Quick Weight Loss & Detoxification". Fitness Through Fasting. Retrieved 21 May 2013.
- David Stipp: How Intermittent Fasting Might Help You Live a Longer and Healthier Life. Scientific American, 17 January 2013
- Emma Young: Hunger games: The new science of fasting. New Scientist, 2 January 2013
- Allison Aubrey: Two-day Diets - How Mini-fasts Can Help Maximize Weight Loss NPR-ME 29 July 2013