Intermittent fasting (IF) is a pattern of eating that alternates between periods of fasting (usually meaning consumption of water and sometimes low-calorie drinks such as black coffee) and non-fasting.
There is evidence suggesting that intermittent fasting may have beneficial effects on the health and longevity of animals—including humans—that are similar to the effects of caloric restriction (CR). There is currently no consensus as to the degree to which this is simply due to fasting or due to an (often) concomitant overall decrease in calories, but recent studies have shown support for the former. Alternate-day calorie restriction may prolong life span. Intermittent fasting and caloric restriction are forms of dietary restriction (DR), which is sometimes referred to as dietary energy restriction (DER).
Scientific study of intermittent fasting in rats (and anecdotally in humans) was carried out at least as early as 1943.
A specific form of intermittent fasting is alternate day fasting (ADF), also referred to as every other day fasting (EOD), or every other day feeding (EODF), a 48-hour routine typically composed of a 24-hour fast followed by a 24-hour non-fasting period.
Several formal and informal studies since the 1930s are discussed in a 2013 Scientific American article. The effects of fasting in general, and in diabetes and on the brain were discussed in New Scientist in 2013.
Animal studies 
The 1945 study by Carlson and Hoelzel, cited above, found that the apparent life span of rats in the study was increased by intermittent fasting. Tests in which a group of 33 rats were allowed the same food ad libitum and groups of 37, 37, and 30 rats were fasted one day in four, three, and two days, respectively, after the age of 42 days, showed that the optimum amount of fasting appeared to be fasting one day in three; this increased the life span of littermate males by about 20% and littermate females by about 15%. However, the pre-experimental condition of the individual rats was also found to be an important factor in determining the life spans. No drastic retardation of growth was produced by the intermittent fasting, but the development of mammary tumors was retarded in proportion to the amount of fasting.
A number of subsequent studies have shown beneficial effects of intermittent fasting in animals:
- "Reduced serum glucose and insulin levels and increased resistance of neurons in the brain to excitotoxic stress."
- Intermittent fasting was found to "Enhance cardiovascular and brain functions and improve several risk factors for coronary artery disease and stroke including a reduction in blood pressure and increased insulin sensitivity" and that "cardiovascular stress adaptation is improved and heart rate variability is increased in rodents" and that "rodents maintained on an IF regimen exhibit increased resistance of heart and brain cells to ischemic injury in experimental models of myocardial infarction and stroke."
- It may "ameliorate age-related deficits in cognitive function" in mice.
- A correlation between intermittent fasting and significantly improved biochemical parameters associated with the development of diabetic nephropathy.
- Resistance in mice to the effects of gamma irradiation.
- Lifespan increases of 40.4% and 56.6% in C. elegans for alternate day (24-hour) and every second day (48-hour) fasting, respectively, as compared to an ad libitum diet.
- Rats showed markedly improved long-term survival after chronic heart failure via pro-angiogenic, anti-apoptotic, and anti-remodeling effects.
- "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."
But negative effects of intermittent fasting in animals have also been noted:
- ADF in rats "prolongs life span and promotes numerous health benefits in rodents, including tissue protection from ischemic damage" but also has negative effects including causing "myocardial hypotrophy, cardiac fibrosis, diastolic dysfunction, and a reduction of cardiac reserve."
Human studies 
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.
Human diet 
A number of individuals are experimenting with different varieties of intermittent fasting as a dietary regimen. In this context, shorthand such as "20/4" is used to denote a repeating pattern of 20 hours of fasting followed by 4 hours of non-fasting. For example, "Fast-5" is a book promoting a regimen equivalent to "19/5". Other alternatives include "16/8" and "15/9".
In common usage, intermittent fasting describes any diet that includes a period of fasting and a period of non-fasting, even if the diet involves consuming a limited amount of calorie-containing beverages such as coffee or tea during the fasting period. This contrasts with scientific usage of the term, in which no calories are consumed during the fasting period.
Another variation on intermittent fasting is to consume limited calories (e.g., 20% of normal) rather than none at all on fasting days. This regimen may provide many of the benefits of intermittent fasting.
Another possibility is eating only one meal per day without caloric restriction. When overall calorie intake is not reduced, this diet worsens some cardiovascular disease risk factors.
The BBC2 Horizon documentary Eat, Fast & Live Longer showed another plan: during days of fasting, people eat 400-500 kcal (women) or 500-600 kcal (men), and during feed days, the diet was unrestricted. This was done either alternately (one day fasting, one day feeding) or by fasting two days per week: the 5:2 diet.
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