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. 
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 review 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.
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 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. Intermittent fasting may function as a form of nutritional hormesis.
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."
In a specific vulnerable population, ADF may bring harm: 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:
- 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.
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