Intermittent Fasting

Fasting traditions and the research lineage

Fasting at set times or on set days is described in several religious traditions. A British Dietetic Association article describes Ramadan fasting as one of the five pillars of Islam, kept from dawn to sunset for 29 to 30 days according to the lunar calendar, and says children, older adults, people who are sick and pregnant women may seek exemption. A 2025 narrative review also covers Orthodox Christian fasting periods alongside Ramadan. These practices have their own religious meaning, and the sources used here treat them as a separate matter from secular dietary patterns.

The sources reviewed for this page do not give a dated, citable history of Lent, Yom Kippur or other religious fasts, so this page does not describe them in detail. The 2025 review included studies of Ramadan and Orthodox Christian fasting in its scope, which is why only those two are discussed here.

Research-era framing

The current research framing appears in review articles. Valter Longo and Satchidananda Panda published a 2016 review in Cell Metabolism that discusses fasting, circadian rhythms and time-restricted feeding, and describes fasting-mimicking diets as an area for translational research. In December 2019 Rafael de Cabo and Mark Mattson, of the National Institute on Aging and Johns Hopkins University, published a review in the New England Journal of Medicine titled Effects of Intermittent Fasting on Health, Aging, and Disease. The sources available for this page do not trace how caloric restriction research or media popularization, including the 5:2 label, developed, so those histories are not covered here.

Intermittent fasting as a family of patterns

A 2026 Cochrane review describes intermittent fasting as eating patterns in which people take little or no energy for extended periods, alternated with periods of normal food intake. The review counted time-restricted feeding, periodic fasting, alternate-day fasting and modified alternate-day fasting as forms of the approach. Other sources use the label for a similar range of patterns, so the term describes a family rather than one standard method.

Named patterns

Time-restricted eating is described by a British Dietetic Association article as consolidating daily food intake into a consistent window, with the daily fast typically lasting roughly 12 to 20 hours; the article states that calorie restriction is not a requirement of the approach. Alternate-day fasting alternates fasting days with ordinary eating days. An NHS-branded page describes 5:2 as eating less on two days a week and eating a balanced diet on the other five. These are definitions drawn from the sources and are not schedules to follow.

Related Machipedia pages

Intermittent fasting is a member of the Machipedia family page named Dietary Approaches. It is distinct from Water Fasting and Juice Fasting, which are separate pages, and from the Ketogenic Diet and the Calorie Restrictive Diet, which are also covered separately. Mindful Eating Therapy and Dietetics are related pages on eating behavior and the profession that advises on diet. A 2025 narrative review notes that variation in fasting protocols across studies limits comparison between them.

What the pages describe

In the sources reviewed, intermittent fasting is described by what changes in the timing of eating, not by which foods are eaten. A British Dietetic Association article notes that time-restricted eating is meant to align eating with circadian rhythms and does not require calorie restriction. Practitioners and enthusiasts describe it as simpler to follow than counting calories, an idea the same article attributes to a separate perspective paper without testing it.

Who delivers advice

The NHS-branded page summarizes a British Dietetic Association view that anyone following a 5:2 plan should choose an evidence-based plan written by a dietitian, and should tell their general practice team about any medication before changing their diet. This page does not describe how to carry out any pattern. Dietitians and physicians are the professional groups named in the sources, and the sources reviewed do not describe a separate credential specific to intermittent fasting.

Weight management and metabolic aims

Reviews describe intermittent fasting mainly as a method for weight loss, and sometimes as an approach to glycemic control and cardiometabolic markers. The Cochrane review treats weight loss as its main outcome, and a 2026 review in type 2 diabetes examined HbA1c and weight. Longo and Panda's review frames time-restricted feeding as having potential benefits for healthspan, a claim they present as warranting further research.

Religious and spiritual aims

Religious fasts such as Ramadan are kept for spiritual reasons. A narrative review reports that Ramadan and Orthodox Christian fasting were associated with greater perceived spiritual meaning in the studies it summarized, with cultural context moderating the effects. These are findings reported for people who fast for religious reasons and are not claims that the practice is a health intervention.

Systematic reviews

A 2026 Cochrane review of randomized trials in adults with overweight or obesity included 22 studies with 1,995 participants. Compared with regular dietary advice, intermittent fasting may make little to no difference to percentage weight loss (low-certainty evidence, 21 studies). Compared with no intervention or a waiting list, the estimate was a mean difference of -3.42 percentage points of baseline weight (95% CI -4.95 to -1.90; 6 studies, moderate certainty), which the review authors nonetheless summarized as likely little to no difference. The review included only follow-up up to 12 months and no included study reported diabetes status or participant satisfaction.

A 2026 systematic review in Food Science & Nutrition, covering 26 studies of 3 to 50 weeks, reported statistically significant weight reductions and no significant difference from continuous calorie restriction in several trials. Its authors rated 16 of 24 randomized trials as low risk of bias. Systematic reviews in particular populations are described in the safety section.

Individual randomized trials

A 12-month trial reported in the New England Journal of Medicine in 2022 randomized 139 adults with obesity to an eight-hour daytime eating window with calorie restriction or to calorie restriction alone. Weight fell by 8.0 kg and 6.3 kg respectively, and the difference was not statistically significant. Adverse events did not differ substantially between groups.

A 12-month trial in the Annals of Internal Medicine randomized 90 adults with obesity to an eight-hour eating window without calorie counting, daily calorie restriction, or a control group. Both active arms lost more weight than control, and the authors reported no significant difference between time-restricted eating and calorie restriction. They noted that the trial was not blinded and not powered to detect relatively large differences.

Observational signal and what was not found

In March 2024 the American Heart Association issued a news release on a preliminary abstract (P192) analyzing about 20,000 US adults from the 2003 to 2018 National Health and Nutrition Examination Surveys. It reported an association between an eating window under eight hours and higher cardiovascular mortality. The association was observational, rested on two 24-hour dietary recalls, and had not been peer reviewed; the senior author said it does not show that time-restricted eating caused the deaths. The abstract is not a peer-reviewed publication and its findings are unreplicated.

The British Dietetic Association article on time-restricted eating describes the trials it reviewed as mostly small and short and states that the evidence base needs to expand before wide advocacy. NICE guideline NG246 lists a research recommendation on the effectiveness and cost effectiveness of intermittent fasting for adults, and does not make a recommendation for or against it.

Groups named as unsuitable or needing medical input

An NHS-branded page says that people with a history of eating disorders such as anorexia or bulimia, and pregnant or breastfeeding women, should not attempt intermittent fasting, and advises people on medication to tell their general practice team before changing their diet. A narrative review reports that people with eating-disorder features showed increased preoccupation with food and a higher risk of relapse, and that mood destabilization, anxiety and rare manic or psychotic episodes were reported in vulnerable individuals.

The sources reviewed do not report on gout, underweight status or children specifically, so this page makes no statement about those groups. Absence of a statement in these sources should not be read as a finding that fasting is suitable or unsuitable for them.

Diabetes medicines and older adults

The British Dietetic Association advises people with type 1 or type 2 diabetes to speak to their diabetes team before fasting in Ramadan and to get advice on adjusting insulin or medication. A 2026 review of 12 trials in type 2 diabetes reported a pooled HbA1c reduction but very high heterogeneity and no significant pooled change in weight. A 2026 network meta-analysis in adults aged 60 and over reported that eating windows of 10 hours or less and prolonged fasting were associated with adverse outcomes, including lower cognitive scores and a 58 percent increase in cardiovascular mortality, and its authors urged moderate approaches and further long-term research.

Reported effects and gaps

The British Dietetic Association article reports mild, early and transient effects including headaches, diarrhea, increased thirst and dizziness, and flags unknown effects on lean mass and nutrient absorption. The Cochrane review rated adverse-event evidence as very low certainty. NICE lists the adverse events of dietary approaches, including intermittent fasting, as a research gap. None of this replaces advice from a clinician or dietitian, and necessary medical care should not be delayed.