Skip to content
ContentLora

    Tip: press / anywhere to search.

    Explainer

    Intermittent fasting explained: 16:8, 5:2 and what trials show

    Intermittent fasting means cycling between periods of little or no food and normal eating, for example eating only within an 8-hour window or fasting on two days a week.[1][2] Randomised trials show it helps weight loss about as much as ordinary calorie restriction, not more; a February 2026 Cochrane review found little to no difference versus standard dietary advice.[2][3]

    Editor reviewedStrict sourcingUpdated ExplainerHealth and medicine, GLP-1 and metabolic medicine, Aging and longevity biology

    Intermittent fasting is a widely promoted diet with a contested evidence base.[2] This page explains the main types, what randomised trials have found, and why a 2024 headline linking 8-hour eating to heart deaths needs careful reading. A deeper look at the competing views is in the fasting evidence analysis.

    This page is general information, not medical advice. Talk to a clinician before changing how you eat, especially if you have a medical condition or take medicines.

    The main types

    All forms of intermittent fasting alternate periods of eating with periods of little or no food.[1]

    • Time-restricted eating (TRE): eating only within a daily window. Windows typically run from 4 to 12 hours; “16:8” means eating within 8 hours and fasting for 16.[4]
    • 5:2 or 4:3 fasting: cutting calories sharply on two or three days a week and eating normally on the others.[1][5]
    • Alternate-day fasting: very low intake every other day.[1]

    The Cochrane review classed time-restricted feeding, periodic fasting, alternate-day fasting and modified alternate-day fasting as intermittent fasting.[2] A frequently cited proposed mechanism is “metabolic switching”: fasting shifts the body’s fuel from liver glucose to ketones made from fat. A 2019 NEJM review by NIA researchers argued this, rather than weight loss alone, drives the benefits seen in studies. That remains a hypothesis built largely on animal and short-term human data.[6][1]

    What randomised trials show

    Randomised controlled trials (RCTs) are the strongest evidence here, because people are assigned to a diet by chance. The main findings:

    • Little or no edge over normal dieting. A February 2026 Cochrane review of 22 RCTs (1,995 adults with overweight or obesity) found little to no difference in weight loss versus regular dietary advice, with low-certainty evidence.[2]
    • About the same as calorie restriction. A 2025 BMJ network meta-analysis of 99 RCTs found every fasting and calorie-restriction diet beat unrestricted eating. Only alternate-day fasting had a small edge over continuous restriction, about 1.3 kg.[3]
    • Time windows add little on their own. Adding an 8-hour window to a calorie-restricted diet did not significantly increase weight loss over 12 months.[7] In a 12-week US trial, 16:8 eating without other changes did no better than three meals a day.[8]
    • Some exceptions. A 4:3 schedule with intensive behavioural support produced 2.89 kg more weight loss than daily restriction over a year.[5] In women with polycystic ovary syndrome, a 6-hour window matched calorie restriction.[9]

    Fasting and cholesterol

    The 2024 AHA release noted earlier research suggesting time-restricted eating can improve blood pressure, glucose and cholesterol.[10] Head-to-head trials are less encouraging. In a year-long trial, alternate-day fasting raised LDL by 11.5 mg/dL more than daily calorie restriction.[11] The BMJ analysis found time-restricted eating slightly raised total and LDL cholesterol compared with whole-day fasting. It concluded that fasting diets have broadly similar cardiometabolic effects to calorie restriction.[12] For proven ways to lower LDL, see the cholesterol explainer.

    The 8-hour eating headline

    In March 2024 a conference abstract reported that, among about 20,000 US adults, eating within less than 8 hours a day was linked to a 91% higher risk of cardiovascular death.[13] This was observational evidence, not a trial. Eating windows came from two days of self-reported diet, and the senior author said the link does not show that time-restricted eating caused the deaths.[14] The peer-reviewed paper (2025) reported a hazard ratio of 2.35 for cardiovascular death but no link with overall death. It said residual confounding could explain the result.[15]

    Open questions

    Most fasting trials last a year or less. The Cochrane authors called for longer follow-up.[16] NIA researchers noted that human trials have not shown long-term effects, and that safety for people at a healthy weight, or for younger and older people, needs study.[1] Medicines are now a separate route to weight loss; see obesity as a disease.

    Next in the crash course

    More in Health & medicine

    Developing in Health

    Go deeper

    Questions readers ask

    What is intermittent fasting?

    An eating pattern with hours or days of little or no food. Commonly studied versions are alternate-day fasting, 5:2 fasting (two fasting days a week) and daily time-restricted eating, such as eating only within a set window each day.[1]

    Is intermittent fasting better than a normal diet for weight loss?

    Mostly no. A 2026 Cochrane review of 22 trials found little to no difference versus regular dietary advice, and a 2025 BMJ network meta-analysis found only alternate-day fasting had a small edge (about 1.3 kg) over continuous calorie restriction.[2][3]

    Does intermittent fasting lower cholesterol?

    Trials show no consistent extra benefit. In one year-long trial, alternate-day fasting was linked to higher LDL than daily calorie restriction, and a 2025 meta-analysis found time-restricted eating slightly raised LDL compared with whole-day fasting.[11][12]

    Does 16:8 fasting cause heart disease?

    That has not been shown. An observational study linked eating within less than 8 hours a day to higher cardiovascular death, but its authors and the AHA stressed it shows an association, not cause, and relied on two days of dietary recall.[13][14][15]

    Is intermittent fasting safe for everyone?

    Long-term safety is not established. NIA researchers noted that most studies involve overweight, middle-aged adults and that safety for people at a healthy weight, or younger or older people, needs study. Talk to a clinician first.[1]

    Sources

    · 16 claims from 9 publishers

    Each numbered claim is a statement we checked against the sources listed with it. Tap a number in the text to see its sources in place.

    1. [1]

      A 2019 NEJM review by NIA researchers described intermittent fasting regimens (alternate-day, 5:2 and daily time-restricted feeding) and possible benefits mainly from animal and short-term human studies, but said human trials have not shown long-term effects, that most clinical research has involved overweight, middle-aged adults, and that safety in people at a healthy weight, younger or older people needs study. confirmedas of 2026-10-11

    2. [2]

      A February 2026 Cochrane review of 22 randomised trials (1,995 adults with overweight or obesity, follow-up up to 12 months) covering time-restricted feeding, periodic fasting and alternate-day fasting, found intermittent fasting may result in little to no difference in weight loss compared with regular dietary advice (low-certainty evidence). confirmedas of 2026-10-11

    3. [3]

      A 2025 BMJ network meta-analysis of 99 randomised trials (6,582 adults) found all intermittent fasting and continuous calorie-restriction diets reduced weight versus unrestricted eating; alternate-day fasting was the only form with a small benefit over continuous restriction (-1.29 kg), and longer trials (24 weeks or more) showed benefit only versus unrestricted eating. confirmedas of 2026-10-11

    4. [4]

      Time-restricted eating, a type of intermittent fasting, limits eating to a set number of hours each day, typically a window of 4 to 12 hours; many followers use a 16:8 schedule, eating within 8 hours and fasting for the remaining 16. confirmedas of 2026-10-11

    5. [5]

      In a 12-month US trial of 165 adults with overweight or obesity, all receiving intensive behavioural support, a 4:3 intermittent fasting schedule (80% calorie restriction on three non-consecutive days a week) produced 2.89 kg more weight loss than matched daily calorie restriction. confirmedas of 2026-10-11

    6. [6]

      The NIA-led review proposed metabolic switching, in which fasting shifts the body's energy source from liver glucose to ketones from fat, as the key mechanism behind benefits seen in studies. reportedas of 2026-10-11· interpretation

    7. [7]

      In a 12-month randomised trial of 139 people with obesity in China, all on a calorie-restricted diet, adding time-restricted eating (8 a.m. to 4 p.m.) did not significantly increase weight loss (-8.0 kg vs -6.3 kg; net difference -1.8 kg, P = 0.11). confirmedas of 2026-10-11

    8. [8]

      In the 12-week TREAT trial (116 US adults with overweight or obesity), 16:8 time-restricted eating without other changes produced no significant difference in weight loss versus three structured meals a day (-0.26 kg between groups). confirmedas of 2026-10-11

    9. [9]

      In a 6-month randomised trial of 76 women with polycystic ovary syndrome published in 2026, 6-hour time-restricted eating reduced body weight by 4.32% relative to controls, similar to daily calorie restriction. confirmedas of 2026-10-11

    10. [10]

      The AHA release noted that previous research has found time-restricted eating improves cardiometabolic measures such as blood pressure, blood glucose and cholesterol. confirmedas of 2026-10-11

    11. [11]

      In a one-year trial of 100 adults with obesity, alternate-day fasting produced similar weight loss to daily calorie restriction, had the highest dropout rate (38%), and was associated with higher LDL cholesterol at 12 months than daily restriction (by 11.5 mg/dL). confirmedas of 2026-10-11

    12. [12]

      In the same BMJ network meta-analysis, time-restricted eating produced a small increase in total, LDL and non-HDL cholesterol compared with whole-day fasting, and the authors concluded intermittent fasting has broadly similar benefits to continuous calorie restriction. confirmedas of 2026-10-11

    13. [13]

      In March 2024 researchers presented a conference abstract analysing about 20,000 US adults from NHANES (2003-2018) which found that people whose eating was limited to less than 8 hours a day had a 91% higher risk of cardiovascular death than those eating across 12 to 16 hours. confirmedas of 2026-10-11

    14. [14]

      The AHA noted the 2024 result was a preliminary abstract; its limitations included self-reported diet, eating windows based on just two days of dietary recall, and factors outside eating duration not included in the analysis, and the senior author said the association does not mean time-restricted eating caused cardiovascular death. confirmedas of 2026-10-11

    15. [15]

      The peer-reviewed version, published in August 2025 (19,831 NHANES adults, median follow-up 8.1 years), reported that eating duration under 8 hours was associated with higher cardiovascular mortality than 12 to 14 hours (hazard ratio 2.35) but not with all-cause or cancer mortality, and said residual confounding could not be ruled out. confirmedas of 2026-10-11

    16. [16]

      The Cochrane authors said the included trials focused on effects up to 12 months, that longer follow-up is needed, and that clinicians and patients may need to weigh willingness, practicality and sustainability when considering intermittent fasting. confirmedas of 2026-10-11

    Revision history (1)
    1. Page created.

    Created Oct 11, 2026. Last reviewed by an editor on Oct 11, 2026. Next scheduled review: Jan 11, 2027.

    Cite this page

    "Intermittent fasting explained: 16:8, 5:2 and what trials show." ContentLora, updated Oct 11, 2026. https://contentlora.com/explain/intermittent-fasting-explained

    Spotted an error? Suggest a correction or emailcorrections@contentlora.com.

    ☰ Contents