GradedHealth

Does fasting increase longevity? What the human studies show

Fasting can move some aging markers in trials, but no study has shown it extends life. Short eating windows and long nightly fasts were linked to more deaths.

Published October 2026

Short answer

Human trials have not shown that fasting extends life. Two years of calorie restriction slowed one biological aging marker by about 2–3%, and mortality data suggest moderate eating windows rather than extreme fasting. Evidence grade C (Limited). Trials have tested only aging markers rather than deaths: the two-year CALERIE trial slowed one DNA methylation clock but not two others, fasting-mimicking diet findings are exploratory, and mortality cohorts disagree; downgraded for surrogate outcomes and inconsistency.

Key takeaways

  • No human trial has shown that fasting extends lifespan — a Cochrane review of 18 trials found none reported deaths, heart attacks or strokes [1].
  • In the two-year CALERIE trial, about 220 healthy adults cut calories by roughly 12% and slowed one DNA-based aging measure by about 2–3%; two other aging clocks did not change [2].
  • Three monthly cycles of a 5-day fasting-mimicking diet were linked to a 2.5-year lower biological age estimate in a 100-person trial, but the analysis was exploratory and the diet's maker funded the study [3].
  • US studies link very short eating windows (8 hours or less) and long nightly fasts (over about 12 hours) to higher death rates; the lowest mortality sat at moderate patterns, not extremes, and these studies cannot show cause [4][5].
  • In 960 genetically diverse mice, calorie restriction and intermittent fasting both extended lifespan in proportion to how much food was restricted [6].

The short version

No human study has shown that fasting extends life, but trials do suggest it can nudge the biology of aging: in the two-year CALERIE trial, calorie restriction slowed one DNA-based aging measure by about 2–3% and improved heart risk factors [2][7]. A 2021 Cochrane review of intermittent fasting trials found none reported deaths, heart attacks or strokes [1]. US mortality studies link both very short eating windows and long nightly fasts to higher death rates, so more extreme fasting does not look better [4][5].

What the studies show

Why no one has tested lifespan directly. A trial that could show fasting extends life would need to follow thousands of people for decades, so researchers measure instead "biological age": DNA and blood markers that predict death risk in large observational studies [2]. The human trials below tested those markers, not longer life.

The main human test: calorie restriction. In the two-year CALERIE trial, 220 healthy adults were randomly assigned to cut calories by 25% or to eat normally; on average they managed about 12% and lost about 10% of their body weight [2][7]. Calorie restriction slowed the DunedinPACE measure of aging pace by about 2–3%, but two other aging clocks, PhenoAge and GrimAge, showed no change [2]. Based on earlier observational data, the researchers estimate that the slowdown could correspond to as much as 10–15% lower mortality risk, though the trial did not measure deaths [2]. The metabolism hormone T3 and the inflammation marker TNF-α also fell [7].

Fasting-specific trials. In a 100-person randomized trial, three monthly cycles of a 5-day fasting-mimicking diet were associated with a 2.5-year lower median biological age on a validated blood-marker measure, along with lower insulin resistance and liver fat; a second 44-person study found nearly identical results [3]. The researchers reported the biological-age change was independent of weight loss, but the analysis was exploratory and the company that makes the diet supplied it, with two authors holding equity [3]. Four weeks of alternate-day fasting cut calorie intake by about 37% and lowered LDL cholesterol, T3 and sICAM-1, a marker of age-related inflammation [8]. In people with multiple sclerosis, secondary analyses found intermittent calorie restriction lowered a metabolomic aging score while daily calorie restriction did not [9]. A six-month trial in 121 adults with obesity found a possible increase in autophagy, a cell-cleaning process linked to aging, with intermittent fasting plus time-restricted eating, though the authors call the result uncertain [10].

Animal context. Eating less extends life in many species. In 960 genetically diverse mice, graded calorie restriction and intermittent fasting both extended lifespan roughly in proportion to how much food was restricted; the strictest 40% restriction cost lean mass, fasting did not extend life in mice that started out heavier, and genetics mattered more than diet [6].

Deaths: what the large studies find. These are observational, so they can show links, not cause and effect. Among 33,052 US adults followed for a median of eight years, the association between eating window and death was U-shaped: risk was lowest at about 11–12 hours, windows of 8 hours or less were linked to 30% or more higher all-cause mortality (most strongly in older adults), and windows of 15 hours or more were linked to 25% higher mortality [4]. In 10,561 US adults over 60, nightly fasts longer than 12.38 hours were linked to higher cardiovascular mortality, with the lowest risk at 11.49 hours [5]. A meta-analysis of 9 cohorts (242,095 adults) found that skipping breakfast was linked to 27% higher all-cause mortality, though the evidence was rated very low certainty and some sensitivity analyses disagreed [11]. A 2026 review of adults over 60 found 16:8 eating and Islamic Sunnah fasting were the most effective fasting patterns for weight among 7 pooled trials, but also noted that very restrictive eating windows and prolonged fasting were linked to adverse outcomes including lower cognitive scores [12].

The nuances

"Fasting" covers very different patterns: daily time-restricted eating, alternate-day fasting, the 5:2 diet — and also calorie restriction and fasting-mimicking diets, which are not fasting in the usual sense [2][3]. Results from one approach do not prove another works.

Biological age is not lifespan. Even in CALERIE the slowing was small (2–3%), the two groups' results still overlapped by close to 90%, and no one knows whether it translates into fewer deaths [2]. The fasting-mimicking diet biological-age findings are exploratory and were not the trial's main outcome [3].

The mortality findings cannot prove cause: people who are already ill often eat less, eat within shorter windows or skip breakfast [4]. And no fasting pattern has been shown to lower death rates in a randomized trial [1].

People taking diabetes medicines can get low blood sugar while fasting [13]. A review found that in vulnerable people fasting was linked to mood destabilisation, anxiety and, rarely, psychotic or manic symptoms, and that people with eating-disorder traits showed more preoccupation with food and a higher risk of relapse [14]. Very short eating windows look riskier for older adults [12]; the intermittent fasting topic page covers pregnancy, breastfeeding, eating-disorder and other safety details.

What this means for you

If you are fasting to live longer, the honest answer is that no human study supports it yet: the main long-term trial tested calorie restriction rather than fasting, and measured markers rather than lifespan [2]. The most solid benefit of intermittent fasting in trials remains weight loss, and even there it works about as well as simply cutting calories [1]. What human data exist point to moderate patterns: the lowest death rates in US studies sat around an 11–12 hour eating window or an 11.5-hour nightly fast, not extreme restriction [4][5]. A 177-person randomized trial is now testing time-restricted eating against 25% calorie restriction, with molecular markers of aging among its outcomes, so better answers are coming [15]. If you take diabetes medication, are pregnant or breastfeeding, are over 60, or have a history of an eating disorder, talk to a doctor before making fasting a long-term habit [13][14][12].

Go deeper

  • Intermittent Fasting — Intermittent fasting helps adults with overweight or obesity lose weight: pooled trials show about 3 kg more loss than control groups, and it works about as we…

References

  1. Intermittent fasting for the prevention of cardiovascular disease.. Cochrane Database Syst Rev, 2021.
  2. Effect of long-term caloric restriction on DNA methylation measures of biological aging in healthy adults from the CALERIE trial.. Nat Aging, 2023.
  3. Fasting-mimicking diet causes hepatic and blood markers changes indicating reduced biological age and disease risk.. Nat Commun, 2024.
  4. Association of Eating Window With Mortality Among US Adults: Insights From a Nationally Representative Study.. Aging Cell, 2025.
  5. A National Study Exploring the Association between Fasting Duration and Mortality among the Elderly.. Nutrients, 2024.
  6. Dietary restriction impacts health and lifespan of genetically diverse mice.. Nature, 2024.
  7. A 2-Year Randomized Controlled Trial of Human Caloric Restriction: Feasibility and Effects on Predictors of Health Span and Longevity.. J Gerontol A Biol Sci Med Sci, 2015.
  8. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans.. Cell Metab, 2019.
  9. Fasting Mimicking Diets Reverse Accelerated Biological Aging in Multiple Sclerosis.. Ann Neurol, 2025.
  10. Intermittent time-restricted eating may increase autophagic flux in humans: an exploratory analysis.. J Physiol, 2025.
  11. Breakfast skipping and risk of all-cause, cardiovascular and cancer mortality among adults: a systematic review and meta-analysis of prospective cohort studies.. Food Funct, 2024.
  12. Intermittent Fasting and Healthy Aging in Older Adults: A Systematic Review of Cardiometabolic, Mental Health and Cognitive Outcomes with a Network Meta-Analysis of Anthropometric Measures.. Nutrients, 2026.
  13. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial.. Diabet Med, 2018.
  14. Psychological and Psychiatric Consequences of Prolonged Fasting: Neurobiological, Clinical, and Therapeutic Perspectives.. Nutrients, 2025.
  15. Time-restricted eating versus calorie restriction for improving biomarkers of age in adults with overweight or obesity and incipient fatty liver disease: protocol for the ENSATI randomized controlled parallel groups trial.. Front Endocrinol (Lausanne), 2026.