Sources

Every paper behind the stages.

Each body stage describes what studies measured in healthy adults, not a reading of your own body, with the paper cited and a confidence label. These are all 24 sources: primary studies and reviews plus two clinical guidelines (ASPEN 2020, NICE CG32).

The same papers are cited on each stage and explainer card in the app. Autophagy, hunger, sleep and mood, inflammation and longevity are explainer cards, never stages, so the papers behind them are listed here with the card that cites them.

Where a stage's text rests on one paper's finding, a short note under the paper says what it measured; otherwise there is no note. Each paper links out to its full record.

The confidence words

Measured in humans · high
Direct measurement in healthy adults across several studies.
Measured in humans · moderate
Fewer or smaller studies, or wider variation between people.
Animal or indirect evidence · low
Used only for explainer cards, which is why autophagy is never a stage.

The papers

24 papers

  1. Deru 2021

    Randomised trial, 20 people

    The Effects of Exercise on β-Hydroxybutyrate Concentrations over a 36-h Fast: A Randomized Crossover Study

    Med Sci Sports Exerc · 2021 Sep 1

    The balance of insulin to glucagon fell markedly and more fat was released into the blood, while ketones stayed near baseline.

    PubMed 33731648, Deru 2021

  2. Cahill 2006

    Fuel metabolism in starvation

    Annu Rev Nutr · 2006

    PubMed 16848698, Cahill 2006

  3. Rothman 1991

    Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR

    Science · 1991 Oct 25

    The share of the body's glucose that is newly made early in a fast: about half at first, 64% averaged over the first 22 hours.

    PubMed 1948033, Rothman 1991

  4. Service and O'Brien 2005

    34 people

    Increasing serum betahydroxybutyrate concentrations during the 72-hour fast: evidence against hyperinsulinemic hypoglycemia

    J Clin Endocrinol Metab · 2005 Aug

    Blood ketones through a fast: 74% of people were above 2.7 mmol/L by 72 hours.

    PubMed 15886243, Service and O'Brien 2005

  5. Ho 1988

    6 people

    Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man

    J Clin Invest · 1988 Apr

    Growth hormone pulses, about three times larger by day 5.

    PubMed 3127426, Ho 1988

  6. Nilsson and Hultman 1973

    Liver glycogen in man--the effect of total starvation or a carbohydrate-poor diet followed by carbohydrate refeeding

    Scand J Clin Lab Invest · 1973 Dec

    PubMed 4771102, Nilsson and Hultman 1973

  7. Owen 1998

    Protein, fat, and carbohydrate requirements during starvation: anaplerosis and cataplerosis

    Am J Clin Nutr · 1998 Jul

    PubMed 9665093, Owen 1998

  8. Zauner 2000

    Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine

    Am J Clin Nutr · 2000 Jun

    Noradrenaline about doubled and resting metabolism was 14% higher by day 3, not lower.

    PubMed 10837292, Zauner 2000

  9. Jordan 2019

    Healthy volunteers

    Dietary Intake Regulates the Circulating Inflammatory Monocyte Pool

    Cell · 2019 Aug 22

    Fewer inflammatory monocytes circulating in the blood, already lower from about 19 hours into a fast.

    Explainer card
    • Inflammation

    PubMed 31442403, Jordan 2019

  10. Felig 1969

    Amino acid metabolism during prolonged starvation

    J Clin Invest · 1969 Mar

    PubMed 5773094, Felig 1969

  11. Pietzner 2024

    12 people

    Systemic proteome adaptions to 7-day complete caloric restriction in humans

    Nat Metab · 2024 Apr

    Much of the early drop in lean mass during a long fast was water, largely regained within 3 days of eating again.

    PubMed 38429390, Pietzner 2024

  12. Wilhelmi de Toledo 2019

    1,422 people

    Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects

    PLoS One · 2019

    Fasts under medical supervision: ketones level off and uric acid peaks.

    Explainer card
    • Sleep and mood

    PubMed 30601864, Wilhelmi de Toledo 2019

  13. Ogłodek 2021

    Is Water-Only Fasting Safe?

    Glob Adv Health Med · 2021

    PubMed 34414015, Ogłodek 2021

  14. Owen 1969

    Liver and kidney metabolism during prolonged starvation

    J Clin Invest · 1969 Mar

    PubMed 5773093, Owen 1969

  15. NICE CG32

    Clinical guideline

    Nutrition support for adults: oral nutrition support, enteral tube feeding and parenteral nutrition (CG32)

    NICE clinical guideline · 2006 Feb

    nice.org.uk, NICE CG32

  16. ASPEN 2020

    Clinical guideline

    ASPEN Consensus Recommendations for Refeeding Syndrome

    Nutr Clin Pract · 2020 Apr

    PubMed 32115791, ASPEN 2020

  17. Mizushima 2004

    In vivo analysis of autophagy in response to nutrient starvation using transgenic mice expressing a fluorescent autophagosome marker

    Mol Biol Cell · 2004 Mar

    Explainer card
    • Autophagy

    PubMed 14699058, Mizushima 2004

  18. Chaudhary 2022

    Intermittent fasting activates markers of autophagy in mouse liver, but not muscle from mouse or humans

    Nutrition · 2022 Sep

    Explainer card
    • Autophagy

    PubMed 35660501, Chaudhary 2022

  19. Espelund 2005

    Fasting unmasks a strong inverse association between ghrelin and cortisol in serum: studies in obese and normal-weight subjects

    J Clin Endocrinol Metab · 2005 Feb

    Explainer card
    • Hunger

    PubMed 15522942, Espelund 2005

  20. Natalucci 2005

    Spontaneous 24-h ghrelin secretion pattern in fasting subjects: maintenance of a meal-related pattern

    Eur J Endocrinol · 2005 Jun

    Explainer card
    • Hunger

    PubMed 15941923, Natalucci 2005

  21. Michalsen 2003

    Effects of short-term modified fasting on sleep patterns and daytime vigilance in non-obese subjects: results of a pilot study

    Ann Nutr Metab · 2003

    Explainer card
    • Sleep and mood

    PubMed 12748412, Michalsen 2003

  22. BMJ 2025 network meta-analysis

    Network meta-analysis of 99 trials

    Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials

    BMJ · 2025 Jun 18

    Across 99 trials, intermittent fasting forms were roughly equal to continuous daily restriction for body size.

    Explainer cards
    • Inflammation
    • Longevity and body size

    PubMed 40533200, BMJ 2025 network meta-analysis

  23. Cochrane 2021

    Cochrane review

    Intermittent fasting for the prevention of cardiovascular disease

    Cochrane Database Syst Rev · 2021 Jan 29

    Explainer card
    • Longevity and body size

    PubMed 33512717, Cochrane 2021

  24. Mao 2025

    Association of Eating Window With Mortality Among US Adults: Insights From a Nationally Representative Study

    Aging Cell · 2025 Nov

    Explainer card
    • Longevity and body size

    PubMed 40944446, Mao 2025

Back to the eleven stages, each with its sources and confidence label.

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