What Is Fasting, TRE and IF? A Definition for Intermittent, Modified and Therapeutic Fasting
- Alastair Hunt
- Jun 25
- 6 min read
Updated: Jul 10
In recent years, fasting has surged in popularity as both a lifestyle trend and a therapeutic tool. Yet, the terminology surrounding different fasting methods has remained murky, confusing not only the public but also healthcare professionals and researchers. An international team of 38 experts from five continents recently stepped forward with a consensus on how we define fasting. This standardisation aims to bring clarity to a complex and often inconsistent field.
As ever, please talk to your doctor or medical practitioner most familiar with your medical history before implementing any changes in diet, exercise or lifestyle, especially if you are under treatment. Links to all studies at bottom of page.
Why Standard Terminology Matters
Over the past two decades, scientific interest in fasting has exploded, especially as researchers uncover its potential benefits for longevity, metabolic health, and disease prevention. But with this surge in interest comes a tangle of terminology. Words like "intermittent fasting" "time-restricted eating" and "fasting-mimicking diet" are often used interchangeably, even though they describe very different practices. This makes it difficult to compare study outcomes, guide clinical practice, and inform the public accurately.
Key Definitions
Among the most critical outcomes of the consensus process were definitions for commonly used fasting terms:
Fasting: Defined broadly as voluntary abstinence from some or all foods or beverages for health, cultural, or religious reasons.
Modified Fasting: Limits intake to around 25% of one’s energy needs on fasting days.
Time-Restricted Eating (TRE): Eating is confined to a specific daily window (e.g. 10 hours), resulting in at least 14 hours of fasting.
Intermittent Fasting (IF): Involves repeated fasting periods of up to 48 hours.
Short-Term Fasting (STF): Lasts 2 to 3 consecutive days.
Prolonged Fasting (PF): Any fasting lasting 4 days or more.
Fasting-Mimicking Diet (FMD): A low-calorie, plant-based diet designed to trigger the same metabolic responses as fasting.
The definitions also extend to traditional and religious fasting forms, such as dry fasting (no food or drink), Buchinger therapeutic fasting, the FX Mayr therapy (a very low-calorie regimen with digestive training) and intermittent dry fasting, as practised during Ramadan.
Other noteworthy categories include alternate-day fasting (ADF), where a day of regular eating alternates with a fasting day, and alternate-day modified fasting (ADMF), where the fasting day includes a reduced caloric intake. Preventive fasting refers to any fasting protocol followed primarily for maintaining general wellbeing, while therapeutic fasting is used as an intervention in treating chronic conditions.
The Impact on Science and Medicine
This new lexicon will likely streamline future fasting research, enabling better comparisons and meta-analyses. For clinicians, it provides a clearer framework to recommend fasting protocols tailored to individual needs. Importantly, it also brings uniformity to discussions around fasting in both public health and personal wellness.
The standardisation avoids imposing arbitrary calorie counts in favour of contextual, individualised recommendations. For example, the definition of caloric restriction now refers to "a reduction in energy intake below what's needed to maintain current body weight, without malnutrition" rather than prescribing a rigid percentage. This broader, more flexible approach allows clinicians to adapt caloric restriction to a person’s age, sex, body composition, physical activity and clinical goals.
Notably, caloric restriction is no longer limited to a specific range (such as 20 to 40% below maintenance needs), but is instead based on outcomes and context. It may include continuous energy restriction or daily energy restriction, which are interchangeable terms describing sustained daily caloric deficits. Importantly, caloric restriction is now clearly distinguished from modified fasting: while both involve reduced energy intake, modified fasting is episodic and more extreme (≤25% of energy needs on fasting days), whereas caloric restriction is a continuous, long-term approach aimed at gradual metabolic adjustment.
Another important clarification lies in differentiating fasting from starvation. While both involve energy deficit, starvation is involuntary and often harmful, whereas fasting is deliberate and controlled. Similarly, dietary restriction is now clearly defined as encompassing limitations on energy, macronutrients, or food types, with or without fasting components.
Recommendations for Everyday Use
What does this mean for someone curious about trying fasting? First, understand that not all fasts are the same. This consensus helps you and your healthcare provider choose a fasting regimen that fits your health status, goals, and lifestyle:
If you're exploring fasting for weight management or metabolic health, intermittent fasting or time-restricted eating might be accessible starting points.
For more intensive effects, short-term or prolonged fasting should only be attempted under professional supervision.
Interested in a more food-inclusive option? The fasting-mimicking diet offers flexibility while still engaging key metabolic processes. Another popular caloric restriction practice - the (original) 5:2 diet - is also a popular alternative; read more here.
These clear-cut categories also help demystify fasting's relationship with nutrition. For instance, fluid-only fasts can include up to 500 kcal per day from vegetable broths and juices, while dry fasting allows no intake whatsoever, often practised for cultural or religious reasons. Therapeutic regimens like Buchinger or FX Mayr typically include bowel cleansing and specific dietary reintroduction phases.
Final Thoughts
This international consensus brings much-needed clarity to the fasting landscape. Whether you're a researcher, a clinician, or someone curious about improving their health through fasting, these definitions offer a solid foundation. As fasting continues to gain traction in both clinical and everyday settings, having a shared language ensures that we're all on the same page - scientifically, therapeutically and personally.
And remember, fasting isn't a one-size-fits-all solution. The diversity of defined regimens reflects the diversity of people's needs and goals. Use these definitions as a guide, but always consult with a healthcare professional to tailor any fasting practice safely to your own circumstances. Some of the latest research (Hamsho et al, 2025) indicates that there is no superior form of dieting when it comes to health outcomes, caloric restriction and intermittent fasting produce similar results. So do what works for you, consistency in the long-term!
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Related Studies and Resources
Koppold DA, Breinlinger C, Hanslian E, Kessler C, Cramer H, Khokhar AR, Peterson CM, Tinsley G, Vernieri C, Bloomer RJ, Boschmann M, Bragazzi NL, Brandhorst S, Gabel K, Goldhamer AC, Grajower MM, Harvie M, Heilbronn L, Horne BD, Karras SN, Langhorst J, Lischka E, Madeo F, Mitchell SJ, Papagiannopoulos-Vatopaidinos IE, Papagiannopoulou M, Pijl H, Ravussin E, Ritzmann-Widderich M, Varady K, Adamidou L, Chihaoui M, de Cabo R, Hassanein M, Lessan N, Longo V, Manoogian ENC, Mattson MP, Muhlestein JB, Panda S, Papadopoulou SK, Rodopaios NE, Stange R, Michalsen A. International consensus on fasting terminology. Cell Metab. 2024 Aug 6;36(8):1779-1794.e4. doi: 10.1016/j.cmet.2024.06.013. Epub 2024 Jul 25. PMID: 39059384; PMCID: PMC11504329.
Hamsho M, Shkorfu W, Ranneh Y, Fadel A. Is isocaloric intermittent fasting superior to calorie restriction? A systematic review and meta-analysis of RCTs. Nutr Metab Cardiovasc Dis. 2025 Mar;35(3):103805. doi: 10.1016/j.numecd.2024.103805. Epub 2024 Nov 23. PMID: 39732588.
Other
Cagigas ML, De Ciutiis I, Masedunskas A, Fontana L. Dietary and pharmacological energy restriction and exercise for healthspan extension. Trends Endocrinol Metab. 2025 Jun;36(6):521-545. doi: 10.1016/j.tem.2025.04.001. Epub 2025 May 2. PMID: 40318928.
Cagigas ML, Twigg SM, Fontana L. Ten tips for promoting cardiometabolic health and slowing cardiovascular aging. Eur Heart J. 2024 Apr 1;45(13):1094-1097. doi: 10.1093/eurheartj/ehad853. PMID: 38206047.
Kraus WE, Bhapkar M, Huffman KM, Pieper CF, Krupa Das S, Redman LM, Villareal DT, Rochon J, Roberts SB, Ravussin E, Holloszy JO, Fontana L; CALERIE Investigators. 2 years of calorie restriction and cardiometabolic risk (CALERIE): exploratory outcomes of a multicentre, phase 2, randomised controlled trial. Lancet Diabetes Endocrinol. 2019 Sep;7(9):673-683. doi: 10.1016/S2213-8587(19)30151-2. Epub 2019 Jul 11. PMID: 31303390; PMCID: PMC6707879.
Das SK, Roberts SB, Bhapkar MV, Villareal DT, Fontana L, Martin CK, Racette SB, Fuss PJ, Kraus WE, Wong WW, Saltzman E, Pieper CF, Fielding RA, Schwartz AV, Ravussin E, Redman LM; CALERIE-2 Study Group. Body-composition changes in the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE)-2 study: a 2-y randomized controlled trial of calorie restriction in nonobese humans. Am J Clin Nutr. 2017 Apr;105(4):913-927. doi: 10.3945/ajcn.116.137232. Epub 2017 Feb 22. PMID: 28228420; PMCID: PMC5366044.
Lee C, Longo V. Dietary restriction with and without caloric restriction for healthy aging. F1000Res. 2016 Jan 29;5:F1000 Faculty Rev-117. doi: 10.12688/f1000research.7136.1. PMID: 26918181; PMCID: PMC4755412.
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