What Is Fasting?

What Fasting Means

Fasting means voluntarily not eating for a set period, with the timing and duration chosen in advance rather than forced by circumstance. That voluntary, planned quality is what separates fasting from simply having no food available.

What Fasting Is Not

Occasionally skipping a meal because you weren't hungry is not the same as a structured fast: a fast has a defined start, a defined length, and a defined purpose, even a loose one. Starvation is a different thing entirely: it is involuntary, typically caused by a lack of access to food, and is a medical emergency rather than a wellness practice. The fasting-mimicking diet is also not true fasting in the strict sense: it involves eating a small, specific amount of low-calorie food each day for several days, not zero food; see what a fasting-mimicking diet is for how it differs from eating nothing at all.

The Main Types of Fasting

Several named patterns show up across fasting research and coverage, differing mainly in how long the fast runs and how often it repeats. This section is a short landscape map, not a full comparison; each pattern has its own dedicated page with the schedule detail and evidence.

• Intermittent or time-restricted fasting, such as 16:8, means eating within a daily window (for example, 8 hours) and fasting for the rest of each day. See how to start intermittent fasting for named schedules and the evidence behind each.
• Alternate-day fasting alternates very-low-calorie or zero-calorie days with days of normal eating, rather than restricting a window within every day. See alternate-day fasting for how it compares with daily patterns.
• OMAD (one meal a day) is the most extreme end of the daily-fasting spectrum: all of a day's food eaten in a single sitting. See OMAD: what to know.
• Prolonged water-only fasting means consuming only water for several days or longer, with no food at all. See prolonged fasting for how multi-day fasts differ from daily patterns.
• The fasting-mimicking diet (FMD) is a low-calorie food program, typically run for about 5 days at a time, designed to approximate some effects of fasting while still providing some food. See what is a fasting-mimicking diet.

What Changes in the Body During a Fast

Fuel Source: From Glycogen to Fat and Ketones

The body's first fuel source during a fast is liver glycogen, stored carbohydrate. Over roughly 12 to 24 hours of fasting, depending on physical activity, liver glycogen is largely used up and the body shifts toward fat-derived fuels, including ketones (Longo and Mattson, 2014) [1]. Ketone production keeps rising over the following days of a longer fast (Rebello et al., 2025) [2]. A review of fasting and performance notes that an overnight fast of 8 to 10 hours is normal, and that Ramadan-style daytime fasting, which also excludes fluids, has only small overall metabolic effects (Maughan, Fallah, and Coyle, 2010) [3].

Hormonal Changes

In general terms, insulin falls as eating stops, part of the broader shift toward using stored fuel (Longo and Mattson, 2014) [1]. This page states this change only in general terms; the size and timing depends on fast length and the individual, and is not quantified further here.

Autophagy: The Most Overstated Part of This Story

Autophagy is a cellular process that clears out damaged components, and it is the part of fasting's biology most associated, in popular coverage, with anti-aging claims. It is well documented in animals. In people, a small number of studies have looked for it, mostly through indirect markers in blood cells or muscle, and they disagree: a 4-day trial found higher activity of an autophagy-related gene, alongside higher activity of MTOR, a nutrient-sensing gene that generally restrains autophagy (Jamshed et al., 2019) [5]; an 8-week intermittent-fasting trial found muscle autophagy markers went down rather than up, which the authors link to weight loss (Chaudhary et al., 2022) [7]; and a 6-month trial that measured autophagic activity (flux) in blood cells calls its own result "exploratory" (Bensalem et al., 2025) [6]. See "Where the evidence is thinner than it sounds" above for the full picture, and autophagy and fasting for a dedicated page on this topic.

How Long These Effects Take

Roughly speaking, liver glycogen runs low somewhere in the 12-to-24-hour range of fasting, depending on activity, and the shift toward fat and ketones then continues over the following days (Longo and Mattson, 2014 [1]; Rebello et al., 2025 [2]). The exact timing varies by person, prior diet, and activity level. For a fuller timeline covering fasts that extend across multiple days, see prolonged fasting, which draws on its own dedicated set of citations rather than repeating them here.

Where to Go Next

This page is a starting point for the fasting-basics cluster on this site. From here:

• How to start intermittent fasting: named daily schedules, compared with citations.
• OMAD: what to know: the most extreme daily pattern.
• What breaks a fast: what counts as fasting in practice.
• Prolonged fasting: multi-day water-only fasts.
• What is a fasting-mimicking diet: a low-calorie food program, not true fasting.
• What the research shows about fasting: a fuller round-up of findings, including where they disagree.
• Fasting and gut health: what's known and not known about fasting and the microbiome.
• Autophagy and fasting: the full picture behind the short summary above.

Who Should Talk to a Clinician First

This applies across every type of fasting described above, regardless of length: talk to a clinician before fasting if you are pregnant, under 18, managing diabetes or blood pressure with medication, or have a history of an eating disorder. Each type above has its own more specific cautions. How to start intermittent fasting covers insulin and sulfonylurea specifics; prolonged fasting covers diabetes-specific glycemic-measure findings; the fasting-mimicking diet pages cover medication-adjustment cautions specific to that program.

Safety and Cautions

This is general information, not medical advice. The science behind fasting's basic fuel-switching mechanism is well established; the science behind autophagy in humans specifically is new, small, and not yet tied to a proven health outcome. Talk to a clinician before fasting of any kind or length if you are pregnant, under 18, have a history of an eating disorder, or manage diabetes or blood pressure with medication.