"Fasting" means different things depending on who you ask. Someone keeping to a daily eating window calls it fasting. Someone drinking only water for days calls it fasting. The Prolon 5-Day Fasting Mimicking Diet (FMD), a fasting nutrition program with food for all five days, calls itself fasting too: it's built to mimic fasting itself, so the body gets fasting's effects while you still eat. Two things separate these approaches: how long you go, and whether you eat anything while you're doing it.
How fasting methods differ
Fasting is any stretch of time your body goes without food, long enough to shift into a different metabolic state. Intermittent fasting is short, with food allowed in a daily or weekly window. Water fasting and the Fasting Mimicking Diet are both prolonged, running for several days. Water fasting asks you to eat nothing; the Fasting Mimicking Diet gives you food built to keep your body in a fasting state.
When does the body enter autophagy during a fast?
Autophagy is the body's natural repair process in which cells clean out, break down, and recycle their own components. Autophagy runs at a low level all the time. Fasting of around 72 hours or more ramps it up. Intermittent fasting, lasting up to two days, usually ends too soon to ramp it up; a prolonged fast can.
Intermittent fasting
Intermittent fasting covers approaches that last up to two days: eating only within a set window each day, or cutting calories on set days each week. It may support metabolic health and weight management.
Does intermittent fasting trigger autophagy the same way a longer fast does?
Not usually. Intermittent fasting is usually too short to ramp up autophagy, the cellular cleanup process covered above.
12:12 (a daily eating and fasting window aligned with your circadian rhythm, and Prolon's recommended daily window) and 5:2 (five days of normal eating, and reduced calorie intake on two non-consecutive days a week) are two common patterns, and both are ones Prolon recommends. Neither is the same as a single prolonged fast like the FMD.
Is the Fasting Mimicking Diet the same as intermittent fasting?
No. The FMD is a five-day program built around one prolonged fast, not a pattern of short, repeating ones. The Prolon 5-Day FMD is not an intermittent fast. It mimics a prolonged water fast while you still eat, over five consecutive days rather than a daily or weekly window.
That doesn't make the two competitors. Following intermittent fasting patterns, such as 12:12 or 5:2, in between Prolon cycles doesn't conflict with the FMD.
Prolonged fasting: water-only and the Fasting Mimicking Diet
A prolonged fast runs longer than 72 hours, long enough to ramp up autophagy. There are two ways to do one: eat nothing, or eat food engineered to keep your body in that state anyway.
Water-only fasting
Water-only fasting means consuming nothing but water for the length of the fast. The largest study of a closely related protocol followed 1,422 people on medically supervised fasts of 200 to 250 kcal a day, lasting 4 to 21 days: in that one-year observational study, researchers recorded reductions in weight and abdominal circumference and an increase in physical and emotional well-being (Wilhelmi de Toledo et al., PLoS One, 2019). This study was performed at the Buchinger Wilhelmi Clinic and financed by Amplius GmbH, a company that runs research for the clinic; the paper names the clinics as its funders. The lead author sits on the clinic's board and is managing director of Amplius GmbH. Three other authors are employed by the clinic, were formerly employed there, or consult for it with pay from Amplius GmbH. Water-only fasting carries real risks too: possible muscle loss, nutritional deficiencies, and a slowing metabolism, with more serious risks for people who have certain medical conditions. Anyone considering a water fast should talk to a healthcare practitioner first. For a closer look at water fasting on its own, see our guide to water fasting.
Water fasting isn't a method Prolon sells, and this isn't a how-to for doing one. The FMD was developed to deliver the effects of a prolonged fast while providing food. Our FAQ on water fasting after a Prolon cycle answers a specific question we hear often.
What's the difference between water fasting and the Fasting Mimicking Diet?
Water fasting means eating nothing and drinking only water. The Prolon 5-Day FMD mimics a prolonged water fast. Five set days of precisely formulated, plant-based meals keep your body in that fasting state instead of asking it to eat nothing at all. The Prolon 5-Day FMD is the only nutrition program clinically proven to trigger autophagy in humans.
Our page on the Fasting Mimicking Diet covers the formulation and the research behind it in full.
How the three compare
This is a structural comparison, not a ranking.
| Water fasting | Intermittent fasting | Fasting Mimicking Diet (Prolon 5-Day FMD) | |
|---|---|---|---|
| What you eat | Nothing; water only | Food within a set window; nothing outside it (patterns like 12:12 or 5:2) | Precisely formulated, plant-based soups, bars, snacks, and teas, in specific daily quantities |
| Duration | Commonly 2 to 5+ days, by individual approach | Up to 2 days per cycle (daily windows, or a weekly 5:2 pattern) | 5 days per cycle |
| Calories | 0 kcal/day (water only) | No restriction; timing only (daily windows). About 500 to 600 kcal on 2 days (5:2). | About 1,100 kcal on Day 1; 700 to 800 kcal on Days 2 to 5 |
| Typical frequency | No standard schedule; by individual approach | Daily (12:12) or weekly (5:2) | 3 consecutive monthly cycles to start, then 3 to 4 cycles per year for maintenance |
Choosing between them
If you want a short, repeatable daily practice that fits a normal schedule, intermittent fasting asks the least of you day to day: no special food, no multi-day commitment, just a window. If you want the most traditional version of a prolonged fast, and you're comfortable eating nothing at all for several days, water fasting is the original. If you want a prolonged fast without giving up food entirely, the Fasting Mimicking Diet is built to mimic fasting itself, so your body gets fasting's effects while you eat: a fixed five days, specific food, a defined start and end.
None of the three replaces the others. Whichever you're considering, talk to your healthcare practitioner first to make sure it's right for you. If you choose the FMD, the Prolon 5-Day FMD provides every meal for all five days, portioned and planned.
Related reading: Traditional Intermittent Fasting vs Fasting Mimicking Diet: What’s Better for Weight Loss?
