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Fasting Mimmic AOK. 3+ Day Fast is Best.

Jason Fung, M.D., a Canadian nephrologist, author, and low-carbohydrate diet advocate, explains that even short fasts and fasting mimics are beneficial, but the longer the better.

FrugalDoc

10/15/20263 min read

Short Summary: In this conversation (a clip from Dr. Mindy Pelz interviewing Dr. Jason Fung), Fung explains that newer research has shifted his view toward the value of longer fasts and modified approaches over strict water-only fasting alone.

A 7-day water-only fast produces major beneficial changes in protein expression (linked to heart health and more), but many of these only appear after day 3.

Modified options—such as the 5:2 diet (~500 calories on two days) or a fasting-mimicking diet (5 days at roughly 800–1,000 calories, low in protein and sugar to keep insulin and mTOR low)—deliver about 80% of the benefits of pure water fasting while being roughly 50% easier.

Small “fasting aids” (bone broth, tea, olives, avocado, or limited avocado + bacon) can further extend the fast without fully activating nutrient sensors, allowing people to reach the deeper benefits more sustainably.

The core principle is to keep insulin- and protein-driven signals low so the body stays in repair mode rather than growth/storage mode.

Important: Extended fasts - over 2 days should NEVER be done without micronutrient support from sources like fulvic and humic liquids and/or bone broth!

Listen to the short summary here.

Link to Video: https://www.youtube.com/watch?v=BLeSiu-l3vI

Or click on the image.

  • Can't do an extended fast?
  • Can't go an entire day without eating?
  • You can still gain substantial benefits from a fasting-mimicking approach or by skipping at least one meal.
Longer summary (focused on levels/stages of fasting & fasting-mimicking approaches)

Dr. Jason Fung describes an evolution in his thinking: while the core benefits of fasting for weight loss, insulin reduction, and metabolic health (as outlined in his earlier books) still hold, recent studies highlight additional advantages that emerge only with longer durations and show that modified versions can capture most of those advantages with far less hardship.

1. Strict water-only fasting

  • Pure water (or sometimes black coffee/tea) with zero calories.

  • A recent study of a 7-day water fast showed large, largely beneficial shifts in protein expression relevant to heart disease and other conditions.***

  • Critically, many of these bigger changes do not appear until after the third day.

  • This makes multi-day water fasting more powerful for a “full-body reset” but also more stressful and harder to sustain.

*** Fulvic and/or humic liquid and bone broth are a must to prevent bone loss.

2. Modified / fasting-mimicking approaches

Fung emphasizes that traditional water fasting is not the only (or always the best) option. Research now supports structured low-calorie protocols that keep the key nutrient sensors (especially insulin and mTOR) relatively quiet:

  • 5:2 diet (popularized by Michael Mosley): Five days of normal eating + two days restricted to about 500 calories.

  • Fasting-mimicking diet (associated with Valter Longo’s work): Typically five consecutive days at roughly 800–1,000 calories, formulated to be low in protein and sugar/carbohydrates while providing some fat and (cooked) plant-based nutrients. The design deliberately avoids strongly stimulating insulin or mTOR so the body continues to behave as if it is fasting.

Fung’s practical framing: these modified versions give you roughly 80% of the benefits of water-only fasting at about 50% of the difficulty. Because they are easier, people can more readily complete the multi-day periods needed for the deeper effects (inflammation reduction, potential cancer-related benefits, immune and regenerative changes, etc.).

3. “Fasting aids” or “training wheels” (partial calorie intake that still preserves the fasted state)

Even within a modified fast, small amounts of specific foods can make longer durations feasible without fully breaking the metabolic benefits:

  • Bone broth (provides minerals and sodium that help with electrolytes).

  • Tea.

  • Handful of olives.

  • Avocado.

  • Limited “avocado and bacon” style eating (kept low enough in protein and carbohydrate that insulin stays low).

A study mentioned in the discussion compared a 15-hour fast with a 15-hour fast plus a light snack that extended it to 17 hours; the metabolic benefits (weight, glucose, insulin) continued during the extra time. The idea is that a small intake that does not strongly raise insulin or mTOR can let someone go longer and ultimately gain more net benefit than stopping early because of pure hunger.

Underlying mechanism emphasized

The body has nutrient sensors. Carbohydrates raise insulin; protein raises mTOR. When these are elevated, the body shifts into growth and energy-storage mode. When they are kept low, the body enters a rest-and-repair state (autophagy, cellular cleanup, reduced inflammation, etc.). Therefore, the practical rule for any modified or aided fast is: keep total calories moderate and keep protein and carbohydrate relatively low.

To our world of fatophobes, that means consume fats (keto-like foods) during the modified fast.

Practical takeaway on “levels”

  • Shorter daily intermittent fasting remains useful for everyday insulin control and weight management.

  • To access the more profound protein-expression, regenerative, and disease-risk benefits that appear around day 3+, longer periods are needed.

  • Water-only (with micronutrients) can reach those benefits faster (~3 days), while modified/fasting-mimicking protocols typically need ~4–5 (or up to 5–7) days but are substantially more tolerable and therefore more realistic for repeated use.

Fung stresses that research supporting these modified approaches wasn't available years ago; the field has matured, making longer, sustainable fasting more accessible without requiring pure water-only endurance.

Listen to the LONG summary here.