Prolonged Fasting Forces the Body to Rebuild Its Immune System: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers at the University of Southern California wanted to understand what happens to the immune system during prolonged fasting — specifically, whether extended fasting of 72 hours or more could trigger genuine regeneration of immune cells rather than simply suppressing them. The study combined animal experiments with a small human pilot to test whether fasting before and during chemotherapy could protect cancer patients from chemotherapy-induced damage to the immune system — and what the underlying biological mechanism was. The findings have since become one of the most widely discussed results in fasting science.

Who Was Studied

Participant profile: Cancer patients undergoing chemotherapy; various cancer types; ages and gender breakdown not detailed in the abstract.

How the 72-hour fasting protocol worked in this study: Patients consumed only water and clear non-caloric fluids for 72 hours — 24 hours before chemotherapy began and continuing through the duration of each treatment cycle. The protocol was repeated across multiple chemotherapy cycles.

What the Researchers Found

White Blood Cell Counts: A Planned Drop and Recovery

The study's most striking finding was that prolonged fasting caused a significant reduction in white blood cell counts — but this was followed by a pronounced rebound upon refeeding, with the body generating new, functional immune cells.

• Prolonged fasting caused old and damaged immune cells to be cleared from circulation — a form of cellular housecleaning
• Upon refeeding, hematopoietic stem cells (the precursors to all blood and immune cells) became activated and generated new, functional white blood cells
• IGF-1 (insulin-like growth factor 1) levels dropped significantly during fasting — this reduction was identified as the key upstream trigger for the entire regeneration cascade
• PKA (protein kinase A) activity also decreased, which appears to allow stem cells to shift from dormancy into an active regenerating state

The Mechanism: IGF-1, PKA, and Stem Cells

The researchers identified a clear hormonal pathway:

• Prolonged fasting → reduced IGF-1 and insulin → reduced PKA signalling → hematopoietic stem cells activated → new immune cell production

This pathway explained why the immune system didn't just weaken during fasting — it underwent a cycle of removal and renewal.

Protection Against Chemotherapy-Induced Immunosuppression

• Fasting before chemotherapy appeared to offer protection against the typical drop in immune function associated with cancer treatment
• The regenerated immune cells after fasting may be more functional than the cells they replaced

What Did Not Change

• No evidence of long-term immunosuppression from the fasting cycles
• No adverse events specifically attributed to fasting in the pilot

What the Researchers Concluded

The researchers concluded that prolonged fasting creates a cellular environment in which the immune system undergoes a "reset" — clearing damaged or aged immune cells through reduced IGF-1 and PKA signalling, then regenerating new functional cells from hematopoietic stem cells once eating resumes. The authors described this as "hematopoietic-stem-cell-based regeneration" and suggested it may represent a natural mechanism for immune system renewal.

What This Means If You Fast

• Prolonged fasting (72+ hours) may trigger genuine immune renewal, not just temporary suppression — the body appears to use periods of extended fasting to clear old immune cells and generate new ones via stem cell activation.
• This is not an everyday fasting effect — the mechanism described requires extended fasting of at least 72 hours, well beyond the 16:8 or 18:6 intermittent fasting protocols most people use. Occasional extended fasts may carry different benefits than daily time-restricted eating.
• IGF-1 reduction is central to the mechanism — IGF-1 drops when you fast, which is one reason fasting is associated with longevity research. This study adds immune regeneration as another downstream effect of that drop.
• The findings suggest fasting may be protective of immune function, not damaging — provided the fast is broken correctly and nutritional repletion is adequate. Breaking a fast the right way is particularly important after extended fasting.
• Stem cell activation during re-feeding means that how and what you eat after a prolonged fast may matter for how well your body rebuilds. Protein, in particular, is needed to fuel new cell construction.
• For most people, the practical takeaway is that periodic extended fasting — once per month or quarter — may support immune health in ways that daily shorter fasting does not.

Study Limitations

• Primarily preclinical: The bulk of this research was conducted in mice; the human pilot was small (n=11) and used as proof-of-concept data, not a powered clinical trial
• Cancer patient population: The human participants were undergoing chemotherapy — a population with very different immune dynamics than healthy adults; direct translation to healthy fasters requires caution
• No randomisation in the human component: The pilot lacked a control group of healthy adults doing the same fast without chemotherapy; this makes it impossible to isolate fasting's effect from the chemotherapy context
• Fasting duration (72 hours): The mechanism demonstrated requires very extended fasting, which is beyond what most intermittent fasting protocols use; whether shorter fasts trigger a meaningful version of this pathway is unknown
• Single institution: Findings from USC require replication in larger, independent cohorts
• Self-reported tolerance: Patient tolerability of the 72-hour fast was not rigorously quantified

Source

Cheng CW, Adams GB, Perin L, Wei M, Zhou X, Lam BS, Da Sacco S, Mirisola M, Quinn DI, Dorff TB, Kopchick JJ, Longo VD. (2014). Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression. Cell Stem Cell, 14(6), 810–823. PMID: 24905167

Frequently Asked Questions

Does fasting reset the immune system?

This study suggests that prolonged fasting (72 hours) can trigger a cycle of immune cell clearance and regeneration via stem cell activation — a process researchers describe as an immune system "reset." However, this has primarily been demonstrated in animal models and a small cancer patient pilot; large-scale trials in healthy adults are needed.

How long do you need to fast to trigger immune regeneration?

Based on this study, the mechanism appears to require fasting of approximately 72 hours. This is far longer than the typical 16–18 hour windows used in daily intermittent fasting. Shorter fasts may offer other benefits, but the specific stem cell activation pathway described here requires extended fasting.

Is it safe to fast for 72 hours to boost immunity?

Extended fasting of this length carries risks, particularly for people with diabetes, low blood pressure, eating disorder history, or who are pregnant. The human data in this study was in cancer patients under medical supervision. If you're considering a 72-hour fast, doing so without medical guidance is not recommended.

Can fasting protect you from getting sick?

The study showed a reduction in immunosuppression in chemotherapy patients who fasted — suggesting fasting may help maintain immune function under chemotherapy-induced stress. Whether this translates to better resistance to common infections in healthy adults is a separate question not addressed by this research.

Does this mean fasting weakens the immune system?

The drop in white blood cells during fasting is temporary and part of the regeneration cycle — old cells are cleared, making way for new ones upon refeeding. This is different from the kind of immunosuppression that makes people vulnerable to infection. The net effect, as the researchers describe it, is regeneration, not weakening.

Related Research and Articles

• Intermittent fasting and inflammation: the research explained
• How intermittent fasting promotes autophagy
• Intermittent fasting and longevity: what the science says
• What is extended fasting and is it safe?
• Is it safe to fast for 24 hours or longer?
• What happens to your body during intermittent fasting?

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