Can Fasting Regenerate Your Immune System? What the 2014 USC Study Found and What Human Trials Show

In 2014, headlines announced that California researchers had discovered how to regenerate the entire immune system in 72 hours, and that the method was cheap: fasting. The stories were based on a study from the University of Southern California published in the journal Cell Stem Cell. The research was genuinely interesting, but the headlines went well beyond what it showed. Most of the striking results came from mice, and the idea that people can rebuild their immune systems by skipping food for three days was not established.

Since then, the same research group and others have tested a less extreme approach, a “fasting-mimicking diet,” in human trials, and research on intermittent fasting more broadly has grown. This article explains what the 2014 study found, what human research shows, what the evidence means for immune health, and who should avoid fasting.

Can fasting regenerate your immune system in 72 hours? Not exactly. A widely shared 2014 USC study found that cycles of prolonged fasting in mice, and some human data, could trigger stem cells to renew immune cells, which headlines framed as “rebooting” immunity in three days. The research is promising but early, mostly in animals and alongside chemotherapy. Prolonged fasting carries risks and should not be attempted as a self-treatment without medical supervision.

What Did the 2014 Fasting Study Actually Find?

A 2014 USC study found that cycles of prolonged fasting in mice reduced white blood cells and then triggered stem cells to regenerate immune cells; some human data were included, but the dramatic “reboot” framing went beyond what was shown. The study by Chia-Wei Cheng, Valter Longo, and colleagues, published in Cell Stem Cell in June 2014, examined how prolonged fasting affects hematopoietic stem cells, the cells in bone marrow that produce all blood and immune cells. In mice, cycles of fasting lasting two to three days reduced levels of IGF-1, a growth factor, and dampened a signaling pathway called PKA. This shifted stem cells toward self-renewal and regeneration. When mice received chemotherapy, which damages immune cells, fasting cycles protected stem cells and helped restore white blood cell counts and immune function. In older mice, fasting cycles also appeared to shift the balance of immune cells toward a more youthful pattern.

The paper included limited human data from a small early-phase trial in patients receiving chemotherapy, in which fasting for about 72 hours around treatment was associated with some protection of white blood cells. These observations were preliminary.

The research did not show that fasting “regenerates the entire immune system” in healthy people, and the 72-hour figure referred to fasting periods used around chemotherapy, not a time in which the immune system is rebuilt. Researchers themselves cautioned against unsupervised prolonged fasting, particularly for cancer patients.

From Water Fasting to Fasting-Mimicking Diets

Prolonged water-only fasting is difficult and can be risky. To make the approach more practical, the USC group developed a fasting-mimicking diet: a few days of a low-calorie, low-protein, plant-based diet designed to produce some of the metabolic effects of fasting while providing some nutrition. The diet was later commercialized, and the researchers have disclosed financial interests in the company that sells it, which readers should consider when evaluating the research.

A 2017 randomized trial in Science Translational Medicine by Min Wei and colleagues compared generally healthy adults who continued their usual diet for three months with those who followed the fasting-mimicking diet for five consecutive days each month for three months. The diet cycles were associated with reductions in body weight, body fat, blood pressure, and IGF-1, with larger benefits among participants who had elevated risk factors at baseline. The study measured risk markers, not disease outcomes, and some participants did not complete the regimen.

A 2024 analysis in Nature Communications by Sebastian Brandhorst and colleagues examined participants from two clinical trials and reported that three monthly cycles of the fasting-mimicking diet were associated with reduced insulin resistance and other prediabetes markers, lower liver fat measured by MRI, an increased ratio of lymphoid to myeloid immune cells, which the authors describe as an indicator of younger immune age, and a decrease of about 2.5 years in estimated median biological age. Biological age estimates are calculated from blood markers and remain a research tool rather than a validated predictor of individual health outcomes.

Does Fasting Help With Cancer Treatment?

Research has explored fasting or fasting-mimicking diets alongside chemotherapy to protect healthy cells and improve outcomes, with some promising early results, but this is experimental and must be done only under medical supervision. The original interest in fasting and the immune system came from cancer treatment, where chemotherapy damages blood and immune cells. The DIRECT trial, published in Nature Communications in 2020 by Stefanie de Groot and colleagues in the Netherlands, randomly assigned women with HER2-negative breast cancer receiving chemotherapy before surgery to a fasting-mimicking diet around each chemotherapy cycle or their regular diet. The trial found no difference in severe chemotherapy side effects between groups, its primary goal. In analyses limited to patients who followed the diet, tumor responses on imaging and in surgical specimens were more favorable in the fasting-mimicking group. Because adherence was difficult and these were secondary findings, the authors called for larger trials.

Cancer patients should not fast or follow restrictive diets during treatment without their oncology team’s guidance, because weight loss and malnutrition can be harmful, and some treatments require specific nutritional support.

What About Intermittent Fasting?

Intermittent fasting is different from the prolonged multi-day fasting in the study; it has its own potential metabolic benefits but is not shown to “regenerate” the immune system the way headlines claimed. Intermittent fasting, including time-restricted eating, alternate-day fasting, and the 5:2 pattern, is far more common than prolonged fasting. A 2019 review in the New England Journal of Medicine by Rafael de Cabo and Mark Mattson summarized animal and human research suggesting that intermittent fasting can trigger metabolic switching and cellular stress responses associated with improved insulin sensitivity, blood pressure, and inflammation markers. However, many human trials have found that intermittent fasting produces weight loss and metabolic changes similar to those of standard calorie reduction, suggesting that much of its benefit may come from eating less overall. Evidence that intermittent fasting specifically strengthens immune defenses in people is limited.

Approach	What it involves	State of evidence
Prolonged fasting	Several days of water-only fasting	Striking mouse findings; limited, preliminary human data; higher risks
Fasting-mimicking diet	Several days per month of a low-calorie, low-protein, plant-based diet	Small human trials show biomarker changes; long-term outcomes unknown
Intermittent fasting	Daily eating windows or periodic low-calorie days	Many trials; benefits often similar to calorie reduction
Balanced diet without fasting	Regular meals rich in plant foods and adequate protein	Strong evidence for overall health, including immune function

How Does the Immune System Renew Itself?

The immune system constantly renews itself through blood-forming stem cells in the bone marrow; the research suggested fasting might stimulate this process, but the body replaces immune cells continually regardless. The immune system constantly renews itself. Hematopoietic stem cells in the bone marrow produce billions of new blood cells every day, including white blood cells such as lymphocytes and neutrophils that fight infection. Some immune cells live only days, while memory cells can persist for decades. With age, stem cells tend to produce more myeloid cells, involved in inflammation, and fewer lymphoid cells, such as T and B cells, which contributes to weaker responses to infections and vaccines in older adults. This shift is part of what researchers call immunosenescence. The 2014 mouse study was notable because fasting cycles appeared to partially reverse this shift, but similar effects in people have only been suggested by biomarker changes.

IGF-1, PKA, and Autophagy Explained

IGF-1 (insulin-like growth factor 1) is a hormone that promotes growth and is influenced by protein and calorie intake. Lower IGF-1 levels during fasting appear to signal cells to shift from growth toward maintenance and repair.

PKA (protein kinase A) is a signaling enzyme involved in nutrient sensing. Reducing its activity in stem cells was linked in the 2014 study to self-renewal.

Autophagy is a process by which cells break down and recycle damaged components. Its mechanisms were the subject of the 2016 Nobel Prize in Physiology or Medicine awarded to Yoshinori Ohsumi. Fasting stimulates autophagy in animal studies, but measuring it in humans is difficult, and claims about how many hours of fasting are needed to “activate autophagy” in people go beyond current evidence.

Is Prolonged Fasting Safe?

Prolonged fasting can cause dehydration, nutrient deficiencies, muscle loss, and dangerous drops in blood sugar or blood pressure, and is risky for many people, so it should not be undertaken without medical guidance. Long-term calorie restriction, eating less every day without malnutrition, has extended lifespan in many animal studies. In people, the CALERIE trial, funded by the U.S. National Institutes of Health, tested moderate calorie restriction over two years in healthy, non-obese adults. Participants reduced their calorie intake less than planned but still showed improvements in some cardiometabolic risk factors. Calorie restriction also produced some reductions in muscle and bone density, highlighting potential trade-offs. Whether long-term restriction or periodic fasting extends healthy lifespan in humans remains unknown.

A Note on Commercial Products

Some fasting research has been conducted by scientists with financial ties to companies selling fasting-related products, including packaged fasting-mimicking diets. Such ties do not invalidate research, and they are disclosed in publications, but they are a reason to look for independent replication. Consumers should also be cautious about supplements and programs marketed with claims of “immune reset” or “stem cell regeneration,” which go well beyond the evidence.

What Actually Supports Immune Health

The immune system is complex and does not simply need “rebooting.” Strong evidence supports several everyday factors: getting enough sleep, as sleep deprivation impairs immune responses; regular physical activity; a diet with adequate protein, vegetables, fruits, whole grains, and sources of nutrients such as vitamin D, zinc, and vitamin C; not smoking; limiting alcohol; managing chronic stress; and staying up to date on recommended vaccinations, which train the immune system to recognize specific threats. For older adults, whose immune function naturally declines, these measures and vaccination are particularly important.

Malnutrition, including insufficient protein and calories, weakens immune function. This is one reason prolonged or repeated fasting without medical guidance can backfire, especially for older adults, people with chronic illnesses, and those who are already underweight.

Fasting, Inflammation, and Infection

Research in animals and small human studies suggests that fasting can lower some markers of inflammation, such as C-reactive protein and certain inflammatory signaling molecules, possibly by reducing the activity of inflammatory immune cells. Some researchers have explored whether this could help in autoimmune or inflammatory conditions, with early studies in conditions such as multiple sclerosis and rheumatoid arthritis. Results so far are preliminary. Fasting during an acute infection is a different matter: appetite loss during illness is common, but deliberately restricting food while sick has not been shown to help in people and may be harmful for those who are frail, have diabetes, or are at risk of dehydration. People who are ill should follow medical advice about eating and drinking.

Religious and Cultural Fasting

Many people fast for religious or cultural reasons, such as during Ramadan, Lent, Yom Kippur, or various Hindu and Buddhist observances. Studies of Ramadan fasting have found modest changes in weight and metabolic markers, generally returning to baseline afterward, and have provided guidance for people with diabetes and other conditions who wish to fast safely. Religious authorities typically exempt people for whom fasting would be harmful, such as those who are ill, pregnant, or elderly. Anyone with a chronic condition who plans to fast for religious reasons can work with their clinician to adjust medications and monitor health.

Questions to Ask Before Trying Any Fasting Approach

What is my goal, and is fasting the best way to achieve it? Do I take medications that could cause problems without food? Do I have a history of disordered eating or a complicated relationship with food? Am I getting enough protein and nutrients overall? How will I know if fasting is harming me, for example through dizziness, fatigue, or worsening mood? Would changes such as better sleep, more physical activity, or a balanced diet achieve the same goals with fewer risks? Discussing these questions with a clinician or registered dietitian can help.

Who Should Not Fast Without Medical Supervision

People with a history of eating disorders. Fasting can trigger or worsen disordered eating.

People taking diabetes medications. Insulin and some oral medications can cause dangerously low blood sugar during fasting.

Pregnant or breastfeeding people. Nutritional needs are higher, and fasting is generally not advised.

Children and teenagers. Growing bodies need consistent nutrition.

People who are underweight or frail, including many older adults.

People with certain medical conditions, such as kidney disease, heart rhythm problems, or a history of gout, or those taking medications that must be taken with food.

People undergoing cancer treatment, unless fasting is part of a supervised clinical plan.

Common side effects of fasting include hunger, fatigue, dizziness, headaches, irritability, and difficulty concentrating. Prolonged fasting can also cause dehydration, electrolyte imbalances, and, when eating resumes after extended fasting in malnourished people, a potentially dangerous condition called refeeding syndrome.

Why the Headlines Overstated the Science

The “regenerate your immune system in 72 hours” story is a classic example of how early research can be distorted. A careful study in mice, with preliminary human observations in a specific medical setting, became a promise of a cheap, quick fix for anyone. The researchers’ own caution was lost. When reading health headlines, it helps to ask whether a study was conducted in animals or people, how many people were involved, what was actually measured, and whether the results have been replicated. In this case, the honest summary is that fasting affects stem cells and metabolism in intriguing ways, and research in people continues.

Key Terms

Hematopoietic stem cells: Bone marrow cells that produce all types of blood and immune cells.

IGF-1: A growth hormone influenced by diet; lower levels are associated with fasting.

Fasting-mimicking diet: A short, low-calorie, low-protein, plant-based diet designed to produce some effects of fasting.

Intermittent fasting: Eating patterns that alternate periods of eating with periods of fasting or very low intake.

Immunosenescence: Age-related decline in immune function.

Refeeding syndrome: Dangerous shifts in fluids and electrolytes that can occur when malnourished people resume eating.

The Bottom Line

The 2014 USC study showed that repeated fasting cycles could help regenerate blood-forming stem cells and restore immune function in mice, particularly after chemotherapy. It did not show that people can regenerate their entire immune system by fasting for 72 hours. Later human trials of a fasting-mimicking diet have found changes in metabolic and immune-related biomarkers, but long-term health benefits are unproven, and fasting carries real risks for some people. The most reliable ways to support immune health remain sleep, physical activity, balanced nutrition, avoiding smoking, and recommended vaccinations, with any fasting approach discussed with a clinician first.

How to Read Fasting Headlines

Fasting research continues to generate headlines promising rejuvenation, weight loss, or disease prevention. A few questions can help readers judge them: Was the study done in animals, cells, or people? If in people, how many participants, for how long, and was there a control group? Did researchers measure actual health outcomes, such as infections or heart attacks, or biomarkers that may or may not predict them? Do the researchers have financial interests in a related product? Have the findings been replicated by independent groups? Applying these questions to the 2014 study shows why a mouse experiment with preliminary human data could not support the claim that people can reset their immune system in three days.

Older Adults and Fasting

Older adults are often the intended audience for anti-aging claims, yet they face some of the greatest risks from fasting. Age-related loss of muscle, known as sarcopenia, increases the importance of adequate protein intake, and periods of very low intake can accelerate muscle loss. Older adults are also more likely to take medications that require food or that can cause low blood pressure or low blood sugar, and they may be more vulnerable to dehydration. For this group, maintaining muscle through resistance exercise and sufficient protein, staying up to date on vaccines recommended for older adults, and managing chronic conditions are especially important for immune resilience.

What Researchers Are Studying Next

Ongoing research is examining whether periodic fasting or fasting-mimicking diets can reduce chemotherapy side effects in larger trials, improve responses to cancer immunotherapy, help manage autoimmune conditions, or slow age-related decline. Other studies are investigating how the timing of meals, including eating earlier in the day, affects metabolism and inflammation. Researchers are also exploring drugs that mimic some cellular effects of fasting without requiring people to stop eating. Results from these studies over the next several years will clarify whether early promise translates into practical, safe recommendations.

Until then, major medical organizations do not recommend prolonged fasting for immune health, and people interested in fasting-related research may be able to participate in clinical trials, which offer careful monitoring.

Balanced Eating Patterns With Strong Evidence

For people seeking to improve metabolic and immune health through diet, eating patterns with strong evidence offer a safer starting point than prolonged fasting. The Mediterranean diet, rich in vegetables, fruits, legumes, whole grains, nuts, olive oil, and fish, has been associated with lower rates of cardiovascular disease and other chronic conditions in large studies and trials. Diets that limit ultra-processed foods, added sugars, and excess alcohol while providing adequate protein and fiber support a healthy weight and gut microbiome, which interacts closely with the immune system. Regular meal timing, avoiding very late eating, and consistent sleep also support metabolic health. These approaches can be sustained for life without the risks that accompany extended fasting.

For people who find time-restricted eating convenient, such as eating within a 10- to 12-hour window each day, the approach appears safe for most healthy adults and may help some people reduce snacking. As with any eating pattern, overall food quality and quantity matter more than timing alone.

Fasting and Weight Loss Medications

Interest in fasting has grown alongside new weight loss medications, such as GLP-1 receptor agonists, which reduce appetite. Some people combine these medications with intermittent fasting or very low-calorie eating. Because these drugs already reduce food intake, adding prolonged fasting may increase the risk of inadequate protein intake, muscle loss, dehydration, and gastrointestinal side effects. People taking these medications should discuss eating patterns with their prescribing clinician and aim for adequate protein and fluids. Similarly, people taking medications for diabetes, blood pressure, or other conditions should not change their eating patterns dramatically without medical guidance, since doses may need adjustment.

Practical Takeaways

Treat dramatic claims about fasting with skepticism, especially promises of quick immune “resets.” If you are healthy and curious about intermittent fasting, focus on overall diet quality and discuss it with a clinician if you take medications. Avoid prolonged fasting unless it is medically supervised, particularly if you have chronic health conditions, are older, or have a history of disordered eating. Prioritize the well-established foundations of immune health: sleep, physical activity, balanced nutrition with enough protein, stress management, not smoking, limited alcohol, and recommended vaccinations.

Common Myths About Fasting

“Fasting resets your immune system.” Mouse studies showed stem cell effects; human evidence is limited to biomarker changes.

“Longer fasts are always better.” Longer fasts carry greater risks, including muscle loss, electrolyte problems, and refeeding complications.

“Fasting detoxifies the body.” The liver and kidneys handle detoxification continuously; fasting is not required.

“Everyone can fast safely.” Many groups, including people with eating disorder histories, diabetes on medication, pregnancy, or frailty, should not fast without supervision.

“Fasting cures cancer.” Research on fasting alongside cancer treatment is early and does not support fasting as a treatment.

Key Facts at a Glance

The 2014 study in Cell Stem Cell found that repeated two- to three-day fasting cycles in mice lowered IGF-1, promoted blood stem cell regeneration, and reversed chemotherapy-induced immune suppression, with only preliminary human data. A 2017 trial found that a five-day monthly fasting-mimicking diet for three months lowered weight, body fat, blood pressure, and IGF-1 in generally healthy adults. A 2024 analysis reported about 2.5 years lower estimated biological age after three cycles. The 2020 DIRECT trial found no reduction in severe chemotherapy side effects but possible improvements in tumor response among adherent patients. None of these studies shows that fasting regenerates the human immune system in 72 hours, and fasting is not safe for everyone.

A Balanced View

The fasting research described here is scientifically interesting and may eventually lead to useful clinical applications, particularly in supporting patients through chemotherapy or in addressing metabolic risk factors. It is worth following, and well-designed trials should continue. But interest should not be mistaken for proof. Until larger, longer, independent studies show clear health benefits and define who can safely benefit, the most responsible position is cautious curiosity: learn about the research, discuss it with a clinician if interested, and avoid extreme approaches marketed with promises the science cannot yet support.

For readers drawn to fasting by headlines about immune regeneration, the encouraging message is that the basics of immune health are accessible to nearly everyone: adequate sleep, regular movement, nourishing food, social connection, and timely vaccinations. These habits do not promise a dramatic reset, but they are supported by far stronger evidence than any fasting protocol currently available, and they carry little risk for most people.

Anyone who has had a bad experience with fasting, such as feeling faint, losing control around food afterward, or becoming preoccupied with eating, should stop and talk with a healthcare professional. Those reactions are signals that a fasting approach is not right for them, and support is available.

Registered dietitians can help people design eating patterns that fit their health conditions, medications, preferences, and budgets, and many insurance plans cover nutrition counseling for conditions such as diabetes and kidney disease.

Community health centers and hospital nutrition services can also connect people with these professionals when cost or access is a concern.

Frequently Asked Questions

Can fasting regenerate your immune system in 72 hours?

Not in the dramatic way headlines suggest. A 2014 USC study found that cycles of prolonged fasting in mice, with some supporting human data, could prompt stem cells to regenerate immune cells. This was reported as “rebooting” the immune system in three days, but the research was early and largely in animals or alongside chemotherapy. There is no proven way to simply fast for 72 hours and renew your immunity.

What did the famous fasting and immune study actually show?

The study, led by Valter Longo at USC, found that in mice, repeated cycles of prolonged fasting lowered white blood cell counts and then activated blood-forming stem cells to produce new immune cells. It suggested fasting might help the body clear old immune cells and regenerate them, and included some human data from people fasting before chemotherapy. The findings were promising but preliminary, not a proven human therapy.

Is fasting safe to do for immune or health benefits?

Short, moderate fasting is safe for many healthy adults, but prolonged multi-day fasting carries real risks, including dehydration, nutrient deficiencies, muscle loss, and dangerous drops in blood sugar and blood pressure. It is especially risky for people who are pregnant, underweight, diabetic, elderly, or have certain medical conditions. Prolonged fasting should only be done under medical supervision, not as a self-directed immune “reboot.”

Is intermittent fasting the same as the fasting in the study?

No. Intermittent fasting, such as eating within an 8-hour window or alternate-day eating, is different from the prolonged, multi-day water fasting or fasting-mimicking cycles used in the immune research. Intermittent fasting has its own potential benefits for weight and metabolic health, but it has not been shown to regenerate the immune system. Confusing the two can lead to unrealistic expectations.

Can fasting help during chemotherapy?

This is an active area of research. Some studies suggest that fasting or fasting-mimicking diets around chemotherapy might protect healthy cells and reduce side effects while making cancer cells more vulnerable, with some encouraging early results. However, this remains experimental, results are not conclusive, and fasting during cancer treatment can be dangerous without oversight. Patients should never fast during treatment except as part of medical care directed by their oncology team.

References

1. Cheng CW, Adams GB, Perin L, et al. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression. Cell Stem Cell. 2014;14(6):810–823. PMID 24905167
2. Wei M, Brandhorst S, Shelehchi M, et al. Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease. Science Translational Medicine. 2017;9(377):eaai8700. PMID 28202779
3. de Groot S, Lugtenberg RT, Cohen D, et al. Fasting mimicking diet as an adjunct to neoadjuvant chemotherapy for breast cancer in the multicentre randomized phase 2 DIRECT trial. Nature Communications. 2020;11(1):3083. PMID 32576828
4. Brandhorst S, Levine ME, Wei M, et al. Fasting-mimicking diet causes hepatic and blood markers changes indicating reduced biological age and disease risk. Nature Communications. 2024;15(1):1309. PMID 38378685
5. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. 2019;381(26):2541–2551. PMID 31881139

Last updated: October 4, 2026