Intermittent Fasting and Cancer: Prevention, Treatment, and Recurrence

Perhaps the most active area of research on intermittent fasting and cancer is its role during cancer treatment – that is, can fasting periods be used to improve the effectiveness of treatment or reduce side effects? Conventional cancer treatments like chemotherapy and radiation are very demanding on the body. The idea of using fasting to "soften the blow" on healthy cells while keeping the pressure on cancer cells is being studied in clinical trials. Here, we will look at how fasting might support chemotherapy, immunotherapy, and radiation therapy, based on current evidence.

Fasting Around Chemotherapy

Chemotherapy drugs target rapidly dividing cells, which is why they kill cancer cells but also damage fast-dividing healthy cells (like those in hair follicles, gut lining, and bone marrow). Fasting may shift this balance by slowing the division of normal cells (making them harder to harm) while cancer cells, with their stuck accelerator pedal, keep dividing and remain sensitive. This differential effect was first hinted at in cell and animal studies: for example, in mice, fasting for 2 days before a high dose of chemotherapy protected the healthy cells and bone marrow, but tumors did not get the same protection. Following these findings, early trials in cancer patients have been conducted to test safety and effects.

Early Patient Experiences: The first human evidence came from case reports. In 2009, a case series was published involving 10 cancer patients who voluntarily fasted before and after chemotherapy (fasts ranging ~48–140 hours prior to chemo). These patients noted that when they fasted around their chemo sessions, they experienced fewer side effects – less fatigue, less weakness, and fewer gastrointestinal issues – compared to their earlier chemo sessions without fasting. Importantly, their doctors observed that fasting did not make the chemotherapy any less effective at shrinking tumors or lowering tumor markers. This was an encouraging sign that short-term fasting is feasible and safe during chemo for many patients, at least under careful observation. One patient in that report described feeling unusually energetic during the fasting period of chemotherapy, in stark contrast to the debilitating fatigue she felt when she received chemo on a full diet – a striking anecdote, but of course just one person's experience.

Clinical Trials and Studies: Since those early cases, a number of small clinical studies have been carried out:

• A Dutch study in 2015 randomized 13 women with early breast cancer to either fast for 2 days (60 hours total, spanning before and after chemotherapy) or follow their normal diet during chemotherapy. The fasting group had significantly less damage to blood cells. Specifically, they maintained higher white blood cell and platelet counts after chemo compared to the non-fasting group. This suggested that fasting might shield the bone marrow from chemo toxicity. Both groups had similar tumor responses, indicating fasting didn't hinder the treatment.

• A controlled trial in Germany (2018) had 34 patients undergo alternating cycles of chemotherapy with and without a 60-hour fast (each patient served as their own control in a crossover design). Fasting was generally well tolerated – side effects from fasting itself were mild, mostly hunger, a bit of headache or nausea, and one instance of lightheadedness. Crucially, patients did not lose significant weight during the fasting cycles. When patients fasted, their quality of life during chemotherapy was better: standard surveys showed that fasting patients reported less reduction in well-being and less fatigue during chemo, compared to when they did chemo without fasting. In essence, fasting helped them get through treatment with a higher quality of life.

• A recent randomized trial in 2023-2024 from Iran studied a fasting-mimicking diet (FMD) in 44 breast cancer patients receiving neoadjuvant chemotherapy (chemo before surgery). An FMD is a low-calorie, low-protein diet designed to mimic fasting's effects while still providing minimal nourishment (~600 kcal/day in this case). Patients in the FMD group ate this way for 3 days prior to each chemo cycle (every 3 weeks), while the control group ate their regular diet. The results were impressive: patients on the fasting-mimicking diet had fewer serious side effects – for example, far fewer had severe (Grade 3) vomiting or neutropenia (dangerously low neutrophil counts) compared to the control group. Moreover, the FMD group's metabolic markers shifted in beneficial ways (their IGF-1 and insulin levels went down during treatment, whereas the control group's went up). When doctors evaluated the tumors after chemotherapy, the FMD group was more likely to have a robust tumor response, including some near-complete tumor disappearances, than the regular diet group. In short, this trial suggests that a fasting-like diet can both protect patients from chemo's toxicity and potentially make the chemo attack the cancer more effectively – a very promising dual benefit.

• Multiple phase I/II trials in the U.S. and Europe (several by Dr. Valter Longo's team and collaborators) have consistently found that 24–72 hour fasting around chemo is safe and achievable for patients. In one study of 20 patients on chemotherapy, those who fasted for 48 or 72 hours had reduced DNA damage in their blood cells compared to those who fasted only 24 hours. There was also a trend toward fewer high-grade neutropenia episodes with longer fasts, though the sample was too small for definitive stats. These fasting periods led to significant drops in IGF-1 (up to one-third reduction). Patients did report some hunger and fatigue during fasting, but overall toxicities were limited to grade 1–2 (mild) symptoms like headache or lightheadedness.

Taken together, these studies indicate that short-term fasting or fasting-mimicking diets around the time of chemotherapy are feasible, generally safe, and may reduce side effects while potentially enhancing tumor response. Patients who fast often have less bone marrow suppression, translating to less risk of infection and faster rebound of blood counts. Many also report subjectively that chemotherapy is easier to tolerate when fasting.

It's important to note that these trials were done in controlled settings with medical supervision. Not every patient is a good candidate for fasting – those who are very underweight or already frail might not tolerate it (we will discuss more in the Risks section). But for those who are able, fasting could become a valuable adjunct to chemo. Larger phase III trials (for example, the ongoing "DIRECT" studies in Europe) are currently testing if these promising results hold up in bigger patient populations and whether fasting ultimately improves long-term outcomes like survival or cancer recurrence rates.

Fasting and Immunotherapy

Cancer immunotherapy – treatments that stimulate the patient's own immune system to attack tumors (such as checkpoint inhibitor drugs like anti-PD-1/PD-L1 or CAR-T cell therapies) – has revolutionized cancer care in the past decade. However, not all patients respond to immunotherapy, and it can cause significant immune-related side effects. Researchers are now asking if fasting could be a way to improve immunotherapy's effectiveness and modulate its side effects, based on intriguing early findings.

Immune Activation: In mouse models of cancer, short fasting cycles or fasting-mimicking diets have shown the ability to boost anti-tumor immunity. One 2022 study found that a periodic fasting-mimicking diet rendered an otherwise "cold" (low-immunity) breast cancer tumor susceptible to immunotherapy. Normally, some tumors do not respond to checkpoint inhibitor drugs because they have very few immune cells inside them. But with fasting cycles, these mice's tumors showed increased infiltration of T cells, and the immunotherapy was then able to slow tumor growth dramatically. Essentially, fasting turned up the dial on the immune system in the tumor microenvironment. Another study from 2024 reported that fasting in mice not only delayed cancer growth but also reduced the autoimmune side effects associated with immunotherapy. Mice on an on-and-off fasting diet had less inflammation in organs like the heart and colon during immunotherapy, suggesting fasting might protect against the collateral damage of an overstimulated immune system.

Specific immune cells have been studied as well. Natural Killer (NK) cells, which are a type of white blood cell that can attack cancer cells, appear to be invigorated by fasting. Researchers at Memorial Sloan Kettering Cancer Center showed that fasting periods reprogrammed NK cells to be more efficient at destroying cancer cells in mice. These fasted NK cells had improved cancer-fighting properties, leading to slower tumor growth. This kind of immune enhancement is exciting, because it suggests fasting might work hand-in-hand with immunotherapy drugs: the fasting provides a stronger immune army, and the drugs rally that army to fight the cancer.

Early Human Insight: Direct clinical trial evidence for fasting with immunotherapy is still in early stages. Some cancer centers are beginning pilot trials where patients on immunotherapy also do intermittent fasting or fasting-mimicking diets to see if their response rates improve. While results are not yet published, the rationale is strong given the animal data. Anecdotally, a few patients have tried combining dietary strategies with immunotherapy and reported good tolerance, but we have to wait for systematic studies to know if there is a true benefit.

It is also worth noting that by reducing factors like insulin, fasting may counteract some of the immune-suppressive signals that high insulin and high sugar environments promote. High insulin can lead to more rapid tumor growth and may dampen immune function, so fasting's ability to keep insulin low could indirectly favor the immune system's cancer surveillance.

Overall, while the evidence is preclinical for now, fasting shows potential to both enhance the cancer-killing immune response and reduce certain side effects of immunotherapy. This dual effect – similar to what is seen with chemotherapy – positions fasting as a promising complementary strategy. Ongoing research will tell us if these benefits hold true in patients. If they do, we might one day see dietary protocols integrated into immunotherapy regimens to improve outcomes.

Fasting and Radiation Therapy

Radiation therapy, like chemo, damages DNA in cells – it's directed at tumors but unavoidably affects normal tissues in the radiation field. Here too, scientists are testing whether fasting can protect normal cells (by putting them in a more radio-resistant state) and possibly sensitize cancer cells (which may struggle to repair DNA damage when nutrients are scarce). The evidence here is somewhat limited but growing:

Protecting Normal Tissue: One of the concerns in radiation therapy is collateral damage to healthy tissues, which can limit the dose of radiation doctors can safely deliver. Animal studies have provided proof-of-concept that fasting might increase the tolerance of healthy tissue to radiation. For example, a 2019 study showed that a 24-hour fast in mice reduced intestinal radiotoxicity – meaning the mice's intestines suffered less damage from abdominal radiation. This allowed researchers to escalate the radiation dose to treat pancreatic cancer in those mice without causing fatal bowel injury. Essentially, fasting gave the normal cells in the gut a better chance to weather the radiation, presumably by slowing their division and activating stress-resistant pathways.

There is also evidence that fasting can enhance DNA repair in healthy cells. The German study (Bauersfeld et al.) mentioned earlier noted that peripheral blood cells of patients who fasted had faster DNA damage repair after chemo/radiation exposure than those of non-fasting patients. By a similar token, one could expect fasting to help normal cells more quickly mend the sub-lethal DNA damage from radiation, reducing long-term side effects or organ damage.

Sensitizing Cancer Cells: Tumor cells often live in a nutrient-rich environment (many cancers actually reprogram metabolism to pull in lots of glucose). Sudden nutrient deprivation can weaken a cancer cell's ability to fix radiation-induced DNA breaks. Some cell culture experiments have found that cancer cells exposed to fasting conditions or low glucose are more likely to be killed by radiation, although this line of research is still in early phases. The theory is that fasting might push cancer cells into a vulnerable cell cycle phase or make them less efficient at activating repair proteins, thereby enhancing the radiation's tumor-killing effect.

Clinical Observations: In terms of patient data, there was a pilot where breast cancer patients undergoing radiation tried a 5:2 fasting diet (severely cutting calories two days a week). The results indicated it was safe and feasible – patients managed to do it during weeks of radiation without major issues – and they saw reductions in fat mass and some improved metabolic markers. While that study was more about weight control (since many breast cancer patients on hormonal therapy tend to gain weight), it demonstrated that intermittent fasting did not interfere with radiation treatment and might offer metabolic benefits. Quality-of-life during radiation was not significantly worse with fasting; in fact, some patients felt it gave them a sense of proactive control.

Another angle: patients who are very overweight are at risk of more complications in radiation therapy (and the excess adipose tissue can increase inflammatory cytokines). By helping with weight management, intermittent fasting might indirectly improve radiation outcomes or reduce complications, though this is a hypothesis that needs investigation.

In summary, fasting's role in radiation therapy support is still being clarified. Early data suggest it can reduce toxicity to normal tissues (at least in animals) and is doable for patients. More studies are needed to confirm if fasting before radiation sessions might allow higher dosing or better tumor control. If you are a patient, do not start fasting during radiation without discussing with your oncology team – but know that researchers are actively studying this, and there may be clinical trials available. As always, any complementary approach should be coordinated with medical professionals so that it's done safely and at the right time.