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Intermittent Fasting and Cancer: What Is Known

Does intermittent fasting change cancer risk, and what about fasting during treatment? Studies in people are few and small. Here is what the reviews and trials show, and what they do not.

By Cancer Explained Editorial TeamPublished

Original commentary from the Cancer Explained editorial team.

Review status: AI-assisted, source checked. Not reviewed by a clinician. How we check pages

Sources last checked .

A bowl of salad with carrot, tomato, cucumber and a toasted bread crisp.
A bowl of mixed salad vegetables. Gerda Arendt, via Wikimedia Commons (CC0)

Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.

The short answer

We do not know. No study we opened shows that intermittent fasting raises or lowers the chance of getting cancer. The human evidence on cancer risk is thin, and the studies on fasting during treatment are small.

One 2021 review in a cancer journal said the effect of intermittent fasting on human cancer incidence "remains unknown" because there are no high-quality randomized trials. That is still the picture in the newer reviews we opened.

If you are in cancer treatment, do not start fasting on your own. Talk with your care team first. Treatment can make it hard to eat enough, and that matters.

The idea as it circulates

The idea goes like this. Fasting changes how the body uses energy. In animals, it may slow tumors or make treatment work better. So fasting, or eating only in a short daily window, might lower cancer risk or help treatment.

Parts of this are real. Animal studies do show effects. But the 2021 review noted that results in rodents are "controversial" and may even point to harm in some cancers. Mouse results do not tell us what happens in people.

Here are the common forms of the idea:

  • Time-restricted eating: you eat only within a set window each day, such as 8 hours.
  • Alternate-day fasting: you eat very little every other day.
  • Fasting-mimicking diets: a few days of a low-calorie, planned diet.

What the studies show: cancer risk in healthy people

We found no trial that followed healthy people on intermittent fasting to see who got cancer. The 2021 review says high-quality randomized trials on this are lacking.

A 2025 commentary in the Journal of the National Cancer Institute says that human time-restricted eating studies are "less consistent" than animal ones and are "generally short term (<1 year)." It says NCI funded five intermittent fasting studies in 2020, four on time-restricted eating, to ask how fasting affects cancer incidence, treatment response and outcomes.

The 2021 review said that intermittent fasting "may be considered" for adults who want to manage weight for cancer prevention. But it said it is still an open question whether fasting itself changes cancer-related pathways.

What the studies show: weight and body markers

An umbrella review (a review of reviews) pooled randomized trials of intermittent fasting. It found small changes in body fat, fasting insulin and cholesterol in adults with overweight or obesity. For example, fat mass fell by 0.72 kg on average compared with a usual diet. These are not cancer outcomes. The review did not measure whether anyone got cancer.

The link to cancer is indirect. NCI says obesity raises the risk of several cancers. On weight loss, NCI says that most data come from observational studies. It also says randomized weight loss trials sometimes did not produce much weight change and "did not appear to reduce cancer risk." Some observational studies found lower risks of some cancers in people who lost weight. These studies cannot show cause.

So fasting may help someone lose weight, and weight matters for some cancers. But no source we opened shows that fasting lowers cancer risk by itself.

What the studies show: fasting during treatment

Most studies are small, short and mostly in breast cancer. A few examples:

  • DIRECT trial. 131 people with early breast cancer were randomly assigned to a fasting-mimicking diet or their usual diet. They ate the diet for 3 days before and during chemotherapy. Side effects did not differ between groups. More people on the diet had a response on scans (odds ratio 3.168, p = 0.039). A second pathology measure was better only in a per-protocol analysis, which looks only at people who followed the plan (de Groot 2020).
  • A 44-person randomized trial. Patients on a fasting-mimicking diet had less severe vomiting and low white cells than the control group. Response was also better in the diet group. The trial was small (Front Nutr 2024).
  • Reviews. A 2023 systematic review found eight studies with 379 patients. It said they were mostly low to moderate quality and "insufficiently powered," and that evidence for fewer side effects was "missing." A 2025 review found nine studies. It said fasting was safe and doable, but "no impact on treatment outcomes and chemotherapy-related toxicities was demonstrated." A 2026 scoping review of 20 studies (871 people) found some signs of less fatigue and less toxicity. It said the findings were mixed and came mostly from small studies. Adverse events were "generally mild and transient."

So the results lean slightly positive in some trials, flat in others, and none shows that fasting treats cancer. No trial we opened showed that fasting improved survival.

What official advice says

We did not find an NCI or other official page, which we could open, that recommends fasting for cancer prevention or during treatment.

What we did open is NCI's page on nutrition during treatment. It says people with cancer "may need extra protein and calories" to keep strength up and to prevent malnutrition. It says cancer and its treatment can affect appetite and the ability to eat enough or absorb nutrients. NCI says a registered dietitian can help.

The 2021 review's authors said they "would not currently recommend" that patients in active treatment try intermittent fasting outside a clinical trial. A 2023 review said that unintended weight loss can harm outcomes, so fasting "is not indicated" in that situation. These are review authors' views, not official guidelines.

What about cancer?

Fasting is not a treatment for cancer. No source we opened says it is. Trials that test fasting alongside chemotherapy are testing it as an add-on, and the results so far are too small and mixed to guide care.

If you are curious about fasting during treatment, bring it up with your oncology team or a registered dietitian before you change how you eat. Do not fast on your own during treatment. Our pages on fasting during cancer treatment and whether fasting can prevent or treat cancer have more.

What this story cannot tell you

  • It cannot say whether fasting changes your chance of getting cancer. No trial we opened tested that.
  • It cannot say whether fasting helps or harms treatment for a given cancer, drug or person. Trials were small and mostly in breast cancer.
  • Fasting forms differ a lot between studies, so results do not add up neatly.
  • It cannot tell you whether any benefit comes from fasting itself or from eating fewer calories and losing weight.
  • Most studies were short. Long-term safety is not known.
  • We could not open the American Cancer Society page on fasting (it blocked our access), so we did not use it.

Sources

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed. We opened the sources above, except where a limit is stated in the text.

See an error, old source, or unclear wording? Tell us.

Put the story in context

Prevention, possible warning signs, screening, and diagnosis

This story relates to Diet and cancer. The information below is general, and not every point applies to every cancer. Personal advice depends on age, symptoms, family history, exposures, and medical history.

  • Prevention and risk reduction

    Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.

    NCI prevention information ↗

  • Symptoms and possible early signs

    Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.

    NCI signs and symptoms ↗

  • Screening and early detection

    Screening looks for certain cancers before symptoms begin. Recommended tests exist only for some cancers and depend on age and risk. Screening can have benefits and harms; it is not the same as evaluating a new symptom, and there is no single routine scan or blood test that reliably screens for every cancer.

    NCI cancer screening information ↗

  • How cancer is diagnosed

    Diagnosis may involve a history and exam, imaging, laboratory tests, and often a biopsy. Pathology can identify the cancer type and may test biomarkers that guide treatment. Symptoms, screening results, tumor markers, or online stories alone cannot confirm cancer.

    NCI diagnosis information ↗

Learn more about diet and cancer

A news story may encourage questions, but it should not be used to estimate your risk or choose testing. Contact a healthcare professional about a persistent or concerning change. Seek urgent care for severe or rapidly worsening symptoms.

Related Cancer Explained resources

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