Can Fasting Prevent or Treat Cancer?

Fasting is widely promoted as a way to prevent cancer, and sometimes as a way to treat it. It can mean skipping meals, eating only inside a set window, or going without food for a day or more. The strongest versions suggest fasting can shrink tumors, or stand in for conventional treatment.

Fasting is free. It needs no prescription. And it is entirely within your control. When you are waiting on scan results, or sitting through a treatment you did not choose, doing something today has real pull. The story attached to it sounds mechanical and satisfying: starve the cancer cells, reset the body, make chemotherapy work better. That pull is a reasonable response to feeling powerless. It is not a failure of judgement.

Most of what is known about fasting in humans concerns weight and metabolism, not cancer. Some people find that eating within a shorter daily window helps them eat less overall. And keeping to a healthy weight is one of the best-evidenced ways to lower the risk of several cancers. Those include endometrial, esophageal, liver, kidney, colorectal and postmenopausal breast cancer.

That is a benefit of the weight and the overall diet, though. It is not a benefit of the clock. What has not been shown is that the timing itself adds anything beyond that. No fasting schedule has been demonstrated in clinical outcome trials to prevent cancer or to treat it, and none is recommended for either purpose.

Here the stakes change. Here the two questions have to be kept apart.

Research does exist. Small trials have tested short fasts and fasting-mimicking diets around chemotherapy. Mostly they asked whether people can tolerate them, and whether side effects shift. One systematic review looked at time-restricted eating in people with cancer. It described the findings as preliminary: feasible and acceptable, possibly helpful for quality of life. It also noted that most studies were small single-arm feasibility trials, not randomised trials with survival endpoints. Researchers in the field say it is too early to recommend fasting outside a study.

The risk on the other side is not preliminary at all. Malnutrition is common among people with cancer. The figures quoted in NCI's clinician nutrition summary span a wide range — studies have reported it in anywhere from roughly a third to the large majority of patients — because they cover very different cancers, stages and definitions, so no single number describes any one group. Weight loss during treatment is associated with more severe side effects and higher infection risk, and loss of lean muscle is associated with poorer outcomes independently of weight. Those are associations drawn from observation; they do not prove that the weight loss causes the harm, though the direction is consistent enough that teams act on it. Many people in treatment are already fighting to eat, because of nausea, mouth sores, altered taste or exhaustion. Choosing to eat less, in that situation, can turn a manageable problem into a serious one. And the people most drawn to fasting are often those who can least afford to lose weight.

Does fasting interest you? The useful next step is to raise it with your oncology team, or with an oncology dietitian. They can tell you whether your weight and muscle mass leave any room for it. They can tell you whether it clashes with your drugs or steroid schedule. And they can tell you whether a trial is open near you.

It does not mean fasting research is nonsense. It is an active, legitimate field, and some findings may hold up. It also does not mean short fasts before a procedure are a problem. Those are planned and supervised by your team.

What it means is that the honest answer today is "we do not know yet". And "we do not know yet" is not a reason to eat less while your body is doing hard work.

There is no evidence that fasting prevents cancer apart from its effect on body weight. There is no evidence that it treats cancer, or replaces any part of standard care. During treatment, the risks of eating too little are well documented. The benefits remain unproven. Keeping your strength up is not giving in. For most people it is the part of the plan that makes the rest of it possible.

• NCI - Nutrition During Cancer Treatment (PDQ), Patient Version
• NCI - Diets, Supplements, and Cancer
• The Clinical Impact of Time-restricted Eating on Cancer: A Systematic Review (Nutrition Reviews)
• World Cancer Research Fund - Be a healthy weight: evidence for our recommendations