Intermittent Fasting and Cancer: What the Evidence Shows

Intermittent fasting and cancer is a topic attracting growing scientific interest, yet the evidence remains at an early stage. Intermittent fasting involves cycling between defined eating windows and periods of caloric restriction, and researchers are exploring whether its metabolic effects, including reductions in insulin, IGF-1, and inflammation, might influence cancer risk or treatment. This article explains how intermittent fasting works, what current research shows, what UK health guidelines recommend for cancer prevention, and when to speak to your oncology team before making any dietary changes.

What is intermittent fasting and how does it work?

Intermittent fasting cycles between eating windows and fasting periods, triggering metabolic changes such as reduced insulin, increased fat oxidation, and autophagy. It is not suitable for everyone, and medical advice is essential for those with diabetes, eating disorders, or other relevant conditions.

Intermittent fasting is an eating pattern that cycles between defined periods of eating and deliberate caloric restriction or complete abstinence from food. Rather than specifying which foods to eat, it focuses on when eating occurs. Several structured approaches exist, including:

• 16:8 method: eating within an eight-hour window and fasting for sixteen hours each day

• 5:2 diet: eating normally for five days per week and restricting calories to around 500 to 600 kcal on two non-consecutive days

• Alternate-day fasting: alternating between unrestricted eating days and fasting or very low-calorie days

During a fasting period, the body depletes its glycogen stores and begins to shift towards burning fatty acids and producing ketone bodies for energy, a state sometimes called metabolic switching.^[8] Insulin levels fall, and cellular processes such as autophagy, the body's mechanism for clearing damaged cellular components, may become more active. These physiological changes have attracted scientific interest in relation to metabolic health, weight management, and, more recently, cancer biology.

It is important to note that "complete abstinence from food" does not mean avoiding fluids. Staying well hydrated with water and other non-caloric drinks during fasting periods is important for safety. Prolonged or multi-day fasts should not be attempted without clinical oversight.

Intermittent fasting is not suitable for everyone. People who are pregnant, breastfeeding, underweight, or living with certain medical conditions should seek medical advice before adopting any fasting regimen. Groups who should be especially cautious and seek medical supervision before fasting include:

• People with type 1 diabetes or type 2 diabetes managed with insulin or sulfonylureas, due to the risk of hypoglycaemia

• People taking SGLT2 inhibitors (such as dapagliflozin, empagliflozin, or canagliflozin), due to the risk of euglycaemic ketoacidosis

• Frail or older adults

• Children and adolescents

• Anyone with a current or previous history of an eating disorder

The approach should always be considered in the context of an individual's overall nutritional needs and health status.

Could intermittent fasting reduce the risk of cancer?

No definitive causal link between intermittent fasting and reduced cancer risk has been established in humans. Maintaining a healthy body weight through any sustainable dietary approach remains the most evidence-supported strategy for lowering cancer risk.

There is emerging but still preliminary evidence suggesting that intermittent fasting may influence some biological pathways associated with cancer risk, though no definitive causal link has been established in humans. Much of the interest stems from the metabolic effects of fasting, particularly reductions in circulating insulin and insulin-like growth factor 1 (IGF-1), both of which have been associated with cell proliferation in laboratory and animal studies. Elevated insulin levels and obesity are recognised risk factors for several cancers, including colorectal, breast, and endometrial cancers, according to Cancer Research UK.^[4][5]

Fasting-induced autophagy is another area of scientific interest. By promoting the clearance of damaged proteins and organelles, autophagy may theoretically reduce the accumulation of cellular abnormalities that can precede malignant transformation.^[8] Some fasting studies have also observed reductions in markers of chronic low-grade inflammation, another recognised contributor to cancer development. However, these mechanistic findings come largely from laboratory and animal research, and it is not yet established that they translate into meaningful cancer risk reduction in people.

The majority of supportive evidence comes from animal studies and short-term human trials with small sample sizes. Researchers have not yet demonstrated that intermittent fasting independently reduces cancer incidence in large, well-controlled human cohort studies.

Cancer Research UK and the World Cancer Research Fund note that excess body fat is linked to an increased risk of at least thirteen types of cancer.^[4][5] Maintaining a healthy body weight through any sustainable dietary approach, including balanced caloric intake and regular physical activity, remains the most evidence-supported dietary strategy for reducing cancer risk. Intermittent fasting may assist with weight management for some individuals, which could indirectly contribute to risk reduction, but it should not be viewed as a standalone cancer prevention strategy.

It is also worth noting that other lifestyle factors, including not smoking, attending recommended cancer screening, and accepting relevant vaccinations (such as HPV and hepatitis B vaccines), have strong evidence for reducing cancer risk and should not be overlooked in favour of dietary approaches alone.^[15][16]

What does the current research say about fasting during cancer treatment?

Evidence on fasting during cancer treatment is preliminary, with no UK clinical guidelines recommending it as standard care. Malnutrition risk is significant, and any fasting during treatment must be supervised by an oncology team and registered dietitian.

Research into fasting during active cancer treatment is at an early stage, and no UK clinical guidelines currently recommend fasting as a standard component of cancer care. Some preclinical studies and small human trials have explored whether short-term fasting around chemotherapy cycles might reduce side effects or improve treatment tolerability, based on the hypothesis that fasting may protect healthy cells while leaving cancer cells more vulnerable to cytotoxic agents, a concept known as differential stress resistance.^[8][9]

A small number of early-phase clinical trials have reported that short-term fasting before chemotherapy was feasible and associated with reduced fatigue and gastrointestinal side effects in some participants. However, these studies have been limited by small sample sizes, heterogeneous cancer types, and a lack of long-term outcome data. It is not yet known whether fasting during treatment affects tumour response, survival, or recurrence rates in a clinically meaningful way.

Importantly, malnutrition and unintentional weight loss are significant concerns during cancer treatment. Many patients already struggle to maintain adequate caloric and protein intake due to treatment-related side effects such as nausea, mucositis, and appetite loss. Introducing deliberate caloric restriction in this context carries a real risk of worsening nutritional status, impairing immune function, and reducing treatment tolerance. Macmillan Cancer Support provides patient-facing guidance on eating well during treatment and the importance of avoiding unintentional weight loss.

People on glucose-lowering medicines, including insulin, sulfonylureas, or SGLT2 inhibitors, face additional risks during fasting, including hypoglycaemia or euglycaemic ketoacidosis, and require particularly close medical supervision.

For anyone undergoing cancer treatment, any dietary change, including fasting, should only be considered under the direct supervision of the oncology team and a registered dietitian. Self-directed fasting during chemotherapy, radiotherapy, or immunotherapy is not recommended without specialist guidance.

If you experience urgent or worrying symptoms during treatment, contact your hospital's 24-hour acute oncology or chemotherapy advice line if one has been provided to you. You can also contact your GP or call NHS 111. For severe or life-threatening symptoms, attend your nearest accident and emergency department or call 999 immediately.

What UK health guidelines say about diet and cancer prevention

UK guidelines from Cancer Research UK, the WCRF, and the NHS do not recommend intermittent fasting for cancer prevention. They emphasise healthy weight, a diet rich in wholegrains and vegetables, limiting red and processed meat, and avoiding alcohol.

UK health guidelines on diet and cancer prevention are primarily shaped by Cancer Research UK, the World Cancer Research Fund (WCRF), and NHS guidance, all of which draw on large bodies of epidemiological and clinical evidence. These guidelines do not currently recommend intermittent fasting as a specific cancer prevention strategy, but they do emphasise several dietary and lifestyle principles with strong supporting evidence.

Key recommendations include:

• Maintaining a healthy body weight, as Cancer Research UK and the WCRF note that excess body fat is linked to at least thirteen types of cancer^[4][5]

• Eating a diet rich in wholegrains, vegetables, fruit, and pulses, which provide fibre, vitamins, and phytochemicals associated with reduced colorectal cancer risk, as highlighted by Cancer Research UK and the WCRF Cancer Prevention Recommendations

• Limiting consumption of processed and red meat, with NHS guidance advising no more than 70 g of red or processed meat per day on average^[10]

• Avoiding or limiting alcohol, which is a recognised carcinogen associated with cancers of the mouth, throat, oesophagus, liver, breast, and bowel

• Limiting foods high in saturated fat and added sugar, in line with NHS Eatwell Guide principles

Regarding ultra-processed foods: the Scientific Advisory Committee on Nutrition (SACN) has highlighted associations between high consumption of ultra-processed foods and adverse health outcomes, but has not issued prescriptive limits on ultra-processed food intake as formal UK dietary guidance.^[11][12] Readers should be aware that this remains an active area of research and policy development.

NICE guidance on weight management in adults (including NICE guideline NG246 on obesity) supports dietary approaches that help individuals achieve and sustain a healthy weight, which has downstream implications for cancer risk.^[13][14] The NHS recommends that anyone seeking to make significant dietary changes for health reasons consult their GP or a registered dietitian, particularly if they have an existing medical condition.

Intermittent fasting may align with some of these principles if it supports weight management and encourages healthier food choices, but the dietary pattern itself is not endorsed by any UK body as a cancer prevention measure.

When should you speak to your oncologist before changing your diet?

Anyone with a current or previous cancer diagnosis should consult their oncologist before starting intermittent fasting, especially during active treatment or if taking medications affected by caloric restriction such as insulin or SGLT2 inhibitors.

Anyone with a current or previous cancer diagnosis should speak to their oncologist or specialist nurse before making significant changes to their diet, including starting any form of intermittent fasting. This conversation is particularly important in several circumstances:

• Before, during, or after active treatment, including chemotherapy, radiotherapy, surgery, hormonal therapy, or immunotherapy, as nutritional needs change significantly at each stage

• If you have experienced unintentional weight loss, reduced appetite, or difficulty eating, which may indicate nutritional risk requiring dietetic assessment

• If you are taking medications that require food for absorption or that interact with significant changes in caloric intake, including insulin, sulfonylureas, or SGLT2 inhibitors, where fasting carries a risk of hypoglycaemia or euglycaemic ketoacidosis

• If you have a cancer type associated with metabolic effects, such as pancreatic cancer or cancers affecting the gastrointestinal tract, where dietary management is especially complex

• If you have co-existing conditions such as type 2 diabetes, where fasting can affect blood glucose levels and medication requirements

Your oncology team may refer you to a specialist oncology dietitian, who can provide personalised guidance based on your diagnosis, treatment plan, and nutritional status. In the UK, dietetic support is available through NHS cancer services, and referrals can be made by your GP or hospital team. Macmillan Cancer Support also provides information on accessing specialist dietary support during and after cancer treatment.

If you experience urgent or worrying symptoms, contact your hospital's 24-hour acute oncology or chemotherapy advice line if one has been provided to you. You can also contact your GP or call NHS 111 for advice. For severe or life-threatening symptoms, attend your nearest accident and emergency department or call 999 immediately. Do not delay seeking medical advice in favour of dietary self-management during cancer treatment.

Scientific References

1. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
2. Life-threatening dapagliflozin-associated euglycaemic diabetic ketoacidosis in a postoperative patient.
3. The peri-operative implications of sodium-glucose co-transporter 2 inhibitors: a narrative review.
4. Obesity Biomarkers, Metabolism and Risk of Cancer: An Epidemiological Perspective.
5. Obesity as a risk factor in cancer: A national consensus of the Spanish Society for the Study of Obesity and the Spanish Society of Medical Oncology.
6. Obesity as a major risk factor for cancer.
7. Quality of life and illness perceptions in patients with breast cancer using a fasting mimicking diet as an adjunct to neoadjuvant chemotherapy in the phase 2 DIRECT (BOOG 2013-14) trial.
8. A review of fasting effects on the response of cancer to chemotherapy.
9. Reduced levels of IGF-I mediate differential protection of normal and cancer cells in response to fasting and improve chemotherapeutic index.
10. Meat in your diet.
11. Processed foods and health: SACN's rapid evidence update – summary.
12. SACN statement on processed foods and health – summary report.
13. Overweight and obesity management (NG246).
14. Overweight and obesity management (NG246) – Physical activity and diet chapter.
15. HPV vaccine.
16. HPV vaccination guidance for healthcare practitioners.
17. Updating the epidemiologic evidence for the WCRF/AICR cancer prevention recommendations.

Frequently Asked Questions