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Fasting and Cancer

When I tried Googling “fasting and cancer”, there were nearly 3 billion results – it’s obviously a very hot topic! There is increasing evidence that fasting, in various forms, may be very useful both in the prevention of chronic diseases such as cancer, as well as alongside cancer treatment. In this blog, we’ll look at the potential benefits of fasting during treatment, and explore whether there is a role for fasting once active treatment is finished.

Forms of fasting

There are many different types of fasting, so it’s important to clarify a few terms first.

Periodic fasting refers to abstaining from all food and calorie intake for a few days at a time, typically 2-4 days. Whilst this has traditionally been practised by many religions for hundreds of years, after having had a cancer diagnosis it is very important only to do periodic fasting under the supervision of a suitable healthcare practitioner who can take your individual health needs into account.

Fasting-mimicking diets (FMD) recreate many of the biological effects of periodic fasting on the body whilst still allowing some food, although calories are considerably reduced from an individual’s normal daily intake. Work into FMDs and cancer has been pioneered by Dr Valter Longo, a scientist studying longevity, aging and age-related diseases such as cancer. As well as being low in calories, a FMD is low in protein and the proportions of macronutrients (carbohydrates, fats and protein) are strictly controlled.

Intermittent fasting (IF) involves short periods of fasting or calorie restriction interspersed with periods of eating freely. There are many different formats. Examples are alternate day fasting and the popular 5:2 diet, which involves strict calorie restriction for 2 days a week and unrestricted eating for the other 5 days.

Time restricted feeding (TRF) refers to eating all your food within a certain daily time window. Again, there are different models, such as eating in a 12 hour window and fasting 12 hours overnight (12:12), or eating within an 8 hour window and fasting for 16 hours overnight (16:8). TRF is often practised every day.

How can fasting impact your biology?

Different forms of fasting have been studied in humans as well as animals with regard to how they may increase longevity or reduce the risk of several chronic diseases(1). One study showed that healthy people who followed a 5 day FMD once a month for 3 months experienced reductions in body weight, total body fat, blood pressure, fasting glucose, IGF-1 (a growth factor related to cancer), triglycerides, total and LDL cholesterol and the inflammation marker C-reactive protein(2). This would be expected to reduce the risk of developing type 2 diabetes and cardiovascular disease as well as possibly cancer.

During fasting, the body cannot continue to be fuelled by glucose as its stores run out. Instead, it enters a state of ketosis, which has anti-inflammatory effects. It also has beneficial effects on the gut microbiome, which can lead to improved immunity. Longer term, these changes in the gut microbiome may result in a better response to cancer immunotherapy(3).

An interesting point is that many animal studies on fasting show benefits that have not been replicated in subsequent human studies. It appears that fasting only has benefits for metabolic health (blood glucose, blood pressure etc) in humans if it results in weight loss, whereas in animals, metabolic benefits occur even if weight loss does not. However, some of the benefits of fasting in conjunction with cancer treatment, which we will explore below, were seen without significant weight loss occurring.

Dr Longo suggests that healthy people who are interested in using fasting to increase longevity and healthy aging may wish to do a 5 day FMD a few times a year, combined with a nightly overnight fast of 12-13 hours(1). If you feel that you are back to optimum health after cancer treatment is finished, then this could be an approach to consider, with support. Always seek your doctor’s advice if you have an ongoing health condition (and in particular if you are taking any anti-diabetic medication) before considering fasting.

Fasting during cancer treatment

Most of the research in this area has been spearheaded by Dr Longo using a 5 day FMD which significantly restricts calories. The research has now progressed from animal studies to human trials, with the safety of using a 5 day FMD in combination with a variety of cancer treatments (including chemotherapy, endocrine therapy and immunotherapy) and across a range of cancer types having now been established(4).This study also explored some of the mechanisms by which an FMD may improve treatment outcomes. These include metabolic changes such as a reduction in blood glucose and insulin, and an improvement in anti-tumour immunity.

The DIRECT trial(5) looked at the effect of the FMD on outcomes of standard neoadjuvant chemotherapy (FEC-T or AC-T) given to 131 women with stage II-III HER2 negative breast cancer. Compared to controls, women randomised to the FMD were more likely to have a good partial response, with a tumour cell loss of 90-100%. The FMD also reduced chemotherapy-induced damage to the DNA of immune cells, and allowed the steroid dexamethasone to be omitted from treatment without any resultant increase in toxicity. This may be because, in a fasted state, normal cells enter a stress response and decrease their metabolic rate, protecting them from the toxicity of chemotherapy. Cancer cells, by contrast, tend to ignore the body’s anti-growth signals, and this means that they continue to be vulnerable to chemotherapy(3).

Periodic fasts have also been studied around chemotherapy(6). These lasted between 24 hours and 72 hours. The benefits seen included an improvement in quality of life and a reduction in fatigue, as well as a protective effect on immune cells. During a periodic fast, broths or vegetable juices may often be consumed.

In these trials, it is important to note that patients with a body mass index below 19kgm-2 were typically excluded due to concerns over progressive weight loss. It’s also important to note that trial participants who followed the FMD had intensive support and guidance from doctors and nutrition professionals. The FMD studied in trials has been carefully designed for its macro- and micronutrient content, so please seek support from a nutritional therapist experienced in fasting alongside cancer treatment if you are thinking about taking this approach. The same goes for periodic fasting.

It is also important to note that fasting increases a cellular process called autophagy, which allows for damaged parts of cells to be cleared away. Autophagy can be beneficial in cancer because it can sensitise cancer cells to chemotherapy(3), but in advanced cancer it may drive cancer growth(7). If you are living with advanced cancer, it is important only to undertake FMDs or longer periodic fasts under the guidance of an experienced practitioner.

Fasting and cancer recurrence risk

I have mentioned already that if you are back to full health after cancer treatment and are not taking any contraindicated medications, it might be beneficial for your general health, longevity and risk of new cancers or other chronic diseases to follow a 5 day FMD a few times per year. However, we do not currently have any data on whether this has any effect on cancer recurrence risk.

In 2016, the most relevant study that we have to date on the subject of fasting and cancer recurrence was published. It was a small study, and much more research is warranted, but the findings were interesting.

2413 women who had taken part in the much larger Women’s Healthy Eating and Living Study and who had been diagnosed with early stage breast cancer were asked to estimate how long their regular overnight fast was. This meant the time between their last food (or calorie) intake of one day and the first food (or calorie) intake of the following day. The researchers found that women who fasted for less than 13 hours overnight had a risk of breast cancer recurrence that was 36% higher than the women who fasted for 13 hours or more. The researchers surmised that the recurrence risk reduction might have been due to improved blood glucose control and an increased sleep time, as these were also observed in women who fasted for longer(8).

Whilst we certainly need more research, the findings of this study can be easily implemented by most people. It is not too difficult to fast for at least 13 hours overnight; this could simply mean not eating breakfast until 8am if you finish your dinner at 7pm.

What can you do now?

I recommend an overnight fast of 13-14 hours to anyone who has had a cancer diagnosis. Some women may be able to fast for longer overnight, but it can be too stressful for other people’s bodies, so I only recommend longer regular fasts once I understand someone’s health very well.

If you are about to go through active cancer treatment, then there are definite benefits to fasting, or following a fasting-mimicking diet, during chemotherapy. Please seek a practitioner’s support before trying this yourself!

Remember that we are all individuals, and while one person may find fasting easy, another may find it really quite difficult. If you find longer fasts too difficult, be kind to yourself. It’s encouraging to know that even a regular overnight fast may have significant benefits!


  1. Longo, V.D and Anderson, R.M. (2022). ‘Nutrition, longevity and disease: From molecular mechanisms to interventions’, Cell, 185(9), pp1455-1470. Available at (Accessed 1 November 2023).

  2. Wei, M., Brandhorst, S., Shelehchi, M. et al. (2017). ‘Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease’, Science Translational Medicine, 9(377), eaai8700. Available at (Accessed 1 November 2023).

  3. Tiwari, S., Sapkota, N. and Han, Z. (2022). ‘Effect of fasting on cancer: A narrative review of scientific evidence’, Cancer Science, 113(10), pp3291-3302. Available at (Accessed 1 November 2023).

  4. Vernieri, C., Fucà, G., Ligorio. F. et al. (2022). ‘Fasting-Mimicking Diet Is Safe and Reshapes Metabolism and Antitumor Immunity in Patients with Cancer’, Cancer Discovery, 12(1), pp90-107. Available at (Accessed 31 October 2023).

  5. De Groot, S., Lugtenberg, R.T., Cohen, D. et al. (2020). ‘Fasting mimicking diet as an adjunct to neoadjuvant chemotherapy for breast cancer in the multicentre randomized phase 2 DIRECT trial’, Nature Communications, 11(3083). Available at (Accessed 31 October 2023).

  6. Plotti, F., Terranova, C., Luvero, D. et al. (2020). ‘Diet and Chemotherapy: The Effects of Fasting and Ketogenic Diet on Cancer Treatment’, Chemotherapy, 65(3-4), pp77-84. Available at (Accessed 1 November 2023).

  7. Amaravadi, R.K., Kimmelman, A.C. and Debnath, J. (2020). ‘Targeting Autophagy in Cancer: Recent Advances and Future Directions’, Cancer Discovery, 9(9), pp1167-1181. Available at (Accessed 1 November 2023).

  8. Marina, C.R., Nelson, S.H, Breen, C.I. et al. (2017). ‘Prolonged Nightly Fasting and Breast Cancer Prognosis’, JAMA Oncology, 2(8), pp1049-1055. Available at (Accessed 1 November 2023)

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