To fast or not to fast? Intermittent fasting and cancer

Hippocrates, the father of Western medicine, believed that the practice of fasting enables the body to heal itself; and modern research has shown this theory may be correct! Intermittent fasting (IF) is an age old method which has gained much traction in today’s nutrition world!

Let’s go back to basics: what do we consider IF?

Intermittent fasting (IF) is not a diet, rather it is a term used for cycling between periods of eating sufficient and insufficient kilojoules. There are many different ways to implement an IF regimen. A popular method is the ‘16:8 method’, or time restricted eating which involves fasting for 16 hours on 3-4 days of the week (e.g. 8pm-12pm) and only eating within an 8 hour window. There is also the ‘24 hour fast’ once per week, which needs little explanation, however, should only be attempted by seasoned fasters (and with support from a trained medical practitioner).

Another commonly used method known as ‘intermittent calorie restriction’ or ‘The 5:2 protocol’ was developed by well-known doctor Michael Mosely. This involves consuming 2100kJ (500kcal) for women and 2500kJ  (600kcal) for men on 2 non-consecutive days each week.

Each of these methods have been validated by research and if implemented with suitable individuals with a sound baseline diet can lead to improvements in body composition, energy levels and enhanced hunger-satiety awareness. However, it is important that the practice of fasting is only recommended to patients where it is safe and clinically indicated.

What happens to our body whilst fasting?

In a normal state, the body uses carbohydrates as it’s primary source of fuel. Dietary carbohydrates are broken down into glucose and either used by our body for energy, or transported to our liver and muscles to be stored as glycogen.

During a fast, this metabolic process changes. After approximately 8-12 hours of fasting, once our blood glucose stores have been utilised and the liver glycogen stores depleted, our body switches to using fat as its energy source. This enhances our fat-burning potential and body composition changes.

Potential benefits of fasting
  • Improved insulin sensitivity
  • Reduced cholesterol levels and blood pressure
  • Reduced inflammation
  • Enhanced mobilisation of fat stores for fuel
  • Promotes weight loss while maintaining lean body mass
  • Reduced risk of chronic diseases such as diabetes, cardiovascular disease, liver disease, Alzheimer’s disease and some cancers
  • Reduced free radical damage, slowed ageing and enhanced cellular regeneration
  • Some studies demonstrate enhanced impact of some anti-cancer therapies (I will elaborate on this in more detail later on)

According to one of my favourite researchers, Dr. Valter Longo (I recommend listening to his podcasts if you’re interested in fasting research), fasting or caloric restriction, in the absence of malnutrition, is one of the most effective physiological intervention for prolonging one’s lifespan and protecting against cancer (1). It has the ability to reduce levels of stress hormones, growth factors and inflammatory cytokines, reduce oxidative stress, induce autophagy (cell death) and repair our damaged DNA (1).


The caveat… IF during cancer treatment

One question I get asked regularly in my practice is “should I fast intermittently during chemotherapy or radiotherapy?”

There have been many small scale studies that have investigated the practice of IF during anti-cancer therapies (e.g. chemotherapy or radiotherapy), however, the evidence is still unclear. Some studies have shown that cancer cells become more susceptible to destruction as a result of IF, therefore making them more sensitive to cancer treatment. It has been hypothesised that cancer cells are unable to adjust as quickly to altered metabolic pathways which makes them more vulnerable. Other studies have shown a negative outcomes associated with IF, as the risk of malnutrition and weight loss during treatment far outweigh the currently unclear benefits of IF during cancer treatment.

What’s the evidence?

Dr. Longo and his colleagues demonstrated positive outcomes after 48 to 72 hours of fasting whilst receiving chemotherapy. They proposed that a synergistic effect appears when combining chemotherapy and fasting, rather than chemotherapy alone (3).  It’s important to note, this trial used an animal model (mice), and as we are all aware, the human body is vastly different from that of a mouse. Therefore it is difficult to determine if this method of fasting would be beneficial for humans

In 2017, a large systematic review was undertaken to investigate fasting and its impact on cancer treatment (4). Fifteen studies identified tumor suppression in the fasting groups (between 48 to 72 hours as established by Dr. Longo) and ten studies showed chemoprotective outcomes (4). Most of the studies were conducted using animal models (rather than humans), which as mentioned before, is a huge limitation when making changes to clinical practice.

Fasting has also shown some promising effects against treatment-related toxicities.  A small study undertaken in 2010 showed that periods of fasting, followed by recommencing a balanced healthy diet prior to treatment, reduced the severity of side effects from treatment. There was a reported reduction in weakness, fatigue and gastrointestinal symptoms (5), all of which have a profound impact on the quality of life and overall health. However given this study had a very small sample size, it is again difficult to recommend this practice in a clinical setting.

How do we discuss this with our patients?

IF can be dangerous and is most definitely not a recommended intervention for everyone, in particular patients with cancer who are already nutritionally vulnerable. There are some patients who will be set on pursuing this practice during their anti-cancer treatment and it is imperative that if they choose to do so (despite education), that their treating teams are aware. It is important to convey the lack of good quality evidence in the literature regarding IF during treatment and that the risk of malnutrition and muscle depletion can further exacerbate toxicities and reduce treatment tolerance.

As mentioned malnutrition and weight loss can have a profound negative effect on treatment tolerance and both short and long term outcomes (6). What’s also important to note is that some drugs (particularly pain medication) will often require patients to have food prior to administration. Many oral chemotherapy drugs can not be taken on an empty stomach, as taking them in conjunction with food helps to increase absorption. Other medications require specific amounts of fats, proteins and carbohydrates to achieve the desired therapeutic outcome.

Our recommendation

Ongoing research is needed to establish if IF is a safe and effective method to be implemented during chemo and/or radiotherapy. There are multiple studies currently underway and we eagerly await these results.  IF is not a safe practice for everyone, in particular, those who are malnourished, at risk of malnutrition, underweight or acutely unwell. If undertaken inappropriately or for long periods, IF has the potential to do more harm than good, which can result in treatment dose reductions, treatment delays and increased toxicities.

In the context of reducing the risk of cancer or cancer recurrence (post-treatment and acute recovery phase), intermittent styles of fasting may be beneficial; however only under guidance from a trained medical practitioner.

OnCore’s healthy tips
  1. Eat a diet rich in fruits, vegetables, nuts, seeds and whole grains.
  2. Include lean proteins – poultry, fish, meat, tofu, beans, legumes, dairy.
  3. Select low glycaemic index (GI) carbohydrates
  4. Include healthy fats, such as nuts, seeds, fish, avocado and extra virgin olive oil.
  5. Aim for less than 500g red meat per week and limit processed meats.

For specific advice or guidance for you or your patients, reach out to us today. Tailor your plate with OnCore – Oncology and Preventative Nutrition


Elise Den

Senior Oncology Dietitian  |  Accredited Practising Dietitian

Co-founder – OnCore Nutrition

[email protected]

T A I L O R  Y O U R  P L A T E   |   B U I L D  Y O U R  B E S T  Y O U



  1. Longo VD, Fontana L. Calorie restriction and cancer prevention: metabolic and molecular mechanisms. Trends in pharmacological sciences 2010;31:89-98.
  2. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Research Reviews 2017; 39: 46-58
  3. Lee C, Raffaghello L, Brandhorst S, et al. Fasting cycles retard the growth of tumours and sensitize a range of cancer cell types to chemotherapy. Science translational medicine 2012; 4:124-27.
  4. Sun L, Li YJ, Yang X,Gao L, Yi C. Effect of fasting therapy in chemotherapy-protection and tumour-suppression: a systematic review. Translational Cancer Research 2017; 6.
  5. Raffaghello L, Safdie F, Bianchi G, Dorff T, Fontana L, Longo VD. Fasting and differential chemotherapy protection in patients. Cell cycle 2010;9:4474-6.
  6. Bauer JD, Ash S, Davidson WL, Hill JM, Brown T, Isenring EA, Reeves M. Evidence-based practice guidelines for the nutritional management of cancer cachexia. Nutrition & Dietetics 2006; 63: S3-S32.

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