Intermittent Fasting and Cancer

Hippocrates, the father of Western medicine, believed that fasting enabled the body to heal itself; and modern research is proving his theory right! The dietary practice of intermittent fasting (IF) is an age old method which has gained much traction in today’s nutrition world!

So let’s go back to basics: what actually is fasting?

Intermittent fasting (IF) is not a diet, rather it is a term used for cycling between periods of eating and fasting. There are many different ways to implement an IF regimen. The most common method is the ‘16 hour fast’ which involves fasting for 16 hours on 3-4 days of the week (e.g. 8pm-12pm). There is also the ‘24 hour fast’ once per week, which needs little explanation, however should only be attempted by seasoned fasters.

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 in our experience have shown fantastic results with our clients! It is, however, important that you are under the guidance of trained medical professional when embarking on your fasting journey. We have written comprehensive guides to help support you and give your tips and tricks to make fasting easier, jump over to our OnCore Store if you’re interested.

What happens to my body while I’m fasting?

In a normal state, the body uses carbohydrates as it’s primary source of fuel.Dietary sources of carbohydrates include grains, rice, pasta, fruit, starchy vegetables, dairy, legumes and sugar. Complex 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 hours of fasting, our liver glycogen stores are depleted and the body switches to using fat to as its primary energy source. This enhances our fat-burning potential.

Potential benefits of fasting

  • Improved insulin sensitivity
  • Reduced cholesterol levels and blood pressure
  • Reduced inflammation
  • Encourages the break down of fat stores for fuel
  • Promotes weight loss while maintaining lean body (muscle) mass
  • Reduced risk of chronic diseases such as diabetes, cardiovascular disease, liver disease, Alzheimer’s disease and cancer
  • 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 calorie restriction, in the absence of malnutrition, is the most effective physiological intervention for prolonging one’s lifespan and protecting against cancer1. 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 DNA1.

Image source: Impact of intermittent fasting on health and disease processes. Mattson et al2

Fasting 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 studies that have investigated the practice of fasting 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 fasting, therefore making them sensitive to  the 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 fasting, as the risk of malnutrition and weight loss during treatment far outweigh the beneficial mechanisms of fasting during cancer treatment.

So what is the answer?

The evidence…

Our beloved 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 alone3.  It’s important to note, this trial used a 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 treatment4. Fifteen studies identified tumor suppression in the fasting groups (between 48 to 72 hours as established by Dr. Longo) and ten studies showed chemoprotective outcomes4. 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 symptoms5, all of which have a profound impact on quality of life and overall health. However given this study had a very small sample size, it is again difficult to use this data in clinical practice.

Precautions and safety

Fasting can be dangerous and is most definitely not a recommended practice for everyone, in particular cancer patients. If this is a practice you wish to partake in, it is important to do so under guidance from a trained medical professional. Implementing a fasting regimen under certain circumstances can be dangerous and can have a negative impact on the patient and their ability to tolerate cancer treatment.

Malnutrition and weight loss can have a profound negative effect on treatment tolerance and both short and long term outcomes6. What’s also important to note is that some drugs (particularly pain medication) will often require patients to have food prior to their administration. Many oral chemotherapy drugs can not be taken on an empty stomach, as taking 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 fasting is a safe and effective method to be implemented during chemo and/or radiotherapy. There are multiple studies currently underway and we can’t wait to see the results. This is a very exciting area with huge potential and as we will continue to update you as new literature and recommendations are released. As mentioned above, fasting is not a safe practice for everyone, in particular those who are malnourished, at risk of malnutrition, underweight or acutely unwell. If undertaken for long periods, fasting can do more harm than good, which can result in treatment dose reductions, treatment delays and increased toxicities. Ensure you ask your oncologists or dietitian before commencing a fasting regimen.

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; feel free to reach out or book an appointment if you think this may be a suitable method for you.

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 – see our OnCore Nutrition GI resource for a comprehensive guide on how to make carbs your friend! 
  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 (That’s approximately 2 servings per week).
  6. If intermittent fasting is a practice of interest to you, this should only be implemented under the supervision of a qualified medical practitioner, so feel free to reach out if you need!

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 

Elise Den

Accredited Practising Dietitian

  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 growth of tumors 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 tumor-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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