Intermittent fasting is another way of eating that many people explore when they want to support their health through their diet. There are a number of different intermittent fasting programmes, but they all involve drastically reducing (or stopping) intake of calories for a defined period of time.

Examples include consuming no calories for 12 or more hours per day and eating normally for the rest of the day or reducing intake of calories to a fraction of normal on alternate days or two days per week (as in the well-known ‘5:2’ diet) and eating normally on the other days. You can find more information about these plans here.

If you are considering making any significant changes to your diet, it's really important to get medical advice about whether this is appropriate for you at this time. 

The theory behind these approaches is that our bodies evolved over thousands of years to survive periods when food was scarce. Our early ancestors evolved to go into ‘growth’ mode when food was available and ‘repair’ mode during times of scarcity. Our bodies haven’t yet ‘caught up’ to life in a world where food is always available.

The theory suggests that if we eat enough calories to stay in ‘growth’ mode all the time, it doesn’t give our bodies time to rest and repair, which isn’t the best way for us to stay healthy. 

With regard to cancer risk, one theory is that ‘repair’ mode involves both slowing the rate at which our cells multiply and finding and destroying damaged cells that could otherwise develop into a tumour. So far, most of the evidence for this approach has come from animal studies or looking at test results in humans rather than actual health outcomes and it hasn’t been conclusively demonstrated in humans that fasting reduces cancer risk.

For people undergoing cancer treatment, there is also a theory that fasting around the time of receiving chemotherapy can help to reduce some of the side effects. Again, this is based on the idea that our bodies go into ‘repair mode’ when calorie intake is restricted.

Chemotherapy works by damaging cells that are dividing, causing them to die. This includes cancer cells, which are multiplying out of control, but also other rapidly-dividing cells including those in the hair follicles and the lining of the digestive system, causing side effects including hair loss and diarrhoea. The theory suggests that if the healthy cells are in repair mode and not dividing quickly, they can escape some of the effects of chemotherapy, while the cancer cells can’t slow down and so are still damaged. 

Intermittent fasting can efficiently decrease body mass and has a meaningful impact on the way cancer cells function. Because of this, research looking in to the role of intermittent fasting in the prevention and treatment of cancer is ongoing but there is no conclusive stance on the role of fasting. A recently published review summarises the evidence we have surrounding intermittent fasting in patients with cancer.

This review suggests that intermittent fasting alone, without any reduction in body weight, eating balanced diet and/or exercise, is unlikely to improve the effects of cancer treatment. In fact, the risk and benefits of fasting should be discussed with your healthcare team. If you have a poor appetite, reduced intake of food or have suffered unintentional weight loss, it is possible that intermittent fasting could cause you to become malnourished.

The general advice for people diagnosed with cancer is to follow a healthy, balanced diet. The NHS has produced its ‘Eatwell’ guide to show what this includes and in what proportions. You can find the Eatwell Guide here.

Making large changes to the number of calories we consume can also affect our intake of the nutrients that we need to maintain our health. If you are going to reduce the amount that you eat, it’s important to make sure that you’re drinking enough water and getting the carbohydrates, proteins, fats and other nutrients that you need. In particular, when you’re having chemotherapy you may experience difficulty eating due to nausea, taste changes or mouth ulcers, for example, and it’s important to maintain your weight.

Research hasn’t established a conclusive link between any particular food or diet and the risk of ovarian cancer. Once again, if you’re considering any significant dietary change, it’s important to seek medical advice as to whether it’s safe for you.

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If you have any questions or need any further information please contact our support service team on 0800 008 7054 or email [email protected]

Reviewed by Rhia Saggu, Specialist Dietitian at Imperial College Healthcare NHS Trust

V.1.2. Last reviewed November 2022, due for review November 2025

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