Fasting before cancer treatment

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Fasting before cancer treatment

By Jane Clarke

February 18, 2019

Fasting may not seem the most obvious topic for a blog here. After all, our focus at Nourish by Jane Clarke is how to help with the eating struggles caused by illness, and to suggest strategies to help – and, of course, delicious foods to enjoy and share with family and friends. But when it comes to cancer, there’s increasing evidence that fasting before chemotherapy may increase the impact of the treatment and reduce the side effects of chemo-toxicity. My experience working with patients is that short-term fasting before chemotherapy can certainly reduce post-treatment fatigue and nausea, but it’s not a once-size-fits-all solution.


The case for fasting

Animal and human studies have shown promising results for pre-treatment fasting – that is, cutting out food and virtually all drinks (water and very dilute juice is permitted) for two-to-three days before chemotherapy. So far, research involving people living with cancer has been small but more clinical trials are underway, which should give a clearer picture of any fasting benefits and how to get the best results.

When we eat as normal, we provide our bodies with the fuel for cells to grow and reproduce. When we fast, our body instead shifts to repairing and protecting existing cells. Scientists have proposed that the energy restriction caused by fasting results in cancer cells (which divide and grow at a fast rate) being unable to defend themselves as quickly as usual. As a result, they are more vulnerable to the effects of chemotherapy treatment. At the same time, healthy cells are in maintenance mode and so more resistant to the toxic side effects of the drugs.

A 2017 review of 22 studies found a period of 48-72 hours fasting was needed to see these positive results, which also included improved cardiac and bone marrow function in some patients. The review also showed that pairing short-term fasting with chemotherapy may be more effective than either approach – fasting or chemotherapy – in isolation.


Is fasting right for you?

It can be really tough to fast for 72 hours, even when you’re feeling poorly and don’t have much of an appetite in the first place. There’s the ritual and routine of eating; the way it helps to give order and focus to the day, to gather people around the table and to provide moments of pleasure. If you’re considering fasting, you need to feel strong enough to cope with the calorie restriction and feel confident that you’ve been eating as well as you can before the fast begins.

There are also certain cases when fasting isn’t recommended if you:
have diabetes, kidney, liver, heart or autoimmune disease
low blood sugar or low blood pressure
an eating disorder, malnutrition or cachexia
cancer that causes hypermetabolism, weight and muscle loss
are having oral chemotherapy or other medication that must be taken with food.
Always consult your oncologist and care team before beginning any fast or change to your usual diet.

glass of water


During & after fasting

Fasting can feel empowering as it’s action you can take to enhance the treatment prescribed by your oncologist but be cautious. Fasting for too long (more than 72 hours) or if you’re not fit enough can cause weakness that might delay or reduce your treatment. You’ll also need to drink plenty of water – at least 2.5 litres – throughout your fast to prevent dehydration and protect your kidneys. My patients also tend to like diluted apple juice, or carrot and ginger juice (as long as you don't have more than 200 calories during a 24-hour period, it is counted as fasting).

After a fast, you’ll need to introduce as many nourishing foods as you can, but avoid any dishes that are too calorie dense or high in fat as these can make you feel overloaded and uncomfortable. A simple chicken soup or noodle bone broth will be gentle on the stomach while providing energy and comfort, then you can build up from there.


Recipes to try

Nourish bone broth
Roasted butternut squash and Parmesan soup
Chicken & pasta soup
Seasonal smoothies
Flavoured milks
Green juice & smoothie

Always consult your oncologist and care team before beginning any fast or change to your usual diet.

seasonal smoothie recipe

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