One definition of fasting is “the voluntary act of withholding food for a specific period of time” (Jason Fung and his excellent website). Though still very much in its infancy, the fascinating and fast-changing science of fasting and calorie restriction holds promising opportunities for cancer patients. Intermittent fasting is often associated with the ketogenic diet, which is why I’d like to explore this a bit further in this article.

My fasting story

I stumbled across the benefits of fasting about a year after I started the ketogenic diet, i.e. in 2013. What happened was that I just didn’t manage to have breakfast any more- for various reasons: first of all, my appetite wasn’t the same any more. I didn’t wake up absolutely ravenous like I used to when I was “sugar adapted” (meaning that I used glucose as my main energy source). When I was younger, I struggled leaving the house with something in my tummy. I remember having to go for fasting blood tests and I dreaded them weeks ahead.

But I always put my amazing appetite down to being a semi-professional triathlete who trained 15-20 hours a week. I obviously burnt a lot of calories so I needed to eat all the time- that’s what I told myself. So I used to have a snack just before going to bed and eat as soon as I opened my eyes. I guess what didn’t help was that my parents were convinced that breakfast was the single and most important meal of the day and that it can’t possibly be skipped without suffering from serious health repercussions.

While it can definitely suit some people to have breakfast as soon as they get up, I’ve learned now that this “breakfast myth” is based on clever advertising rather than solid science.

Another reason why I found it hard to eat breakfast was my daughter starting pre-school. I had to make breakfast for the kids while preparing my daughter’s snack and lunch for my son who stayed at home with our minder. It was simply too stressful to sit down and have breakfast myself because, as you hopefully know, wolfing down your meals isn’t a good idea if you want to absorb your nutrients properly.

Then, the third reason was that I found my mind was a lot sharper and my cognitive functions enhanced when I didn’t have anything to digest. That lead to me pushing out breakfast to 11am initially, then 12pm, 1pm, and in the last few months I’ve adapted to having most of my food intake within a narrow window of 4-5 hours. This not only seems to suit me in terms of wellbeing and health, but also fits my busy lifestyle with business, kids and everything else!

Is intermittent fasting becoming mainstream soon?

Research into fasting has taken off recently and a lot more health professionals are now spreading the word about its benefits. This has given me the confidence to “experiment” with different fasting regimes, too, and to really tune into my body what suits me best. Although there will be an increasing number of studies investigating the practicalities and methods of fasting, I’m convinced it’ll always come down to the individual to determine the best regime. It could also be that you adopt different fasting regimes at different times of the year. Women in particular have to keep a close eye on their hormones as some might be negatively affected by fasting, which could lead to irregular menstrual cycles for instance.

When we shift the focus to cancer, recent studies suggest that fasting can also make chemo- and radiotherapy more effective. In other words, there might be a good reason you don’t feel like eating when you’re sick; a lack of appetite may be just what your body needs.

So, how does fasting or calorie restriction work, and can it work with keto?

The ketogenic diet is a metabolic therapy that changes how cells generate their energy, switching the body to burning fat (ketones) rather than sugar (glucose, found in all carbohydrates). This starves cancer cells of their favourite food – glucose – suppressing tumors in the process.

Target cancer cells, not healthy ones

Fasting, or not eating for a certain period of time, has a similiarly transformative effect, causing healthy cells to slow down their metabolism and go into repair/survival mode until food is available again.

Here’s the really interesting bit. While those healthy cells are ‘hibernating’, they’re at far less risk from the damaging effects of chemo- and radiotherapy, particularly when in high doses.

Valter Longo, Professor of Gerontology/Alzheimer’s Research/Cancer Research/Biological Sciences at the University of California, is a leading fasting expert. Longo and his team have found that “’starvation-dependent’ stress protects normal but not cancer cells against high-dose chemotherapy” (Raffaghello et al, PNAS, 2008)

This is good news given that an estimated 20%+ of cancer-related deaths are precipitated or even caused by the toxic effects of chemo. Those same toxicities also often reduce the overall effectiveness of treatment.

And recent research from 2012 has brought more good news.   It found that as well as slowing the growth of tumors, fasting makes cancer cells more susceptible to treatment (Lee at al, Sci Transl Med, 2012). So, while your healthy cells are getting some shuteye, cancer cells can be treated more effectively.

There is also some evidence that fasting after chemotherapy might also play an important role in reducing the risk of DNA damage by the chemo drug itself.

Seven Types of Fasting – what are they?

There are loads of different types of fasting and, as mentioned earlier, I’d like you to really trust your body’s signals- in combination with monitoring of some important health markers- to tailor your own fasting regime. As I explained above, prolonged intermittent fasting (or the “Warrior Diet”, as you’ll see below) suits me personally although I always adapt a little bit. During menstruation, for instance, I find it harder to fast for that amount of time and usually expand my eating window to 6-8 hours for a few days. This is totally personal, though, and works for me, after doing some trial and error over a few months.

So let’s look into some different definitions of fasting:

Intermittent Fasting (IF) – extended periods without eating on a regular basis.

Alternate Day Fasting (ADF) – eat every second day, leaving 36 hours between each 12-hour feeding window.

Meal-skipping – randomly skip a meal once or twice a week. This could be a good way to start getting used to fasting.

Typical Keto IF – a 16-hour fast, followed by an 8-hour feeding window. Very doable for most keto-adapted people.

Warrior Diet – a 20-hour fast followed by a 4-hour window during which you eat most of your calorie intake.

The 5:2 Diet (Michael Mosley) – 5 days of normal (ketogenic) eating, and 2 days where calorie intake is reduced to 500 calories for women or 600 for men.

4-Day Fast (Valter Longo) – Longo argues that it takes at least 4 days to switch to a fully protective and fat-burning profile that involves every single organ in the body. To be done only under medical supervision.

Fasting on Keto: The Bottom Line

For cancer patients, fasting offers exciting potential. More research trials are currently underway into its benefits and practicalities, but the bottom line is that when combined with other treatment strategies, including a ketogenic diet, fasting has great potential in cancer research.

As with any major changes in your diet and lifestyle, it’s important to start things slowly and get your body used to new habits. Be particularly cautious if you’re very dependent on carbohydrates as it will make fasting a lot more difficult. Also, do prolonged fasts under supervision only if you’re a “beginner”.

In my Keto Course, session 3 of Module 4  is called Fasting and Calorie Restriction. I examine fasting options for patients on a ketogenic diet, and also look at the controversial topic of whether, and how, to restrict calories when following this diet.