Home Metabolic Dietary Therapy: Fasting, Keto and Functional FoodsFasting Complete Guide to Fasting for Cancer: How to Fast (And Why)

Complete Guide to Fasting for Cancer: How to Fast (And Why)

by Maggie Jones

I’ve gotten this question from a few different folks – some who fast intermittently and are looking to explore extended fasting and some who may have never intentionally missed a meal in their life.  I’m eager to share as fasting has been one of the most important tools in my healing. I’ve found that what and when I don’t eat is at least as important as what I do eat.

Why Fast

Some of the benefits I’ve personally experienced from extended fasting are:

  1. Reduced side effects from radiation. The night of my first round of stereotactic radiosurgery I was in sweaty, panting, vomiting agony. It continued through the next day and then turned into three weeks of shaky, clammy vomiting. For my second round, I fasted three days prior. I was able to break fast the next day and head to work the next Monday. Night and day.
  2. Mental clarity. This is a major benefit of ketosis in general but during longer fasts more of my lorlatinib/ brain damage haze seems to fall away. I could write another 80 pages about what lorlatinib can do to an already damaged brain but I’ll never be able to concentrate long enough to fin
  3. Mood Improvements. Bordering on euphoric. Ok, I admit, I’ve felt euphoric at times 3-4 days into a fast but generally I just feel an intense sense of calm and focus.
  4. More time in the optimal GKI zone. No food means lower glucose means higher ketones, baby. Around day 3-4 I’m cruising at a sub 0.5 GKI all day and night.
  5. More time and money. This is a benefit I wasn’t expecting but it’s so stinking nice to have a day when I don’t need to worry about what’s for dinner. Instead of shopping, cooking, and doing dishes I can take an extra yoga class, a walk, or write a blog post.

Here are some of the benefits I can’t see for myself but science can tell me about:

  1. Fasting hurts cancer cells. According to the results of a study by Dr. Valter Longo at USC, “Even fasting on its own effectively treated a majority of cancers tested in animals, including cancers from human cells.”
  2. It protects against immune system damage caused by chemo. Another, more recent study by Longo demonstrates that “fasting also protected against toxicity in a pilot clinical trial in which a small group of patients fasted for a 72-hour period prior to chemotherapy”
  3. It regenerates the immune system in general. That same study showed that, “during each cycle of fasting, this depletion of white blood cells induces changes that trigger stem cell-based regeneration of new immune system cells.”
  4. It triggers autophagy. Autophagy, “self-eating,” is when your body recycles the old parts that were cluttering the place and gumming up the works. “When you activate autophagy, you slow down the aging process, prevent or delay neurodegenerative diseases as well as reduce inflammation.” I also credit it with the reason I don’t have as much loose skin as you’d expect a 40 year-old who’s lost 50 pounds in less than 8 months to have.
  5. It makes my TKIs more effective. According to a study in Oncotarget in 2015, “starvation conditions increase the ability of commonly administered TKIs, including erlotinib, gefitinib, lapatinib, crizotinib and regorafenib, to block cancer cell growth.” I wish I’d spent more time fasting on crizotinib but lorlatinib is getting the treatment!
  6. It makes radiation therapy more effective. As demonstrated by yet another Longo study in 2012.

Read about my first extended fast

How to Fast

  1. With your doctor’s permission and supervision.  Fasting isn’t appropriate for pregnant women, children, anorexics, many diabetics, and others. I don’t know all the “others” but your doctor will know if you’re one.
  2. Start small and build up. Don’t attempt a 10-day water fast first thing. Start by skipping breakfast or dinner a couple of days.  Then skip lunch and see how that feels. Slowly increase the length of your fasting period. I personally feel that 4-5 days is plenty to reap all the benefits above. From there you can refeed and repeat.
  3. Drink lots and lots of water. Then drink more. You can also enjoy sparkling water, tea and coffee black. Be careful with herbal tea, however – many of the fruity types add sweeteners.  Stick with simple black or green tea. I also drink a small amount of apple cider vinegar mixed with water because it puts additional downward pressure on my glucose but I’m a weirdo. 
  4. Continue to take your doctor-prescribed medication.  I personally stop taking all other supplements to give my digestive system and detox organs as much of a break as possible. 
  5. Manage your electrolytes. The usual supplements are sodium, potassium, and magnesium but these may be different for you. For example, I have high potassium as a thing so I avoid adding more.  Electrolytes are critical for extended fasts and for those new to fasting. When you feel tired, dizzy or generally “off”, this usually means you need electrolytes – and more than you think. Many veteran fasters swear by keeping a bottle of “snake juice” handy to sip through the day(s). There are many recipes but I’ve included the most common below.  I usually eat a couple big grains of Himalayan salt each day.
  6. Don’t consume anything else. I avoid flavored waters, sugar-free gum, breath mints, cough drops, diet soda, etc. Some will “dirty fast” with zero-calorie drinks. You do you if that’s your thing. I feel that clean water is the most healing but any kind of fast helps.
  7. Don’t freak out if you “mess up”. Don’t have a cough drop without thinking and decide your entire fast is ruined, it’s time to eat cake. Sure, you may have paused autophagy for a short time but you’re still reaping loads of benefits – including practice, if you keep going. At the same time, do not beat yourself up if you do end up breaking fast early. We’re fasting to do something nice for ourselves so be nice to yourself.
  8. End when it feels right. If you’re feeling unwell and electrolytes don’t make you feel better, feel no shame or guilt ending your fast early. At the same time, if you feel amazing, keep it rolling for another day. Above all, listen to your body. 

Read the results of my 7-day fast

Fasting Tips

  • Fasting will be easier for those already in ketosis. If you eat sugar and grain on the regular, you’ll have those cravings hounding you as well. 
  • Fasting is more fun when you don’t feel alone. There’s a surprisingly large community of people who enjoy fasting that’s worth looking into.
  • You can continue to work out. Fasted workouts are, in fact, the bomb. If you feel faint at all during your workout you need more electrolytes. If it’s not electrolytes, consult your doctor.
  • You won’t feel hungrier. The hunger you feel on day one is the same hunger you’ll feel on day 2 or 3. Hunger comes and goes and is usually gone after day 3 for most including me.
  • Your metabolism won’t shut down and you won’t go into “starvation mode.” Your metabolism and entire body were designed for unfed states. The exact reason humans store excess food energy as fat is so that it can be used as energy when there’s no food. This is another one I could write about endlessly but actual medical doctors have already explained it far better than I can.

How to Break a Fast, aka How to Refeed

“Refeeding Syndrome” is a rare condition where your nutrient-depleted body doesn’t have what’s required to digest your first meal and so pilfers from more critical systems. It’s generally not something you need to worry about for fasts of a week or less. Nevertheless, consult with your doctor – especially if you’re planning a longer fast. General guidelines are:

  • For the first refeed, limit yourself to 10 calories/kg of body weight per day (or 5 calories/kg for fasts longer than 15 days), with a daily increase of 5 calories/kg . For example, if you weigh 60kg, you want your first day of eating after an extended fast to be 600 calories, your second day 900 calories, etc.
  • Take supplemental electrolytes (sodium, potassium, calcium, phosphate, and magnesium) and B vitamins at least 30 minutes before eating. In my case, I take a multivitamin along with my usual salt supplementation if I’ve been fasting 5+ days.
  • Eat something easy to digest. Save the spicy salami for another meal.
  • And, too, you have this beautiful, clean, rested digestive tract. Don’t fill it up with crap.

Brad and I break our weekly fast with a salad made from dark greens, avocado, homemade sauerkraut, and a dressing of flaxseed oil, apple cider vinegar, mustard and raw garlic. This salad, in fact. It’s delicious, supportive of the digestive systems, and very healing.

For an extended fast, I break with homemade bone broth and then follow with the salad or some avocado mashed with sauerkraut and flaxseed oil. 

Fasting Resources

The best thing is, you don’t need any special equipment to fast. Not only is it free, it saves you money. There are dozens of timer apps in the app store if that kind of thing keeps you motivated, I happen to use a free one called LIFE Fasting. I also enjoyed Dr. Fung’s The Complete Guide to Fasting but you can get much of the same information from his free online articles and videos. All you need to do to fast is not eat. 

Snake Juice Recipe

Never personally tested by me.

Snake Juice Electrolyte Drink

I’ve gotten this question from a few different folks – some who fast intermittently and are looking to explore extended fasting and some who may have never intentionally missed a… Fasting Vegan Keto fasting Print This
Serves: 1 Prep Time:
Nutrition facts: 200 calories 0 grams fat


  • 2 liters distilled water
  • 1 tsp potassium chloride (No Salt is one brand)
  • 1/2 tsp sodium chloride (Himalayan pink salt, grey Celtic sea salt or another mineral-rich sea salt)
  • 1 tsp Sodium Bicarbonate (baking soda)
  • 1/2 tsp magnesium sulphate (food grade epsom salts)


Dissolve everything in the water and keep it with you to drink, if that's your thing. 

You may also like


Raquel August 29, 2019 - 12:22 pm

I’ve done intermittent fasting mostly for losing weight, was not aware of the additional cancer-fighting benefits if you do extended versions.

I couldn’t help but chuckle at point #2 of mental clarity, with the “concentrate long enough to fin…” 🙂

What steps do you follow to make your homemade bone broth? Do you use mirepoix?

I’m rooting for you — Go go go

Maggie Jones August 31, 2019 - 1:12 am

Hi Lady! It’s great to hear from you. My classic, tasty broth has always depended on mirepoix. These days for breaking fast I just use bones, leek stems, salt, peppercorns and garlic cloves. xo

Robyn August 30, 2019 - 3:29 pm

Great post, Maggie. So much useful information! I started intermittent fasting 16 hours a day several months ago, with a non-fasting window in the afternoon/evening. One thing, though, I’ve never been able to resist is the milk (almond or soy) in my coffee. I wonder if the benefits would be that much better if I eliminated that? Thanks for the insights. You are so inspiring!

Maggie Jones August 31, 2019 - 1:10 am

Who cares!! You’re reaping so many benefits from what you’re doing – why suffer for negligible extra gains. For prevention and general health, you’re so far ahead of the game. Brad’s started joining me for my weekly 48 hour fast and he still does butter and MCT oil in his coffee to fuel him through. It’s still fasting mimicking which, according to Longo at USC, gives you most of the benefits.

I love you, Robyn! You’ve always been an inspiration to me!

Ida August 30, 2019 - 5:23 pm

Thanks for sharing your knowledge and fasting experience! I am currently undergoing chemo for stage lV ovarian cancer and have fasted 72-86 h in connection to all treatments. Minimal side effects so far and have energy to be active and workout. Going forward I plan to combine intermittent fasting with a weekly 36h fast and a prolonged fast quarterly. The only thing that I worrying about is loosing to much weight as I am a bit thin already. Have that been a concern for you? Any tips?

Maggie Jones August 31, 2019 - 1:06 am

Ida, I’m so sorry about what you’re going through! It’s clear you’re committed to doing whatever it takes to heal yourself and I’m grateful that your experience has mimicked mine as far as side effects go.
Your fasting schedule seems perfect.

I’m at the same point as you where I don’t have much excess body fat left and don’t want to lose more. In theory, we should be able to just eat a bit more during our feeding periods to gain back any weight lost. For me, that means going crazy with the nuts 🙂 I’m also focusing on more muscle-building and so far it’s been working. I’ve been working with a nutritionist and dietitian who’s had major success with other patients using the approach but in a lot of ways you and I are pioneers. I’d love to know how your experience goes. I’ll continue to update as things evolve here, too.

Very best! And thank you for the comments!

#YOF JULY - Going Hungry For 5 Days...On purpose! - July 11, 2020 - 9:28 am

[…] Complete Guide to Fasting for Cancer – https://cancerv.me/2019/08/28/how-to-fast-and-why/ […]


Leave a Comment

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More