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.

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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/ […]

Katherine Lee November 9, 2020 - 1:24 pm

Hello Maggie,
Thank you for all the helpful information to terminal cancer patients. Can you fast even when taking targeted treatment drug like gefitinib? My doctor says to take the drug after food.

Maggie Jones November 9, 2020 - 11:16 pm

Hi Katherine! Doctors normally recommend medicine be taken with food if it will cause nausea or other discomfort. I know when I was first taking crizotinib, I would 100% throw it up on an empty stomach. If I took it with a little food, I had a 50/50 chance of keeping it down =D Back then I would take it with a few almonds. This may not be strictly fasting but is still fasting mimicking (a few hundred calories, low carbohydrate) which Dr. Valter Longo at USC has shown to be almost as beneficial. Over time, my stomach got used to the meds and I no longer needed to eat with them. Of course, your decision should always be based on discussion with your health care team.
Much love to you!

Nadja June 26, 2021 - 10:07 pm

I have stage IV breastcancer with metastasis in both liver, lungs and skeleton. Every 3 month i take Zometa infusion and gets fever and skeleton pain for about 24 hours. Should i try fasting before the infusion?
I try to eat mostly vegan keto and take supplements and i also do alternative treatments for my body, mind and soul to heal.
Thanks for sharing your journey ❤️

Maggie Jones July 1, 2021 - 8:44 pm

Hi Nadja, I’m so sorry for what you’re going through! Of course I can’t give medical advice and the research around fasting and various treatments is woefully thin. I’d definitely recommend working with your metabolically-informed doctor. Personally, I find the benefits of fasting prior to/ during treatment so overwhelmingly helpful that I would fast prior to the infusion but again, this is a decision that needs to be made with your medical team. These days a water fast from Sunday dinner to Thursday lunch is the only way to keep my symptoms and side effects at bay.
It sounds like you have a ton of healing tools your working with, though. I have no doubt you will find healing. Your journey is such an inspiration!!
Much love! Maggie

Barbara September 27, 2021 - 9:42 pm

I was just informed that I have lung cancer that has gone to liver…will see cancer specialist Oct 6…just finished 6 day water only fast…never felt energized or mental clarity. Perhaps due to electrolytes? Any suggestions?

Maggie Jones September 27, 2021 - 9:58 pm

Hi Barbara! I’m so so sorry to hear of your diagnosis but you’re doing the best, most amazing thing for your body. I got chills reading this. At first I was enraged that you had to wait so long for an oncologist but, in truth, I love that you have time to pause and release any sense of urgency. The biggest mistakes in my journey came from letting my panic (or my doctors’ panic) get the better of me. You are going to be such an inspiration for so many.

A 6 day fast is no joke. You are totally amazing. I almost feel sorry for your cancer.

Brad and I both find that electrolytes are the most critical thing to maintain energy levels and manage any brain fog. I aim for 5,000mg sodium in addition to my nightly 300+mg magnesium. He adds potassium in the form of No-salt.

The ol’ stage 4 cancer could also be responsible for some fatigue, of course. And people who’ve been exposed to a lot of toxicity can experience a Jarisch-Herxheimer Reaction (Herx reaction or detox symptoms) starting after 24 hours that can last for days or weeks. I experienced this on my 4th extended fast.

I just finished interviewing Dr. Longo for the upcoming documentary and am meeting with Dr. Fung next week. I’ve never been more confident and excited about the healing benefits of fasting.

Much love to you!

Nya D November 29, 2021 - 6:22 pm

Hi Maggie, Thank you for your blog. Its inspirational. You mentioned, “I usually eat a couple big grains of Himalayan salt each day.” For your choice, whats that recipe look like?
?approx qty salt to how much distilled/filtered water?

Maggie Jones November 30, 2021 - 2:22 am

Hi Nya!
My fasting has evolved and I now fast 3-5 days the first week of each month in addition to every Monday and a prolonged fast 1-2 times a year. These days, I start taking 3-6 grams of Himalayan salt whenever I’m feeling low on energy divided between 1-2 cups of water to sip. I add 1-2 grams of potassium for my husband but I have hyperkalemia and avoid dietary potassium. I always take 500-1200mg of magnesium capsules with water at bedtime.
I personally don’t need to start this until day 3 of my fasting but many need it earlier and with more or less electrolytes. I’m sure you’ll find the right combination for you.
Sending love! Maggie

Steve Donaldson December 20, 2021 - 4:36 pm

Have you found anything about wine/ beer or other drinks containing alcohol, and what effect they have on cancers?

Maggie Jones December 21, 2021 - 9:37 pm

Hi Steve, one thing I know for certain is that alcohol isn’t a healing food. My food philosophy is to eat only foods that actively heal my body. I was a major “alcohol enthusiast” prior to my diagnosis (about a bottle of sauvignon blanc on weeknights, much more on weekend ends) but cut back to just a couple of sips of red wine or champagne each month after my diagnosis. I’ve had to cut cut even those sips and have been completely alcohol free for the past year.

It’s known that alcohol contributes to chronic inflammation which we know is not good for cancer (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2842521/). For me personally, it causes immediate inflammation in my brain leading to seizures.

On the other hand, my perfectly healthy husband will often turn a water fast into a “wine fast” and enjoys a glass or two without ill effects. Beer, or course, if off limits due to the sugar content.

Very best!

Natalie July 10, 2022 - 6:44 am

Hi Maggie
Love your blog. I too have stage 4 NSCLC with an EGFR mutation diagnosed Oct 2017. I have done the COC protocol for the last 4 years. You mentioned that you stop all supplements during your fast but continue on prescription meds. Should I stop metformin and other COC drugs during a fast?

Maggie Jones July 10, 2022 - 4:57 pm

Hi Natalie! It’s so good to hear from you! Oct 2017 – Wow! Exactly one year before my diagnosis.

Of course, talk with your doctor and all that. For me personally, I consider my COC drugs physician prescribed and take them during my fasting. Keep a close eye on your blood glucose taking metformin while fasting. The combo is very individual. I personally can take 2000mg metformin with half a gram of berberine and tbs of apple cider vinegar on a 5 five day fast and never approach hypoglycemia. Others see their blood sugar drop dramatically. Some are okay with that. Check your numbers regularly and pay very close attention to how you feel. Other options are to stop the metformin, take a half dose during fasts… whatever keeps you stable. And most of all, get your doc’s option. (But personally, I know my body better than they do.)

Thank you so much for introducing yourself and the kind words!! Looking forward to chatting again.

John September 2, 2022 - 7:55 am

Hello, Maggie,
I had cancer (sarcoma) and several metastases in the lungs, but after chemotherapy I have had a complete remission. I have done intermittent fasting and extended fasting while receiving chemotherapy. I would like to know how many times a year you do prolonged fasting?
Thank you so much.

Maggie Jones September 4, 2022 - 7:37 pm

Hi John! Congratulations on your incredible recovery! For the year that I had detectible cancer, I fasted ~46 hours every week (Sunday dinner – Tuesday dinner) 3-4 days around starting a new chemo or radiation, and 7 days once in that year. I’m now three years cancer-free and my fasting schedule is Sunday dinner – Tuesday dinner every week extended to Sunday dinner – Thursday midday to Friday (~90-120 hours) dinner the first week of the month. I have other issues these days that prevent me from fasting over 5 days so, if I feel like I need more, I’ll refeed and go again.

On feed days, I still intermittent fast about 16 (weekends)-20 (weekdays) hours a day.

I believe Dr. Nasha fasts 3 days out of the month and Sarona Rameka fasts 5-7 days out of the month.

I hope this is helpful in finding the schedule that works for you! xo

Maria October 21, 2022 - 5:02 pm

Hi Maggie!

I am Maria, I live in Vancouver, BC and was born in Venezuela…I was diagnosed with stage IIIC high-grade serous ovarian cancer with complete surgical resection in June 2022. I am one of the lucky ones it was cut just in time. I am about to finish my chemotherapy treatment, I have one more cycle and I will be done. I also tested negative for the blood genetic testing BRCA1 & BRCA2…wonderful news! Also so good for me is that my tumour tested positive for HRD, meaning that my maintenance therapy (PARP inhibitor) will work wonderfully.

Fasting and eating healthy is not new to me, I have been fasting 16:8 for over a year and on January 2022 I was doing 24 hours of fasting once monthly before I was diagnosed, and I really love it! I was super lean and in shape, I even had a six-pack at 48 years old, that’s how I was able to see a small ball growing in my lower abdomen, along with feeling bloated, a lack of appetite and weight loss.

Chemotherapies have been hard but I don’t have to deal with vomiting or nausea, which allows me to skip the steroids, since Chemo one I started “dirty fasting” by only drinking green juices without sugar, cold press and organic and mineral broth, starting on Sunday and breaking the fast on Thursday the day after my treatment, I get my chemo every 21 days. I couldn’t do full fasting since I lost so much weight after my operation, I am still about 10Lb to 15Lb below my normal weight. For my last chemo, I want to try to fully fast for 96 hours… I tend to feel very weak and dizzy after my chemo, would you know if not having any nutrients for that long would make me feel even weaker, should I fast with mineral broth only?

Thank you for doing all the research, your site has been extremely informative and beneficial for me!
Thank you from the bottom of my heart!

Maggie Jones November 30, 2022 - 7:17 pm

Hi Maria! Your story is so inspiring. I’m so happy you’ve been able to stay off the steroids!

My first thought on your weakness is electrolytes – are you getting enough (way more than you think)? Mineral broth will provide some electrolytes but probably not enough it that’s your only source.

If you continue to have issues and don’t have a supportive doctor (who does), It might be worth looking into a fasting clinic like TrueNorth in California or one of the many in Europe, Thailand and Central/ South America.

It sounds like you have everything under excellent control and I’m so grateful to you for sharing your story here to inspire others!

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