Fasting and Cancer, with Suzanne Dixon, MPH, MS, RDN and Annette Goldberg, MS, MBA, RDN, LDN

October 6, 2016
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To fast is to partially or completely reduce one’s food intake for a period of time. In today’s podcast, Suzanne Dixon and Annette Goldberg discuss the history of fasting, different types of diets, and why some scientists are researching the effects of fasting during or after cancer treatment. They also provide tips for someone considering fasting during treatment.  



ASCO: You're listening to a podcast from Cancer.Net. This cancer information website is produced by the American Society of Clinical Oncology, known as ASCO, the world's leading professional organization for doctors that care for people with cancer.

To fast, is to partially or completely reduce one's food intake for a period of time. In today's podcast, Suzanne Dixon and Annette Goldberg discuss the history of fasting, different types of diets, and why some scientists are researching the effects of fasting during or after cancer treatment. They also provide tips for someone considering fasting during treatment.

Suzanne Dixon is a registered dietitian and epidemiologist with a nutrition and research consulting business in Portland, Oregon. Annette Goldberg is an outpatient dietitian at the Boston Medical Center Cancer Care Center. ASCO would like to thank Ms. Dixon and Ms. Goldberg for discussing this topic.

Suzanne Dixon: Hello. My name is Suzanne Dixon and I'm a registered dietitian and an epidemiologist with a nutrition and research consulting business in Portland, Oregon. I have a long history of working in cancer care, including time as Director of the Outpatient and Oncology Nutrition Program at the University of Michigan Cancer Center. And I'm really passionate about providing accurate information about nutrition and cancer that's based on facts and not fear. Now let me introduce my colleague, Annette Goldberg.

Annette Goldberg: Thanks, Suzanne. My name is Annette Goldberg and I'm the Outpatient Oncology Registered Dietitian for Boston Medical Center. BMC is a nonprofit, 500 bed, academic medical center in Boston. It's the largest safety net hospital and a level-one trauma center in New England. Suzanne, a topic of interest lately within the world of nutrition and cancer in the Boston area, has been fasting and calorie restriction. Have you heard the same interest?

Suzanne Dixon: Yes, I have. I've heard a lot about fasting and calorie restriction, and not just in my local area, but a lot of articles, and newspapers, and on the web. And I've been hearing about it as an adjunct, or an addition to conventional cancer treatment, for a couple of years now. And first, there were the usual animal studies, which is typically how things get started in the research world, in mice. And now we're seeing more interest in studying this approach in humans with cancer, too. And a quick search on the website, which is a website where you can see where trials that get funding from the government are listed, there are numerous studies ongoing on fasting, and they are looking at a whole host of things, including, in conjunction with, conventional cancer treatments. And for a number of cancer types, including breast cancer, prostate, endometrial, and a mix of other tumor types.

And fasting for cancer prevention is also being studied, which is interesting. Annette, why don't you start with a bit of history and explain what we mean by the terms "fasting" and "calorie restriction"? And then I can follow up with some important consideration for patients who may be interested in incorporating fasting, or calorie restriction into their nutrition program during treatment, or into survivorship.

Annette Goldberg: Okay, great. That sounds like a plan. As we know, fasting isn't a new concept. Some religions encourage fasting around their meaningful holidays. Fasting and calorie restriction has been studied in various forms. For example, it's been studied as byproduct of a public health crisis, such as the Great Depression or the Irish Famine. Both of those situations bring about calorie restriction. Both have been studied as a byproduct of a way of life. For example, I'm not too sure if you were aware, but the Calorie Restriction Society International, they're a group of people whose lifestyle practices supports, and contributes, to research of calorie restriction and its effects on health. Fasting's also been studied by a byproduct of other research efforts, such as the Biosphere Project. This was first designed to demonstrate the viability of a closed ecological system to support and maintain human life. This first mission was to support eight humans for two years. And due to the many challenges, which included growing food, the experiment itself created calorie restriction for the study participants that were living within the biosphere.

Finally, as you mentioned, fasting is being studied on its own, as part of research efforts on a variety of diseases and the studies continue today. But before we delve too far into the subject, I think it's important to understand the different terminology when talking about fasting. As you know Suzanne, this has been quite confusing. Often, terms are used that don't truly reflect the meaning of the word. So I wanted to take us through a couple definitions, and realize that as you're looking at studies, or as you're considering diets, make sure you really understand what they're talking about when they talk about fasting, or intermittent fasting, or calorie restriction.

The term "fasting" usually refers to no caloric intake over a period as short as one day, or as long as three to five days. "Intermittent fasting" refers to periodic fasting. It may or may not include a small amount of caloric intake on your fasting days. Your intermittent fasting day might include some caloric budget. It's not truly fasting as we know the word.

"Calorie restriction" refers to eating fewer calories than the body needs to maintain its current weight. Typically, calorie restriction refers to decreasing the calorie intake to 20 to 40% below the daily required intake for weight maintenance, on the average. A person may achieve calorie restriction by simply cutting 20% of calorie intake every day. Or by completely fasting some days and eating more normally on others, so that on average, their calorie intake ends up being about 20% less than what they were eating before the calorie restriction.

I realize this can be pretty confusing. Some diets have been created around fasting, and the intent of these diets have been mostly weight loss, and improving health markers, such as blood glucose and cholesterol. One of the more popular diets that's been talked about is the "Fasting Mimicking Diet," or FMD. And this refers to a diet that is meant to mimic a fast, in terms of its effect on the body. Some research has shown simply reducing calorie intake to a very low amount, around 20% to 40% of your normal calorie intake, will be perceived by the body in a similar way as a complete fast with no calorie intake. And as you can imagine, both fasting and a Fasting Mimicking Diet can be difficult to maintain. So we're getting some encouragement in the fact that, instead of completely fasting, where you're taking in no calories, you might be able to take in a small amount of calories, and have your body mimic as it if were fasting. Which would be a little bit easier for some people to do.

Another popular diet you might have heard of is the 5:2 Diet, and on this diet the two fasting days consist of 500 to 600 calories. The remaining five out of the seven days you eat normally. The major difference is with the 5:2 diet, is the body response time to the calorie restriction, takes maybe months versus days. Until recently, fasting and calorie restriction research was conducted to gain further insight on its effects on aging.  Animal research studies suggest calorie restriction protects many types of cells in the body and delays ageing. But recently some researchers have begun to focus on the complementary approach of fasting, and its protective effects for healthy cells during cancer treatment. And so let us explain that a little bit. The premise of this research is when healthy cells are faced with a restricted energy supply, they switch off signals for growth and reproduction, and enter a metabolically protective mode, sometimes referred to as housekeeping mode.

So they're not always out reproducing, because they realize that they've had some energy restrictions, so they're going to go into housekeeping mode. The cell's energy is diverted to maintaining and repairing itself, and not actively growing and reproducing. Cancer cells are mutated, and this results in uncontrolled growth and the inability to respond to external factors. Because of this, cancer cells cannot protect themselves from the external stress of a restricted energy supply. This continued growth of cancer cells, during times of reduced energy supply, makes the cancer cells more susceptible to radiation, and chemo agents because these treatments specifically target rapidly growing cells. So, in theory, with energy restriction, healthy cells metabolically slow down, and aren't as affected by chemotherapy and radiation. While the cancer cells, which are still growing, and very rapidly reproducing are hit harder by this treatment.

Although there haven't been enough studying in human subjects to consider this approach evidence base, the studies that have been done reveal a couple of things: due to the changes that need to occur within the cells, complete fasting during the time you're receiving chemotherapy appears to be more effective than intermittent fasting during treatment time. For example, if you're repeating chemotherapy every three weeks, you might completely fast for three to five days around the time you receive each infusion. And then eat normally for the two and a half weeks in between each infusion cycle. Small case studies in humans, along with controlled studies in animals, demonstrate fasting appears to reduce the side effect of chemotherapy. And under the watchful eye and care of your oncology team, fasting can be undertaken in a safe, feasible and effective way, for some people during cancer therapy.

Suzanne a lot of this sounds very complicated. How would you advise the patient that's interested in short-term fasting during treatment, or calorie restriction during survivorship?

Suzanne Dixon: You're right in that. With all of the different types of fasting regimens, and all of the differences nutritionally speaking, in terms of what different cancer patients might need, there are some good general guidelines on fasting that can be really helpful for someone who is affected by cancer, who is thinking about this idea of, "Maybe I should try some fasting during my treatment." So the most important thing about fasting is that a person undergoing treatment for cancer should never attempt to do this without first talking to their oncologist, and their entire cancer care team. It's really important if you want to make any major changes to your diet, and that includes adding in things like dietary supplements, herbs, and other over-the-counter products, as well as fasting, you should talk to your doctor before you make this changes.

Different cancer treatments have different effects on the body, and it's important you and your doctor, and your nursing team work together so that you understand what to expect. You want to be very certain that large dietary changes, such as fasting, and that is a really large dietary change, is safe for you at the time that you are going through cancer treatment. Another thing to keep in mind is if you're taking any oral chemotherapy medications, and these are things that you would take by mouth, do not fast unless you know it is safe to take your medications on an empty stomach. Some of the newer oral chemotherapy medications must be taken with food for best absorption. And some even come with very specific guidelines on the amount of fat grams, carbohydrates, and protein to eat in order for the medication to be absorbed properly.

If you receive this type of instructions for taking your oral chemotherapy, do not fast. If you have other chronic diseases, this is a big concern. This could include things like: kidney disease, diabetes, liver disease, gout, heart disease, hypoglycemia, which are also known as episodes of low blood sugar, serious mental health concerns, and autoimmune disease. Or any other chronic health condition, fasting really may not be the best choice for you. Most people who are otherwise healthy can safely try fasting. But if you have other health conditions, the added stress of fasting may make you feel worse. Or fasting may make it harder for you to tolerate your cancer treatments, and other treatments that you need to manage your other chronic conditions.

I also caution people that, if you have a history of an eating disorder, such as anorexia nervosa, bulimia, or something like binge eating disorder, which was recently recognized as an actual psychiatric health condition, I recommend against fasting. And that's because in people with a history of food restricting, or binging episodes, fasting may lead you into some of those behaviors again. As an example, if you restrict your calorie intake severely one day, as perhaps you're doing a Fasting Mimicking Diet, you're just taking in a few hundred calories, this may make you really famished, and then lead you to overeat, or binge the next day.

And while fasting may be a healthy option, yo-yoing, or going up and down on your calorie intake from very few calories one day to very large amounts of calories the next day is not healthy. And that is not going to help your body tolerate cancer treatment either. And it's likely to make you feel a lot worse. So if you get the okay to fast during, or around cancer treatment, ask your doctor for a referral to a dietitian, if one is available at your cancer treatment facility. This person can help you figure out which type of fasting plan, as Annette pointed out, there are many different types of fasting plans. Or it could be something like a Fasting Mimicking Diet will work best for you, and that could depend on what your treatment cycle looks like, the types of medications you're taking, and a few other things.

So, if you can, talk to a dietitian and try to work out what type of fasting plan you're going to do. And if you do try fasting, drink a lot of fluids. I can't stress enough that fasting does not mean avoiding non-caloric liquids. So even if you're doing a complete fast, you can still drink plenty of water, tea, herbal tea, sparkling water, or other non-caloric beverages, and this is really important. The last thing you want to do is show up for your treatments dehydrated, and it is pretty easy to get dehydrated if you're completely fasting. And that's especially important to protect your kidneys from some of the damaging effects of certain chemotherapy medications. And, in fact, if you show up to the clinic and you are dehydrated, your doctor can do a blood draw and they can tell if you're dehydrated. Your doctor may delay your treatment, or send you for some fluid infusions first. So keep in mind, fasting does not mean skipping out on fluids, and be sure to keep yourself well hydrated. And consider your cancer diagnosis to help you decide if fasting is a good option for you.

There's a lot of excitement around fasting and a lot of people saying, "Well, everybody should just try fasting," but some types of cancers, and the treatments that we use for them, have the effect of making the body something that we call hypermetabolic. Basically, this means your metabolism is running very rapidly, and you're more prone to losing a lot of weight unintentionally, or without trying. And if you're in that situation, the likelihood that you lose lean body mass, which can include your muscle tissue, and even white blood cells, red blood cells, and mucous membranes, is much higher than normal. So if you are finding that you're in this hypermetabolism, or unintentional weight loss situation, it's really unlikely you'll lose excess fat. So even if you're overweight, losing weight unintentionally is usually not very helpful. And in these cases, fasting may worsen your health, and may make it more likely that you need to take treatment breaks, or have dose reductions in your medications, which is something we definitely don't want to have happen.

So this is just to give you some basic guidelines. And remember, everybody is different. But there are some cancer types that may be more likely to cause this unintentional weight loss situation. So if you have head or neck cancer, lung cancer, pancreatic, stomach, or liver cancer, ovarian cancer, or any type of cancer that is metastatic, meaning that it does spread to other organs, or to your bones, you may be more prone to this hyper metabolism situation, and to unintentional weight loss. And in this situation, fasting is generally not a good idea. In fact, one recent study of a brand new targeted therapy to treat lung cancer has shown that patients who gained weight during their treatment period experienced a better response to the treatment drug compared with patients who lost weight. So clearly, fasting, which restricts calories and may lead to weight loss, would not be advised in that situation.

Fasting has been best studied in people who have cancers that tend to cause weight gain, rather than weight loss. And believe or not, and a lot of people find this hard to believe, some cancer patients do struggle not to gain unhealthy amounts of extra weight. There may be treatment effects, or things going on that make it more likely that they put on extra weight. And we know, as health professionals, that this excessive weight gain during, or after treatment, isn't good for your health, or for your long-term survival, and  in these cases, fasting may help a patient keep weight from going up into the overweight, or obese range and may help better manage symptoms and side effects. As Annette said, in certain circumstances fasting may make those side effects less intense. So cancers that tend to fall into this category, where you may be more likely to gain weight, rather than lose it, include breast and prostate cancers. Those are the two most common types of tumors that we often see people struggling not to gain unhealthy weight.

So for people with early stages of these cancers, it's true that you could potentially benefit from fasting and you may benefit the most, compared with other tumor types. So though we don't know this, and we won't know it until more research is completed. So again, as I said right at the very beginning, it's really important to involve your cancer team members and everybody else that's working on your treatment, so you're all on the same page, as you go forward. In the end, the best steps you can take, regarding fasting, are to investigate all of the reasons why this approach may or may not be the right choice for you during cancer treatment. And always talk to your doctor to come up with your best plan going forward. So Annette, do you have else to add?

Annette Goldberg: I don't. And Suzanne, that was a great summary. I think it's still very complicated, and as you had mentioned it could be a dangerous situation if you take on fasting without making sure that your oncology care team is involved.

Suzanne Dixon: Absolutely. Well, thank you very much.

Annette Goldberg: Thank you.

ASCO: Thank you Ms. Dixon and Ms. Goldberg. For more expert interviews and stories from people living with cancer, visit the Cancer.Net blog at

Cancer.Net is supported by the Conquer Cancer Foundation, which is working to create a world free from the fear of cancer by funding breakthrough research, sharing knowledge with physicians and patients worldwide, and supporting initiatives to ensure that all people have access to high-quality cancer care. Thank you for listening to this Cancer.Net podcast.