Several years ago, a study suggested that fasting – avoiding most or all calories – for a short time before receiving cancer chemotherapy may enable the treatment to kill cancer cells more effectively. This study was conducted in mice, not people. We do not know if what happened in mice would happen in people with cancer.
Fortunately, there are several well-designed clinical trials on-going, and these trials are focused on determining whether fasting before, during and/or after chemotherapy or radiation treatment is beneficial. Further, these trials should help us sort out which people, with which types of cancer, are likely to benefit the most from this approach.
Understandably, people living with cancer do not want to wait for clinical trial results for an answer. They have cancer today, and for obvious reasons, many of these people are interested in trying fasting now.
What’s the harm in fasting?
Due to the lack of published studies in humans, it is premature to recommend fasting before chemotherapy or radiation therapy for all cancer patients. However, it is reasonable that people with cancer who are otherwise healthy, well-nourished, and not at risk of developing malnutrition try fasting. But this does not mean every person with cancer should try fasting.
The most important thing to remember is that weight loss and muscle wasting are common problems faced by cancer patients. For people with these concerns, deliberately depriving the body of calories and protein is not a good idea.
In fact, recent research on nearly 1,500 lung and gastrointestinal cancer patients found that weight loss and loss of muscle mass were stronger predictors of survival than even tumor type, stage of cancer, the patient’s age, and how well they functioned in their day-to-day life (performance status) [i].
This points to the importance of addressing unintentional weight loss. This is the type of weight loss that simply “happens,” without the patient trying. Unintentional weight loss occurs even though the person is trying to eat enough to prevent weight loss.
Also key: this study clearly showed that even for people who were overweight or obese, losing weight led to poorer survival. In other words, carrying excess weight does not protect against the negative consequences of unintentional weight loss in people being treated for cancer!
Clearly, fasting is not for everyone.
Should I fast?
Remember, if you are having unintentional weight loss due to cancer or its treatment, fasting may cause more harm than benefit. People with unintentional weight loss have more severe side effects, more treatment dose reductions, and generally poorer survival.
The unintentional (not trying) aspect of weight loss during cancer care is important. People in this situation often are very different, physiologically, compared with patients who are not losing weight unintentionally.
When the body is losing weight without the person trying, it means that person is having more inflammation, more loss of lean tissue, more depressed immune function, and is experiencing many other markers of poor outcomes. For people who are not losing weight unintentionally, the picture is very different.
If a person is losing weight with a healthy diet and regular physical activity (including strength training and cardiovascular activities), the small amount of weight loss that may occur with fasting is not as likely to cause harm. Of course, this should only be attempted during cancer care with the approval of the medical team.
In these cases of weight loss “on purpose,” the benefits of fasting may outweigh the downsides. This type of person is in a better place to tolerate some calorie deprivation without losing healthy, lean tissue.
Why would fasting be good?
Normal cells and cancer (malignant) cells grown in test tubes behave differently under conditions of starvation. When deprived of nutrients, normal cells quickly stop growing and switch into a state of “housekeeping.” They perform only functions that are absolutely necessary for survival. Their growth slows, and they metabolize (use) nutrients at a much slower pace than when nutrients are readily available.
In contrast, cancer cells continue to divide under starvation conditions. In many cases, they grow and divide until they exhaust all nutrients, and then die.
This makes sense: the hallmark (key feature) of cancer cells is uncontrolled growth and replication. This means that regardless of the availability of nutrients, or anything else for that matter, cancer cells just go right on growing and dividing.
Unlike healthy cells, cancer cells don’t have the luxury of slowing down just because nutrients are scarce.
Also of interest is that under starvation conditions, normal cells in a test tube are up to 1,000 times better protected against chemotherapy drugs than cancer cells. This also makes sense.
If normal cells are growing very slowly (or not at all) due to a lack of nutrients, they aren’t going to ‘take up’ or absorb the chemotherapy drugs. This may be one explanation as to why fasting could protect normal tissues against damage during cancer treatment.
To summarize: fasting may protect normal cells against damage, while simultaneously making cancer cells more sensitive to the treatments that are aimed at killing them [ii].
Who is studying the possible benefits of fasting during cancer treatment?
One researcher actively pursuing this line of inquiry is Valter Longo at USC.
Dr. Longo has published a human case series. A case series simply reports what is observed. It is not a true test of whether a particular treatment approach (including fasting) is helpful, harmful, or neutral.
The National Cancer Institute has highlighted the work of Dr. Longo and other cancer researchers who are studying fasting.
A few examples of some of the ongoing clinical trials on fasting can be seen at:
You can see more examples of these trials on the ClinicalTrials.gov website. Searching on fasting and chemotherapy, fasting and radiation therapy, dietary restriction and cancer, and other, similar combinations of words will yield dozens of examples of on-going clinical trials on this topic.
What to consider with fasting during cancer care
For people who are interested in fasting, there are some very important things to keep in mind:
- Never attempt fasting without discussing it with your doctor and dietitian. It is in your interest that everyone on your healthcare team is on the same page. Even if your medical team discourages you from fasting, if you plan to do it anyway, do not hide this from them.
- Do not try fasting if you are taking oral chemotherapy medications. These are cancer drugs that are taken by mouth. Many oral chemotherapy medications must be taken with food, or taken with a very specific amount of calories, fat, protein, or other nutrients, to be absorbed fully, and to work properly. This issue hasn’t received much attention in the debate about pros and cons of fasting during cancer treatment. It is very important!
- Hydrate, hydrate, hydrate! Fasting does not mean avoiding all liquids. In fact, if you are dehydrated when you receive cancer treatment, this can cause permanent damage to your body. For example, kidneys can be damaged when chemotherapy is given without the person drinking enough fluids. The liver may not detoxify and excrete the chemotherapy properly.
- Be sure you know what you’re doing and how you plan to fast. To some people, fasting means avoiding all calories, solid and liquid. To others, fasting means cutting way back on what you are eating and drinking, but not going completely without calories. Some people want to fast for 3-5 days, which is a long time! They decide to fast for 1-2 days prior to receiving treatment, the day of treatment, and for 1 day after treatment. Do not do this without talking to your medical team!
- Do not fast if you have other chronic conditions, such as kidney or liver problems, diabetes, heart disease, or other health issues. Fasting under these conditions may not be safe. Don’t risk your health by trying something that may cause more harm than benefit.
- Stop fasting if necessary. For nearly all of the research studies on this topic, the researchers have specific guidelines (protocols) in place to address excessive weight loss. If weight loss reaches a certain point, and if the weight isn’t regained, fasting is discontinued. Even the researchers acknowledge that losing too much weight during cancer treatment can be harmful.
Right now, we don’t know if fasting during cancer treatment is a good idea, but early research suggests this approach may be beneficial to some patients.
If you are interested in this, ask your medical team for more information about the pros and cons of fasting during cancer care.