Intermittent Fasting and Cancer

Intermittent fasting, especially "time-restricted feeding" or "prolonged nighttime fasting" has become very popular, and questions about its potential role in both cancer prevention and treatment have been raised. Early evidence suggests that this strategy has the potential to improve the effectiveness of treatments and reduce side effects, but thus far, only a limited number of studies have been done. With regard to breast cancer, there is evidence that prolonged nighttime fasting may lower the risk of recurrence, a risk we are learning can remain for decades after treatment.

We will take a look at some of the studies that have been done, the potential mechanisms by which it may affect cancer cells, and the potential risks and side effects. Anyone who is living with cancer should talk with their oncologist before considering any dietary regimen, including intermittent fasting.

Defining Intermittent Fasting

Before talking about fasting and cancer it's important to define a few terms, as study results can vary depending on the definition. Fasting, of course, simply means going without food, or "food fasting." This article does not discuss limiting water intake, and for people with cancer, this would be unwise (unless recommended by your doctor for some reason).

Intermittent fasting, in turn, is a dietary pattern in which people abstain from eating for a particular period of time which is alternated with periods of regular eating. During periods of eating, there are no restrictions on the amount of food eaten or a reduction in calorie intake.

Some intermittent fasting regimens involve complete abstinence from food (but not water) for a period of time, while others allow small amounts of food or non-water beverages.

Types of Intermittent Fasting

Subtypes of intermittent fasting include:

• Prolonged nighttime fasting: This regimen is most commonly studied with respect to cancer, and involves simply extending the period of time between dinner and breakfast. This likely was the "normal" diet eaten by our ancestors in the past, when eating was not as convenient as it is today. A common regimen is the 16/8 method, in which food is eaten between 12 noon and 8 p.m. (16 hours of abstaining food and eight hours of unlimited restrictions on eating).
• Time-restricted feeding: This can be the same as prolonged nighttime fasting, and simply defines the hours during which food can be eaten and hours of fasting.
• Short-term fasting: There are a number of varieties of short-term fasting. For example, in alternate-day fasting people alternate between days with no restrictions and days that roughly 25% of average calories are consumed. With whole day fasting, people usually eat normally (without restrictions) five days per week, and consume either no calories or 25% of average daily intake two days per week.

What Intermittent Fasting Is Not

It can be easier to understand intermittent fasting but talking about what it is not.

• It does not restrict water intake: Water, as well as beverages that do not have calories such as calorie-free soda and water drinks, tea, and coffee are generally allowed.
• It does not define what foods should be eaten or not eaten.
• It does not restrict intake of medications or supplements.
• It does not define or restrict the number of calories eaten while not fasting.

History

Historically, intermittent fasting, or at least prolonged nighttime fasting or time-restricted eating, is likely the way our ancestors ate on a regular basis. Until relatively recently (and as is still the case in parts of the world), most people didn't have the convenience of getting up and heading to the fridge to microwave a snack before bed. Likewise, a warm breakfast took time to prepare.

The concept of fasting is common to many of the religions of the world, and has been described in ancient texts. In this context, fasting was often seen as a spiritual practice, though fasting was thought to have health benefits as well.

Looking at other species in the animal kingdom, intermittent fasting (prolonged nighttime fasting) is a common practice as well.

Intermittent Fasting/Short-Term Fasting and Cancer Treatment

Intermittent fasting (prolonged nighttime fasting) may have benefits for at least some people with cancer, though the science is in its infancy.

Theory

Potential mechanisms will be discussed below, but the general theory behind intermittent fasting in cancer is the difference in how cells adapt to stress. Healthy cells are thought to be much better at adapting to fewer nutrients in their environment. Cancer cells, in contrast, just keep growing and thus have a greater need for nutrients. During treatment, such as chemotherapy, this could result in cancer cells being more susceptible to oxidative stress and DNA damage, and therefore being more sensitive to the treatment.

Preclinical Studies

Animal studies, though they can't necessarily be translated to humans, have suggested that restricting calorie intake on an intermittent basis (such as with prolonged nighttime fasting) may be associated with better outcomes with cancer, at least in mice.

Studies looking at human cancer cells grown in the lab have also shown promise. For example, short-term fasting appears to improve stress resistance in normal cells while at the same time making cancer cells more sensitive to toxins. It's thought that the reason is that cancer cells, because they grow and divide so rapidly, are less able to respond to changes in their environment such as a short-term shortage of food.

Studies looking at humans without cancer have also suggested that intermittent fasting may have benefits for people with cancer, and these are discussed below.

Human Studies

Short term fasting may both improve the effectiveness of treatment and reduce toxicity according to early studies on humans, though many of the studies to date have focused primarily on the safety of intermittent fasting in people with cancer.

A 2018 study was done to explore the impact of short-term fasting on chemotherapy. People with ovarian and breast cancer were instructed to begin fasting 36 hours prior to their infusion, and end their fast 24 hours after their infusion. Those who fasted had an improved quality of life and less fatigue during chemotherapy without any adverse effects.

Intermittent Fasting and Cancer Recurrence

Cancer recurrence is not only feared by many people who are diagnosed with early-stage tumors, but a leading cause of death. For example, the majority of women who have stage IV breast cancer were initially diagnosed with early-stage disease and later had a metastatic recurrence. Once breast cancer becomes metastatic, the average life expectancy is only three years, though some people live much longer.

Recent studies highlighting that the risk of breast cancer recurrence does not decrease after five years for women who have estrogen receptor-positive breast cancer has highlighted the need to address ways of reducing the risk of recurrence. In fact, women with hormone-positive breast cancer are more likely to have a recurrence after five years than in the first five years following diagnosis.

A 2016 study looked at the role prolonged nighttime fasting might play in breast cancer recurrence. Over 2,000 women who had been diagnosed with early-stage breast cancer between 1995 and 2007 (and did not have diabetes) were evaluated. It was found women who had a short duration of nightly fasting (defined as less than 13 hours between the evening meal and breakfast) were 36% more likely to experience a breast cancer recurrence than those who had a nighttime fasting duration of more than 13 hours.

The increased risk of recurrence was not associated with increased mortality from breast cancer or overall mortality, however longer periods of follow-up might reveal an association. The authors concluded that prolonging the length of nighttime fasting might be a simple non-medication method of reducing recurrence.

Prevention

As with treatment, studies looking at the role of intermittent fasting in cancer prevention are in their infancy. That said, research looking at the effect of time-restricted feeding on aging and cells suggests a possible benefit. Studies in the lab have found that liver cells exposed to time-restricted feeding are less likely to develop precancerous changes. Certainly there is a large jump from the response of cells in a dish in the lab to humans, but this finding warrants further research.

Mechanisms, Rationale, Actions, and Effects

Until we have studies in humans demonstrating a benefit (or lack thereof) of intermittent fasting, it's important to look at how intermittent fasting may affect cancer, or the potential mechanisms. A number have been proposed that may support the role of time-restricted eating/prolonged fasting in either cancer prevention or treatment.

Decreased Inflammation

There are many studies that have suggested a role of inflammation in both the development of cancer, and the progression and spread of a cancer already present. It's well known that inflammatory markers in the blood are associated with a poor prognosis of cancer, but chronic inflammation can also hinder the treatments for cancer.

A 2019 found that intermittent fasting could reduce inflammation. In the study, both the number of monocytes and inflammatory activity were decreased after a short term fast.

Improved Insulin Sensitivity

Intermittent fasting has become popular as a method for improving sensitivity and reducing blood sugar. In turn, studies have found that with some cancers, such as breast cancer, the presence of diabetes is associated with a poorer prognosis.

Cellular Adaptation and Repair

Evidence to support the theory mentioned earlier in which cancer cells are less able to adapt and survive to environmental stressors has been seen with fasting. Whether the same is true with intermittent fasting (or "fasting-mimicking diets") isn't entirely known, but again, the theory behind this is promising.

During fasting, normal cells in the body undergo a process of repair (the cells equivalent to sleep). One of the processes is autophagy, a term that refers to a process in which cells get rid of old proteins that have built up inside of the cell (like house cleaning). With time-restricted eating, it's possible that normal cells in the body may be better able to tolerate cancer treatments, while cancer cells (which are abnormal in many ways and are not good house cleaners) would be as susceptible or more to the toxic effects of treatment.

Food Choices

While intermittent fasting has nothing to do with food choices, many of the foods that are quickly grabbed late at night, or in a rush in the morning, are low quality. An indirect benefit of intermittent fasting might be the reduction of these processed and fast foods in the diet.

Side Effects, Risks, Contraindications

In general, intermittent fasting (at least prolonged nighttime fasting) appears to be tolerated relatively well, though any dietary changes such as this should be discussed carefully with your oncologist first.

Side Effects

In studies with people who have cancer thus far, only mild side effects have been seen and early on can include "brain fog," headaches, dizziness, nausea, and weakness. People may also experience hunger at first, especially if they are accustomed to late-night snacks and an early breakfast. That said, transient hunger pangs are probably more behavioral than physiological as it's been found that a higher eating frequency (eating smaller, more frequent meals) does not reduce the feeling of hunger.

Medications

If you are on medications, it's important to talk with your doctor and pharmacist. Some foods are absorbed better with food whereas others are absorbed better on an empty stomach. Some vitamins, such as vitamin D require some fat intake in order to be absorbed well, and should be taken during the periods of eating rather than fasting. For those taking medications, it may be helpful to talk to your pharmacist as well as your doctor before changing your eating routine.

Weight Concerns

A major concern among physicians has been weight loss, as intermittent fasting is now being touted for just that. The fear of cancer cachexia, a syndrome of unintentional weight loss accompanied by muscle loss, should be discussed with your oncologist. Cancer cachexia is thought to be the direct cause of roughly 20% of cancer deaths, but the syndrome includes much more than weight loss, and may be present before any weight loss occurs.

Intermittent fasting has been found to result in less loss of lean muscle than calorie restriction.

A different concern has been that fasting can lower metabolic rate, but in contrast to conventional fasting, intermittent fasting may even increase baseline metabolic rate a small amount.

A different concern that has been raised is that intermittent fasting could create a fixation on food. This is probably of less concern for people who have cancer, but time-restricted eating may not be a good option for those who have any history of eating disorders.

Contraindications

Time-restricted eating should not be adopted by those who are pregnant or breastfeeding. It's also not recommended for women who are trying to become pregnant as there may be a risk of infertility. Certainly children and teenagers who are actively growing should not restrict their eating times.

Diabetes: Intermittent fasting could be dangerous for people with diabetes, especially type I, although in some situations an endocrinologist may actually recommend time-restricted eating for weight loss purposes. The effect of intermittent fasting may also differ between men and women, and requires careful monitoring by a physician if tried at all.

Intermittent fasting may not be recommended for people on certain medications.

A Word From Verywell

The science looking at intermittent fasting is relatively new, and despite mechanisms that suggest that it might increase the effectiveness of treatment while reducing side effects, its potential role in people with cancer needs further study.

On the other hand, as a method for reducing risk, especially in those who are overweight, the evidence is stronger. Obesity is currently running head to head with tobacco as the leading preventable form of cancer, and obesity-related cancers are increasing, especially among young adults.

It's also important to note that intermittent fasting says nothing about food choices, and a well-rounded diet rich in fruits and vegetables and minimizing processed food is important for everyone, whether living with cancer or not.