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Can long periods of fasting protect against breast cancer recurrence?

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Previous research in mice suggested that long periods of fasting provide protection against factors that are associated with a poor cancer outcome. A new study associates prolonged fasting (13 hours or more) at night with a lower risk of breast cancer recurrence, but no association between fasting time at night and mortality. While these findings are interesting, more research needs to be done to confirm them. In the meantime, breast cancer survivors should discuss any concerns about nutrition with their health care providers. 05/30/16

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This study is about:

How nighttime fasting affects breast cancer recurrence and mortality. 

Why is this study important?

If longer fasting at night is linked to a better breast cancer prognosis, as the study authors write, “Prolonging the length of the nightly fasting interval may be a simple non-pharmacologic strategy for reducing the risk of breast cancer recurrence.”

Study findings: 

  1. Women who reported fasting fewer than 13 hours per night had an increased risk of breast cancer recurrence compared to women who reported fasting for more than 13 hours per night.

What does this mean for me?

Everyone has a period of nighttime fasting—from their last meal or late night snack until breakfast.  This study looked at the length of that interval, and suggests that late night eating, which shortens the length of the nighttime fast, is associated with breast cancer recurrence. But this study does not definitively conclude that nighttime eating results in breast cancer recurrence; only that there is an association. A large, controlled trial with patients who are randomly assigned to either the nighttime fasting group or a control group is needed to provide more definitive information and confirm this association. Additionally, no current research indicates that late night eating is associated with worse health outcomes. Based on the results of this study, lengthy nighttime fasting is not needed to prevent cancer recurrence.

Research on weight gain and late night eating has produced mixed results—researchers still do not know whether late night eating makes it easier to gain weight. However, dieticians recommend avoiding late night eating because they say it is easy to overdo eating at nighttime (eating out of boredom or stress instead of hunger, for example, and portions are not as well controlled). Ultimately, the results of this research should not stop women from eating a light snack at night when hungry.

Questions to ask your health care provider:

  • How do my eating habits affect my risk for breast cancer recurrence?
  • What foods, if any, are known to reduce risk of breast cancer recurrence?
  • How else can I reduce my risk for breast cancer recurrence?


Study background:

Previous work in mice that were fed a high-fat diet showed that long periods (16 hours) of fasting protected the mice from inflammation and weight gain, two factors that are associated with a poor cancer outcome. However, nothing is currently known about how long periods of fasting affect human breast cancer recurrence and mortality. Catherine Marinac and her colleagues from the University of California, San Diego and other institutions published a report in the journal JAMA Oncology looking at whether the length of time a person goes without eating at night could predict recurrence and mortality for women with early-stage (stages I-III) breast cancer.

Researchers of this study wanted to know:

Does fasting at night help prevent breast cancer recurrence and mortality?

Population(s) looked at in the study:

This research used data from 2,413 women who participated in the Women’s Healthy Eating and Living (WHEL) study. These women had early-stage breast cancer (stages I-III), did not have diabetes, and were between the ages of 27-70 years old. Their dietary assessment consisted of multiple phone calls in a three-week period when they were asked what they ate and when they ate. The women were questioned in the year they enrolled in the study (baseline), the next year, and then again two years later. All 2,413 women completed the baseline assessment, about 91% completed the assessment the next year, and about 80% completed the assessment the last year. 

Study findings:  

  1. Women who reported fasting for fewer than 13 hours per night had a 36% increased breast cancer recurrence risk compared to women who reported fasting for more than 13 hours per night. This increase is modest.
  2. No association was found between fasting time at night and mortality due to breast cancer.


This study used data taken from the Women’s Healthy Eating and Living (WHEL) study; researchers did not produce the questions asked from the dietary assessment, or control for other things they might have wanted to ask. Additionally, the women were asked to self-report their diets and times that they ate. Self-reporting is a definite study limitation, because, memory is not infallible—women may forget the exact time they ate—and is subject to many biases. The study also followed women for a four-year period, so whether this effect lasts over longer periods is unknown.  Finally, it is unclear whether shorter nighttime fast periods are responsible for the increase in recurrence or if other lifestyle factors that accompany shorter nighttime fasting, such as alcohol consumption, amount of sleep a person gets each night, or other lifestyle choices that leads to the increase in recurrence risk play a role.


While this study suggests that long periods of fasting at night are associated with protection against breast cancer recurrence, more work needs to be done to confirm this finding. A large clinical trial would also allow researchers to control for other factors, such as when the women sleep. Many previous studies have found an association in people who sleep at times that disrupt their natural circadian rhythm and an increased risk for breast cancer. Based on this study, women do not necessarily need to avoid eating a light snack at night if they are hungry.

Posted 5/31/16

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Marinac MR, Nelson SH, Breen CI, et al. “Prolonged Nightly Fasting and Breast Cancer Prognosis.” JAMA Oncology, Published online first on March 31, 2016.   

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