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Can Fasting 13 Hours or More at Night Reduce Recurrence Risk?

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A study suggests that waiting 13 or more hours to eat again after the evening meal may reduce the risk of early-stage breast cancer coming back (recurrence). Still, there are some questions about the study.

The research was published online on March 31, 2016 by JAMA Oncology. Read the abstract of “Prolonged Nightly Fasting and Breast Cancer Prognosis.”

Studies in mice have suggested that going longer without eating at night can improve breast cancer prognosis. So the researchers that did this study wanted to see if the same effect happened in people.

Diet information from 2,413 women diagnosed with early-stage breast cancer who were part of the Women’s Healthy Eating and Living Study was analyzed for this study. The women were 27 to 70 years old. None of the women had diabetes.

The Women's Healthy Eating and Living Study wanted to know whether eating a diet high in vegetables, fruit, and fiber and low in fat could:

  • reduce the risk of breast cancer recurrence
  • reduce the risk of being diagnosed with a new, primary breast cancer
  • improve survival

As part of the Women’s Healthy Eating and Living study, the women’s diets were assessed by four prescheduled phone calls asking the women what they ate over the last 24 hours. The calls were done:

  • at baseline, just before the study started
  • 1 year after the study started
  • 4 years after the study started
  • 6 years after the study started

The researchers who did this study analyzed these diet reports to estimate how long the women went without food overnight.

The researchers also looked to see how many breast cancer recurrences and diagnoses of new breast cancers there were during about 7 years of follow up. The researchers also looked to see how many women died from breast cancer or any other cause during about 11.5 years of surveillance.

Overall, half the women fasted for more than 12.5 hours per night and half the women fasted for less than 12.5 hours per night.

The researchers reported that fasting for fewer than 13 hours per night was associated with a 36% higher risk of breast cancer recurrence compared to fasting 13 or more hours per night. This difference was statistically significant, which means that it was likely due to the difference in fasting and not just because of chance.

Fasting for fewer than 13 hours per night was not associated with better survival for breast cancer or any other disease.

"Prolonging the overnight fasting interval may be a simple, non-pharmacological strategy for reducing a person's risk of breast cancer recurrence and even other cancers," said Catherine Marinac, lead author and doctoral candidate at the University of California, San Diego Moores Cancer Center. "Previous research has focused on what to eat for cancer prevention, but when we eat may also matter because it appears to affect metabolic health."

The researchers also found that fasting for fewer than 13 hours per night was associated with less sleep and higher levels of glycated hemoglobin, which is a measure of average blood sugar levels over several months. Higher glycated hemoglobin levels and getting less sleep have been linked to a higher risk of breast and other cancers.

You’ve probably heard about this study because it’s been covered by media outlets around the world and being talked about on social media. And while the results seem very promising, it’s important to know that the researchers relied on only four reports of what and when the women ate, all of which were self-reported.

The findings are dependent on the women accurately remember what they ate and when they ate it. Even misremembering the time breakfast was eaten by 30 minutes could affect the paper’s results. Also, many people are ashamed to admit they get up and eat in the middle of the night. This shame could mean that some of the women misreported what and when they ate.

It’s also important to know that this study wasn’t randomized. In a randomized study, half the women would have been assigned to fast for 13 or more hours per night and half the women would have been assigned to fast for less than 13 hours per night.

It’s also possible that the women who fasted for 13 or more hours per night had other healthy habits that reduced their risk of recurrence. Perhaps more of these women were eating a diet rich in nutritious, unprocessed foods and avoiding alcohol and cigarettes while exercising daily. We just don’t know.

We also don’t know how many women in the study were prescribed hormonal therapy for 5 or 10 years as part of their treatment plan, as well as how many women stuck to this treatment plan. This could dramatically affect how many women had a recurrence.

Because of all these questions, much more research is needed before we really know if fasting for more than 13 hours per night is linked to a lower risk of recurrence.

We do know that getting less than 6 hours of sleep per night can cause ongoing low-grade inflammation, which is linked to many types of cancer, including breast cancer, and heart disease.

So it makes sense to try to get between 7 and 9 hours of quality sleep per night. If you find yourself distracted by television or your mobile devices or worrying about your bills, your job, or your health whenever you try to sleep, here are some tips to help you turn in and tune out:

  • Only do two things in your bed: sleep and have sex. Don’t read, watch TV, check your email, or be witty on Facebook in bed. Do all that in another room.
  • Maintain a regular sleep/wake schedule. Try to go to bed and wake up at the same time every day.
  • Don’t nap if you have trouble falling asleep at night.
  • Avoid caffeine, alcohol, and heavy/spicy/sugary foods 4 to 6 hours before you go to bed.
  • Exercise regularly -- daily if you can -- but not right before bed. Exercising an hour before you go to bed may leave you too amped up to sleep.
  • Make your bedroom as comfortable and as dark as possible. Most people find a cool room best for sleeping. If your pillow has flattened out, you may want to buy a new one. Block out all extra light, including the light from your clock and any outside lights.

For more information on sleep, metabolism, and breast cancer, read the Sleep Tight, Slim Down Think Pink, Live Green column by Marisa Weiss, M.D.,’s chief medical officer.

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