For breast cancer survivors, the risk of recurrence may be tied to how many hours they fast at night, a new study suggests.
Women in the study were more likely to have their breast cancer come back if they fasted overnight for less than 13 hours, researchers found.
While prolonged fasting is likely harmless, the study's lead author cautions that more research is needed before women start fasting to prevent their cancer from returning.
While a healthy diet is thought to improve breast cancer outcomes, researchers have mostly focused on what people eat instead of when they eat.
In mice that are fed high-fat diets, long overnight fasting - about 16 hours - has been found to protect against blood sugar problems, inflammation and weight gain. All those conditions are tied to poor cancer outcomes.
To see if the length of overnight fasting is linked with breast cancer recurrence or death, the researchers tracked 2,413 participants in the Women's Health Eating and Living study.
None of the women had diabetes, but they had all been diagnosed with breast cancer between ages 24 and 70.
During roughly seven years of follow-up, 390 of the women developed a recurrence of their breast cancer.
Overall, 818 women reported fasting overnight for at least 13 hours. The other 1,595 fasted for shorter periods at night - and this group had a 36 percent greater risk of breast cancer recurrence, compared to women who held off on eating for at least 13 hours a night.
Overnight fasting could alter breast cancer risk by influencing sleep and insulin levels
The new study can't say why overnight fasts might influence breast cancer risk, but the researchers found that with every additional two hours of fasting, women's average blood sugar went down and their hours of sleep were increased.
It’s possible that sleep and insulin levels affect breast cancer risk, the researchers speculated.
The new study was limited by the use of food questionnaires that were only collected at three points in time, and the participants' diets may not have been consistent throughout the study.
Experts that more evidence is needed before making a widespread recommendation to women.
They say there is no harm in trying to fast longer at night in order to see if you sleep better and feel better in terms of metabolic health, but it’s too early to know if it will help with breast cancer recurrence risk.
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