Intermittent Fasting and Breast Cancer: What You Need to Know

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photo shows MSK cancer expert Neil Iyengar, who cares for people with breast cancer and is a leading expert on the relationship between breast cancer, nutrition, obesity and exercise.

Neil Iyengar cares for people with breast cancer and is a leading expert on the relationship between breast cancer, nutrition, obesity, and exercise.

Intermittent fasting is one of the most talked-about topics among people looking to lose weight and improve their overall health. The basic idea is straightforward: Stop eating for specified periods of time with the goal of dropping unwanted pounds.  

Over the last few years, intermittent fasting has moved beyond diet gurus to serious consideration by cancer researchers. It is well established that the risk and treatment of some cancers — particularly breast cancer — are impacted by obesity, nutrition, and exercise. Could intermittent fasting be used as a tool against breast cancer?

Memorial Sloan Kettering’s Neil Iyengar cares for people with breast cancer and is a leading expert on the relationship between cancer, obesity, nutrition, and exercise. He answers some of the most common questions about intermittent fasting and breast cancer.

What is intermittent fasting, exactly?

There are many different versions of it. But the bottom line is that it’s a dietary pattern to put the body in a fasting state by not eating for a significant period of time. You aren’t told what kind of foods to eat or not to eat. Instead, it’s about timing.

Some versions have you fast on alternate days. Some versions call for fasting during waking hours, others incorporate sleeping hours into the fasting period. It requires a substantial period of not eating — going without food for a couple of hours won’t do it. 

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Why are cancer researchers interested in intermittent fasting?

Breast cancer is one of at least 13 cancers that are sensitive to obesity and the fat composition of the body. Fatty tissue promotes the development and growth of breast cancer. Therefore, weight-loss strategies that can help lower fat and promote a healthy balance between fat and muscle are of interest to breast cancer researchers.

Weight-loss strategies that can help lower fat and promote a healthy balance between fat and muscle are of interest to breast cancer researchers.

We are also learning a lot about the role of insulin and glucose in the growth of breast cancer. It’s well known that hormones like estrogen fuel the growth of cancer cells in nearly 80% of breast cancer cases. We’re learning that insulin has a lot of interplay with hormones like estrogen, and excess body fat can accelerate insulin production.

The rationale for investigating intermittent fasting in the context of breast cancer is to test whether this strategy will reduce fat and improve insulin levels, which in turn can help lower estrogen levels and slow the growth of breast tumors.

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How strong is the research on intermittent fasting and breast cancer?

This research is in an early phase. There is little research yet on intermittent fasting and cancer that involves humans — instead, it’s what we call pre-clinical research, meaning in mice and other models. The data from pre-clinical research is promising, but it will take rigorous testing to prove whether this will really help people with breast cancer. That research is underway and the data are starting to be reported. One recently published study showed no improvement in weight or fat loss with a specific intermittent fasting approach. Ongoing studies are testing other types of intermittent fasting strategies.  

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What looks most promising for intermittent fasting in terms of fighting breast cancer?

If intermittent fasting turns out to be a successful fat-loss strategy that improves metabolic health, we could potentially harness these effects to reduce the risk that breast cancer will come back after treatment. We know that metabolic disturbances, like too much insulin and blood sugar, increase the risk of breast cancer recurring. By reducing fat and insulin levels, intermittent fasting could help reduce that risk. Other dietary strategies may prove useful in this as well.

Have more questions?

Join a panel of MSK experts on October 14 as they discuss how breast cancer is affected by nutrition and exercise.

Successful fat-loss strategies may also help make treatments work better. For instance, we often give steroids with cancer treatments in order to reduce the side effects of chemotherapy. But the steroids also drive up insulin levels, which we don’t want. Diets that keep insulin in balance may help maintain the benefits of the steroids while reducing the drawbacks.

In terms of reducing the risk for developing breast cancer in the first place, it’s very difficult to design a research trial because there are so many factors that contribute to the development of breast cancer. A lot more long-term research would need to be done before we could make any firm conclusions.

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Are there any side effects to intermittent fasting?

If people get overly enthusiastic about intermittent fasting, they can starve themselves. Especially during breast cancer treatment, that’s bad — some treatments can’t be given if the patient is malnourished.

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What are the next steps for people interested in intermittent fasting?

First, talk to your doctor. If you are in treatment for breast cancer, stick with the proven treatments. We know, for instance, that hormone therapy reduces the risk of cancer coming back by 50% for the majority of breast cancer cases. We have to do more research to see if intermittent fasting, or any dietary approach, can definitely reduce the risk of breast cancer recurrence.

Ultimately, we hope that there will be several dietary strategies that prove to be successful for reducing fat mass, improving metabolic health, and reducing the risk of breast cancer or breast cancer recurrence in the long run. This would allow people to choose a diet with their oncologist that works well for them and that is compatible with their cancer treatment. For now, we know that it is important to maintain a healthy weight, including healthy body fat levels. More data are needed before we can make specific recommendations.

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Dr. Iyengar receives grant funding from the Breast Cancer Research Foundation, the American Cancer Society, and the National Cancer Institute. He has a clinical trial funded by Novartis and has received consulting fees from Seattle Genetics and Novartis.