A diet with periods of calorie restriction, called a fasting-mimicking diet, may make hormone-sensitive breast cancers more responsive to hormone treatments, according to research from the University of Southern California (USC) and Italy’s IFOM Cancer Institute and University of Genoa.

Background: About eight out of 10 breast cancers have female hormone (estrogen or progesterone) receptors on their cells that can drive breast cancer growth. Drugs such as tamoxifen and fulvestrant block female hormones from binding to cancer cells and are an integral part of breast cancer treatment. But these therapies can become less effective as the body develops a resistance to them over time.

Recent finding: Researchers initially worked with mice and found that a fasting-mimicking diet reduces blood levels of binding hormones that increase cancer growth. These binding hormones include insulin, insulin-like growth factor 1 (ILGF1) and leptin. The diet also enhances the effectiveness of tamoxifen and fulvestrant and delays resistance to these drugs. The researchers further described a long-lasting tumor regression in two mouse studies.

In these mouse studies, another benefit was that the diet reduced a known and limiting side effect of tamoxifen called endometrial hyperplasia, which is a thickening of the inside lining of the uterus. Endometrial hyperplasia is hard to treat, may increase the risk of endometrial cancer and limits the use of tamoxifen. This benefit from the diet could extend the use of tamoxifen if it holds up in human trials.

As a follow-up to the mouse studies, a small trial—known as a feasibility study—was conducted in women with hormone-receptor-positive breast cancers receiving hormone therapy. A feasibility study indicates whether further research on humans is warranted. When the women started a fasting-mimicking diet, they experienced reductions in insulin, leptin and IGF1 that paralleled the mouse studies. The results of both the mouse and human studies are published in the journal Nature.

Conclusion: The researchers would like to power up future studies to include about 300 to 400 women. Researchers at USC are currently conducting breast cancer trials that include fasting-mimicking diets. If the results hold up, adding this diet to other therapies would be a safe and non-toxic boost to breast cancer treatment.

Source: Study titled “Fasting-Mimicking Diet and Hormone Therapy Induce Breast Cancer Regression,” by researchers at University of Southern California, IFOM Cancer Institute and University of Genoa, published in Nature.