Your everyday diet may have a bigger impact on your risk of developing breast cancer, new data shows. According to a new, long-term study stemming from the federally funded Women’s Health Initiative, a low-fat diet high in fruits, vegetables, and whole grains may reduce the risk of dying from breast cancer.

Researchers followed 48,835 post-menopausal women over the course of 20 years. Participants didn’t have breast cancer when they first enrolled in the study.

One group of participants adopted a lower fat diet with daily servings of fruit, vegetables, and whole grains. They also cut their fat intake to 25% of total calories.

The other group of participants (the control group) continued their normal diet. According to the American Society of Clinical Oncology, fat accounted for approximately a third of total calories (32%) of the control group’s diet.

Compared to the 23,800 adults diagnosed with brain and spinal cord cancer every year, approximately 22 out of every 10,000 women between the ages of 50 and 54 will be diagnosed with breast cancer in the next year.

During the study, women from both groups were diagnosed with breast cancer. But women who had changed their diets had a 21% lower risk of dying from the disease.

“This is significant because this is the first intervention study targeting breast cancer where a reduction in deaths from breast cancer has been seen,” said Dr. Rowan Chlebowski, the study’s lead author.

However, what researchers don’t know if it was the fat reduction in the participants’ diet that reduced their risk of dying from breast cancer or if it was their increase in fruits, vegetables, and whole grains.

Medical professionals and nutritionists today suggest eating healthy fats from sources such as fish, avocados, olive oil, and other plant-based oils. In fact, up to 67% of millennial consumers say they love ordering healthy food options at restaurants, and as of 2016, 68% of American consumers purchased organic food at least once in the past month, which is how Generation Y became the “avocado toast” generation.

But during the early 1990s when the study first began, the U.S. was at the height of the low-fat era when fat was seen as something we shouldn’t be eating.

“There’s been a lot that we’ve learned in the time since the study started about healthy fats and unhealthy fats,” said Jennifer Ligibel, a medical oncologist at the Dana-Farber Cancer Institute.

That said, researchers aren’t sure whether it was the overall reduction in fat that helped to protect patients from breast cancer-related death or if the benefit came from eating more fruits, vegetables, and whole grains with healthy micronutrients and fiber.

“It is very difficult to disentangle the individual components,” said JoAnn Manson, co-author of the study. “The trial was designed to test reduction in total fat because the evidence at that time was that reducing total fat could lower risk.”

“Now, there’s much more evidence that, especially for preventing cardiovascular disease, the type of fat really matters,” said Mason.

The role of dietary fat has been less clear when it comes to preventing cancer, Mason added. Fat content alone doesn’t determine whether a food is healthy or unhealthy.

Regardless, researchers suggest a diet shift toward more fruits, vegetables, whole grains, and healthy fats not only to reduce the risk of breast cancer but also other diseases such as cardiovascular disease, diabetes, and stroke.