Women who followed a lower-fat diet rich in fruits, vegetables and grains had a lower risk of dying from breast cancer than those on a higher-fat diet, according to results from a major new study released Wednesday.

The conclusions, from the latest analysis of the federally funded Women’s Health Initiative, provide the first randomized clinical trial evidence that diet can reduce postmenopausal women’s risk of dying from breast cancer, the researchers said. Past observational studies – which don’t measure cause and effect – have had inconsistent findings.

The results “are exciting and empowering for the patient,” said Elisa Port, chief of breast surgery at Mount Sinai Health System, who was not involved in the study. “This is a wake-up call for women – there’s something they can do rather than just waiting for the shoe to drop.”

The trial involved more than 48,000 women who did not have breast cancer when they enrolled in the study conducted at 40 centers across the United States. From 1993 to 1998, the women were randomly assigned either to follow their usual diet, in which fat accounted for 32 percent of daily calories on average, or to try to reduce fat intake to 20 percent of calories while consuming daily servings of vegetables, fruit and grains.

The dietary-intervention group fell short of the goal; they managed to reduce their fat consumption to about 24.5 percent, and then “drifted up to about 29 percent,” according to lead study author Rowan Chlebowski of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center. Members of the group lost 3 percent of their body weight on average. Still, the women in that group who developed breast cancer had a lower risk of death than the women who followed their regular diets and developed the disease.

Chlebowski said the study showed that women could improve their health by making modest changes in what and how much they eat. “This is dietary moderation, it’s not like eating twigs and branches,” he said. “It’s what people were eating, say, 20 years ago, before you could pick up 900 calories in one candy bar.”

The dietary intervention lasted for 8.5 years and included several sessions with nutritionists. The latest analysis represents a follow-up of almost 20 years.

Breast-cancer experts generally praised the study but expressed some reservations. For one thing, the study was designed not to determine whether a low-fat diet provided a mortality benefit but whether such a diet could reduce the risk of developing breast cancer in the first place.

Previously released data showed the diet did not result in that in any statistically significant way. In addition, the breast-cancer experts noted, the mortality benefit took almost 20 years to emerge. Some also said it was not clear which dietary component was responsible for the benefit – the reduced fat or the additional fruits, vegetables and grains?

The study authors said the dietary-modification group used a diet similar to one called DASH – for Dietary Approaches to Stop Hypertension – which is designed to prevent or treat high blood pressure.

The new study “adds more evidence on the impact of diet but I wouldn’t rely on it to recommend a specific diet to a patient,” given that people react differently to different diets, depending on their biology, said Neil Iyengar, a medical oncologist at Memorial Sloan Kettering Cancer Center. “I tell patients if they eat more plant-based food, less red meat, decrease alcohol and maintain a healthy weight, they might have a reduced risk of breast cancer recurrence or death.”

The study didn’t look at the impact of diet on risk of recurrence of breast cancer. A separate study is looking at whether weight loss, achieved through cutting back calories and increasing physical activity, leads to a reduction in the risk of recurrence. The study, called the Breast Cancer Weight Loss Study, or BWEL, is being led by Dana-Farber Cancer Institute.

The study comes as more evidence is accumulating about the link between being overweight or obese and a number of cancers. Being obese and overweight – long implicated in heart disease and diabetes – has been associated in recent years with an increased risk of getting at least 13 types of cancer, including stomach, pancreatic, colorectal and liver malignancies, as well as postmenopausal breast cancer.

The study will be presented in coming weeks at the annual meeting of the American Society of Clinical Oncology in Chicago.