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The low-fat diet seemed particularly helpful for early stage breast cancer patients with so-called estrogen receptor-negative (ER-negative) disease. These women had a 36 percent reduced risk of death from any cause over 15 years if they ate a low-fat diet for five years following their diagnosis, said study researcher Dr. Rowan Chlebowski. He is a medical oncologist at the Los Angeles Biomedical Research Institute at the Harbor-UCLA Medical Center.
Women who had both ER-negative and progesterone-receptor negative (PR-negative) cancers had an even greater reduction in death risk during the study. Over 15 years, their risk of dying from any cause was reduced by 56 percent if they ate a low-fat diet during the five years after diagnosis, he found.
Women in the study reduced their dietary fat intake from an average of about 29 percent to 20 percent for the low-fat diet.
Chlebowski is scheduled to present his findings on Friday at a breast cancer meeting in San Antonio. Studies presented at medical meetings are viewed as preliminary until published in a peer-reviewed journal.
The study included more than 2,400 women aged 48 to 79. While nearly 1,600 had ER-positive cancers, the other 800 women had either ER-negative or ER- and PR-negative disease. They all had early stage breast cancer and received treatment between 1994 and 2001, according to the study.
About half of the women were assigned to follow low-fat diets. These women were given nutrition counseling and asked to keep track of their fat intake. The researchers made unannounced calls annually to request 24-hour fat-intake records. The "control" group was not given any counseling or advised to follow a low-fat diet.
Overall, the women in the low-fat group reduced their fat intake by almost 10 percent of total calories. They also lost an average of six pounds, the study authors noted.
When Chlebowski looked at all the women in the low-fat group, regardless of type of cancer, and compared them to those who didn't lower fat intake, he found that the death rate was somewhat lower in the low-fat group -- 14 percent versus 17 percent. That difference was not considered statistically significant, however.
It was only when he focused on the subgroup with hormone-unrelated cancers (ER-negative or ER- and PR-negative disease) that he found the more favorable effect.
The study looked at death from all causes, so the researchers can't say if the low-fat diet directly lowered breast cancer death, Chlebowski said.
Chlebowski can't explain why the low-fat diet helped, but it may be due to the diet and weight loss reducing inflammation, which can affect cancer growth. Chlebowski said it's also not clear from this study why the low-fat diet provided greater benefit to women with estrogen receptor-negative cancers than to those with estrogen receptor-positive cancers.
Commenting on the findings, Dr. David Heber, founding director of the UCLA Center for Human Nutrition, said: "It was the first study of its kind that got women over a long period of time to follow a low-fat diet to influence outcomes in breast cancer patients."
Based on this and other studies, Heber said his advice for all breast cancer patients is to reduce calories from fat to 20 percent or 25 percent of all calories, and to focus on healthy fats, such as omega-3s found in fish such as salmon and other foods.
Dr. Anees Chagpar, associate professor of surgery at the Yale School of Medicine, said the study findings suggest that reducing dietary fat is a relatively simple way to possibly reduce the risk of premature death in women with breast cancer.
"We're not talking about using some really expensive drug or really toxic therapy," said Chagpar, who's also director of The Breast Center at Smilow Cancer Hospital at Yale-New Haven. She reviewed the study findings.
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