For more than three decades, experts have debated whether eating a high-fat diet increases breast cancer risk.
Now, new research suggests it might boost the chances of three common types of breast cancer.
In a large European study evaluating more than 337 000 women in 10 countries over 11 years, researchers found that women who ate the most saturated fat were about 30% more likely to develop breast cancer than those who ate the least.
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While the study showed an association between a high-fat diet and breast cancer risk, it didn't prove cause-and-effect.
"The results of this large study provide support for the hypothesis that breast cancer development is related to dietary fat," said study author Sabina Sieri, a researcher from the department of preventive & predictive medicine at the Fondazione IRCCS Istituto Naziolale dei Tumori in Milan.
Breast cancer subtypes
The study was published in the Journal of the National Cancer Institute.
Sieri and her team found that a high intake of total fat and saturated fat was linked to an increased risk of breast cancer subtypes known as oestrogen receptor-positive (ER-positive) and progesterone receptor-positive (PR-positive). High-fat diets also were tied to a raised risk of developing HER2-negative breast cancer, the investigators noted.
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Cancers that are ER- or PR-positive grow in response to those hormones. Cancers that are HER2-negative have tested negative for the presence of a protein, HER2, which promotes cancer cell growth.
One strength of the new study is its large numbers, said Mia Gaudet, director of genetic epidemiology at the American Cancer Society. The breast cancer subtypes linked with fat intake are common, she said. "The majority of breast cancers in the US and Europe are ER-positive, PR-positive, HER2-negative," she noted.
In the new study, women answered questions about their fat intake and other habits that could affect risk, including smoking, age at first period, pregnancy history, use of hormone therapy and their body mass index (a measurement of body fat based on height and weight).
To correct for measurement errors in the diet questionnaire, the researchers interviewed a random sample of 8% of the women, asking for a 24-hour dietary recall.
After an average follow up of 11.5 years, just over 10 000 women were diagnosed with breast cancer.
Saturated fat association
When the cases of cancer were compared with fat intake, "we found an association between exposure to saturated-fat intake and breast cancer," Sieri said.
Women in the group eating the most saturated fat averaged 48 g a day, compared to 15 g in the group eating the least. A quarter cup of shredded cheddar cheese, for instance, has 10 g of fat, 5 g of it saturated.
The link can't be explained with certainty, Sieri said, but it's possible that the high-fat intake raises the levels of the body's own oestrogen, which can stimulate the growth of breast cancer cells.
"I think it's important these findings be replicated in other studies," Gaudet said. While the researchers ruled out many factors that could explain an increased cancer risk, they didn't rule out all possible factors, she said.
Until more study is done, Sieri tells women: "In general, it is recommended that the percent of total calories from saturated fat should not exceed 10% for women."
Gaudet agreed. "If you have a mainly plant-based diet, that is going to help you keep your fat intake low."
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