"Given that most of the risk factors for breast cancer are not easily modifiable, these findings have potential public health implications in preventing breast cancer and should be evaluated further," wrote lead author Eunyoung Cho in the Archives of Internal Medicine (Vol. 166, pp. 2253-2259).
The research looked at the effects of red meat intake on the incidence of breast cancer among the 90,659 pre-menopausal women enrolled in the Nurses' Health Study.
Over one million women worldwide are diagnosed with breast cancer every year, with the highest incidences in the US and the Netherlands. The National Cancer Institute estimates that 13 percent of American women will develop breast cancer during their lives.
Additionally, the incidence of hormone receptor-positive breast cancer is said to be increasing in the US, particularly among women in the 40 to 69 year old bracket. Hormone receptor-negative breast cancer incidence has not been changing.
"Breast tumors are often characterized by hormone (estrogen and progesterone) receptor status," explained Cho.
"Although the incidence rates of hormone receptor-negative tumors have remained relatively constant, the incidence of hormone receptor-positive tumors has been increasing in the United States, especially among middle-aged."
A hormone receptor-positive tumor indicates that the female hormones estrogen or progesterone actually fuel the cancer growth.
The Harvard researchers assessed red meat intake by food frequency questionnaires in 1991, 1995 and 1999. The participants were then followed until 2003.
After 12 years of follow-up Cho and colleagues documented 1021 cases of invasive breast cancer, of which 512 were estrogen and progesterone-receptor positive, 167 were estrogen and progesterone-receptor negative. The researchers had no data on the other 232 cases.
The highest intake of red meat was not significantly associated with the risk for breast cancer overall or for hormone receptor-negative cancers, but an associated between increased risk for hormone receptor-positive cancer and red meat consumption was observed.
The researchers report that women who ate more than one and one-half servings of red meat per day had almost double the risk of hormone receptor-positive breast cancer compared with those who ate three or fewer servings per week.
The associations remained similar when the researchers calculated red meat intake in grams instead of servings, and also when they split the women into five groups based on how much red meat they ate.
"In this prospective study of pre-menopausal women, we found that red meat intake was strongly associated with an elevated risk of hormone receptor-positive breast cancer, but not hormone receptor-negative cancer," said the researchers.
Cho and co-workers propose several biological mechanisms that could be behind the observations. The first focused on a range of carcinogenic compounds including heterocyclic amines, N-nitroso-compounds, and polycyclic aromatic hydrocarbons, which are found in cooked and processed meats.
Dr Sarah Rawlings, head of policy and information at the British-based charity, Breakthrough Breast Cancer, said: "This study relied on women accurately recalling their diet over the past year and was carried out in the US where animals receive growth hormones not permitted in the EU."
Indeed, the Harvard researchers picked up on this point and noted that hormone residues in beef "theoretically… may preferentially affect hormone receptor-positive tumors."
Another potential cause may be the heme-iron found in red meat, said the researchers, which has been reported to boost estrogen-induced cancers in animal models.
Dr. Rawlings said that more research was needed before any conclusions can be drawn regarding red meat consumption and breast cancer.
"Very little is known about diet and breast cancer risk because we eat a variety of foods and separating out the effect of an individual food is difficult. Previous studies looking at red meat and breast cancer have been inconclusive," she said.
Henry Scowcroft, science information officer at Cancer Research UK, said: "According to this study, a woman would need to eat more than one-and-a-half portions of meat a day, every day, to significantlyincrease her risk of hormone-sensitive breast cancer before the menopause.
"But the overall risk of pre-menopausal breast cancer is low when compared to getting the disease after the menopause. So even at the highest rates of meat consumption this is overall still a relatively small increase."
Randall Huffman, vice president for scientific affairs at the American Meat Institute, is quoted in the Washington Post saying: "[Research into] diet and health is known for its fluid and often contradictory conclusions. This study is a perfect example of that.
"The wisest course of action in the wake of one more contradictory study is to consume the balanced diet recommended by the U.S. Dietary Guidelines," said Huffman.