Eating large amounts of red meat may double young women's breast cancer risk, a study suggests.
Eating too much meat has been linked to bowel cancer
US researchers writing in Archives of Internal Medicine looked at over 90,000 pre-menopausal women.
Having one-and-a-half servings of red meat per day almost doubled the risk of hormone receptor-positive breast cancer compared to three or fewer per week.
UK cancer experts said animals in the UK were not given growth hormones which are used for US animals.
Researchers from Brigham and Women's Hospital and Harvard Medical School in Boston studied women who were part of the Nurses' Health Study II from 1989 to 2003.
The women filled out questionnaires in 1991, 1995 and 1999, on which they recorded how often they regularly consumed more than 130 different foods and beverages.
A portion of meat was defined as beef, pork or lamb as a main dish, in a sandwich or eating a hamburger, bacon, hot-dogs or other processed meat as part of a meal.
Every two years, they reported whether or not they had developed breast cancer. Cases were confirmed through hospital records and pathology reports.
The researchers also looked at the whether the women's tumours were fuelled by the hormones oestrogen or progesterone.
By the end of the study, 1,021 women had developed breast cancer.
There were 512 cases of cancers which were oestrogen and progesterone-receptor positive, 167 that were negative, 110 with mixed status and 232 with unknown status.
Public health implications
When the researchers looked at meat intake and cancer risk, it was found that women who ate more than one-and-a-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.
Writing in Archives of Internal Medicine, the researchers led by Dr Eunyoung Cho, said: "Several biological mechanisms may explain the positive association between red meat intake and hormone receptor-positive breast cancer risk.
They say cooked and processed red meats have been shown to contain cancer-causing chemicals such as heterocyclic amines which are created during the cooking of red meat.
A second potential link is the growth hormones which are given to cattle in the US, although not in Europe.
The researchers also say red meat is a source of heme iron, which previous research has shown fuels the growth of oestrogen-induced tumours.
Dr Cho's team added: "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."
Dr Sarah Rawlings, of the Breakthrough Breast Cancer charity, said: "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."
She added: "Previous studies looking at red meat and breast cancer have been inconclusive.
"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.
And Maria Leadbeater, nurse specialist at Breast Cancer Care, added: "To date we are still a long way off fully determining the many and complex root causes of this disease and it is an area for further research.
"Further studies will need to be done to fully establish the exact nature of any link between a diet high in red meat and breast cancer.
"The benefits of eating a healthy and varied diet are well established and the biggest risk factors for breast cancer remain gender and increasing age."