Eating too much red meat in early adulthood may cause breast cancer, according to a Harvard University study.
The paper, published in the British Medical Journal, analysed data from more than 88,000 women aged 26 to 45 who had filled in surveys in 1991.
Their red meat intake varied from never or less than once a month, to six or more servings a day.
Red meat items included unprocessed red meat (beef, pork, or lamb) and processed red meat (such as hot dogs, bacon and sausage).
Among all the women who took part in the 1991 survey, 2,830 developed breast cancer during 20 years of follow-up.
"Higher intake of red meat was associated with a 22% increased risk of breast cancer overall. Each additional serving per day of red meat was associated with a 13% increase in risk of breast cancer (12% in premenopausal and 8% in postmenopausal women)," the study said.
"In contrast, estimates showed a lower risk of breast cancer in postmenopausal women with higher consumption of poultry.
"Substituting one serving per day of poultry for one serving per day of red meat, in the statistical model, was associated with a 17% lower risk of breast cancer overall and a 24% lower risk of postmenopausal breast cancer."
Scientists suspect proteins in red meat speed up cell division and tumour growth; chemicals such as nitrates in processed meats are already classified as probable carcinogens.
The authors concluded that higher red meat intake in early adulthood "may be a risk factor for breast cancer, and replacing red meat with a combination of legumes, poultry, nuts and fish may reduce the risk of breast cancer."
The study was carried out mainly among educated, white American women, and researchers said that the results were not necessarily applicable to women of other races.
"This underlines the importance of having a healthy diet," Sally Greenbrook, a senior policy officer at the UK charity Breakthrough Breast Cancer, who was not part of the research, said.
She said women should focus on reducing their chances of breast cancer by staying slim, exercising and drinking moderately.
Mia Gaudet, director of genetic epidemiology at the American Cancer Society, said people should eat a "plant-based" diet.
"It's important to have a healthy lifestyle throughout your life and not just as you get older and more worried about cancer," she said. "People should perhaps consider ordering a salad or a vegetarian option sometime."
Valerie Beral, a cancer expert at the University of Oxford, dismissed this Gaudet's claim, pointing out that vegetarians do not have a lower risk of breast cancer than meat-eaters.
"Vegetarians do not have lower risks of breast cancer than non-vegetarians, supporting other evidence that meat consumption is unlikely to play a major role in breast cancer," she said.