Young women who eat a lot of meat, cheese and other foods rich in animal fat may be increasing their risk of breast cancer, a new study has found.

Researchers have long suspected a link between fatty diets and breast cancer, but previous studies couldn't establish one. This study, a large one involving more than 90,000 nurses, found no link between breast cancer and fats from plants, such as olive oil. But it did find that women who ate more red meat and high-fat dairy products showed a modestly increased risk of getting the disease.

"If you think about the risk factors for breast cancer, women don't have many choices to modify those risk factors. Diet is something you can change," said Eunyoung Cho, of Brigham and Women's Hospital in Boston, Mass., and Harvard Medical School.

The study is significant because it involved so many women, and because they were so young, aged 26 to 46 years.

Most previous studies involved postmenopausal women.

Breast cancer is more common after the age of 50, but it may take years to develop, and researchers have been trying to identify factors earlier in life that may play a role. This study suggests that what women eat in their 20s, 30s and 40s may be important.

Animal fat is only one piece of the breast cancer puzzle. Family history is important in a disease that can have a genetic component. Women who started their menstrual periods early have a higher risk, as do those who didn't get pregnant until later in life or never have children.

Breast-feeding may offer some protection. Another study found that women who breast-fed infants for at least 16 months have a reduced risk.

In this study, the women filled out a detailed questionnaire about their eating habits, first in 1991, then again in 1995.

The researchers divided them into five different groups, based on their intake of animal fat.

By 1999, 714 of the women had developed breast cancer; 134 in the high-fat group and 123 in the low-fat group. Dr. Cho said that even though this may seem like a small difference, it is significant.

"When we compared women in the high-fat-intake group with women in the lowest intake group, those with the highest intake had a 33 per cent greater risk of breast cancer," she said in an interview.

She described this as a moderate increase in risk, but said her study indicates it might be wise for young women to decrease the animal fat in their diets. There is another good reason for taking this step, she said.

Reducing the animal fat you eat also reduces the risk of heart disease.

The women in the study who were most at risk obtained about 23 per cent of their total calories from fat, compared to 12 per cent in the lower risk group.

It is not clear why a diet high in animal fat would increase the risk of cancer, Dr. Cho said. In fact, it may be that other substances found in foods containing animal fat are the culprits, not the fat itself.