Eating large amounts of saturated fat or red meat may increase the risk of breast cancer, a study suggests.
Scientists warned against eating large amounts of red meat
Previous studies have also indicated a link. However, scientists have been unsure as to whether other factors, such as being obese, play a role.
This latest study, published in the British Journal of Cancer, suggests eating large amounts of these foods can by themselves increase the risk.
Experts said the findings highlighted the importance of healthy eating.
Dr Norman Boyd and colleagues at the Ontario Cancer Institute in Canada analysed data from all of the published literature on dietary fat and breast cancer.
The looked at 45 separate studies, involving 580,000 healthy women and 25,000 breast cancer patients.
They analysed the data from these studies, taking into account other known and suspected risk factors for breast cancer.
They then compared women with the highest and lowest fat intake, in order to assess whether or not dietary fat was contributing to the disease.
There are two types of fats - saturated and unsaturated fats. Saturated fats are sometimes referred to as "bad" fats. They are found in animal products such as butter,
cheese, whole milk, ice cream, cream, and fatty meats
There are two types of unsaturated fats - monounsaturated and polyunsaturated.
These are sometimes referred to as "good" fats. They are found in olive oil and nuts.
The researchers found that women who ate high amounts of saturated fat were on average around 20% more likely to develop breast cancer than women who ate low amounts.
By comparison, women who ate large amounts of monounsaturated fats were around 10% more likely to develop the disease. This figure is not deemed significant in scientific terms.
Overall, women who ate large amounts of fats were 13% more likely to develop breast cancer, a small but significant figure.
The study also found that women who at large amounts of meat were 17% more likely to develop breast cancer than those who ate little or none.
The researchers said their figures may actually underestimate the true risk.
"We already know that being obese can increase the risk of a range of cancers, but evidence is building that eating large amounts of fat, particularly the saturated kind, can independently increase risk," said Dr Boyd.
"Our analysis of all the available research suggests there is indeed an association with saturated fat and breast cancer.
"The increase in risk seems fairly modest, even among very high consumers of fat, although the difficulties in measuring dietary intake mean we could be under-estimating the true scale of the effect.
"In any case, the effect seems to be over and above the increase in risk from obesity and underlines the message that high consumption of fat is bad for your health."
Dr Lesley Walker, director of cancer information at Cancer Research UK, welcomed the study.
"It's been very difficult to separate out the effects of dietary fat and obesity and previous studies have been inconclusive.
"But by combining data from a wide range of studies using different methods and including a very large number of people, this research provides the strongest indication yet that dietary fat has an independent effect on the risk of breast cancer.
"Tying down the various dietary contributors to cancer is important, as it will allow us to give the best possible advice about how to avoid cancer.
"The effect of dietary fat looks quite small, but the results add weight to the importance of a healthy, balanced diet, low in saturated fat and containing plenty of fruit and vegetables."