Consumption of processed meat may increase the risk of breast cancer in post-menopausal women, a large-scale study by researchers at the University of Glasgow suggests. Researchers did not find a link between processed meat consumption and pre-menopausal breast cancer risk, or consumption of red meat and breast cancer risk.

The study, “Red and processed meat consumption and breast cancer: UK Biobank cohort study and meta-analysis,” was published in the European Journal of Cancer.

Processed meat is made by smoking, curing, or salting with chemical preservatives or by using other processes to enhance flavor. The World Health Organization has warned that consuming processed meat can increase the risk of cancer, particularly colorectal cancer.

One reason processed meats may promote cancer is that nitrates or nitrites are added to enhance flavor and color. These molecules can lead to the formation of potentially carcinogenic chemicals, such as N-nitroso compounds.

While several studies have linked processed meat consumption with multiple cancers of the digestive system, no consensus had existed on whether it also increases the risk of breast cancer.

The team of researchers analyzed data from the UK Biobank, a long-term, comprehensive study that follows the health of 500,000 volunteers. For an average of seven years, they followed 262,195 women in the U.K., ages 40-69, with no previous record of breast cancer. The analysis included whether or not participants developed breast cancer during the study, along with information regarding socio-demographic status, lifestyle factors, and eating habits.

Of the participants, 4,819 (1.8%) developed breast cancer. These women were older, less physically active, more likely to be former smokers, had higher body mass indices, and reported higher intake of red and processed meat.

The team found that processed meat consumption was associated with a significant increase in the risk of breast cancer, specifically when women reached post-menopausal age. The risk was greater as the amount of consumption increased. For example, among women who consumed less than 4 grams of processed meat per day, the risk of breast cancer increased by 15% compared to those who did not consume processed meat. For the group that consumed more than 9 grams per day, the risk increased by 21%.

The team then performed a meta-analysis that incorporated results from their study with 10 other studies, combining data from 40,257 breast cancer cases in over 1.6 million women. The meta-analysis showed that processed meat consumption significantly increased the risk of breast cancer in women by 6% overall. The risk for post-menopausal women was 9%, leading researchers to believe that post-menopausal risk likely is responsible for the overall risk percentage.

Researchers also pointed out that consumption of processed meat occurred throughout the women’s lives, not only during the post-menopausal period. However, detailed information regarding consumption patterns was not available.

For red meat, researchers found that consumption “was not associated with an overall risk of breast cancer either in UK Biobank or the meta-analysis.”