Trans fatty acids (trans fats) have been implicated in heart disease, type 2 diabetes, obesity and cancer. Trans fats are found in many commercially prepared and processed foods, such as French fries and doughnuts, baked goods (pastries, pie crusts, cookies, pizza dough, crackers), and margarine. Hydrogenation stabilizes oils and fats, preventing them from becoming rancid and keeping them solid at room temperature. Trans fats are produced during industrial hydrogenation, but trans fats are also naturally present in butterfat and meat from ruminants such as cattle, sheep and goats.

Trans fat and breast cancer risk

Few studies have examined the association between trans fat intake and risk of breast cancer. One retrospective Canadian study reported that trans fat increases the risk of premenopausal breast cancer. Premenopausal women with the highest levels of trans fat consumption (the highest fourth of intake) had 1.6 times the risk of premenopausal breast cancer as women in the lowest quartile of intake.

A study of women in the French arm of the European Prospective Investigation into Cancer and Nutrition measured circulating levels of fatty acids to assess fatty acid intake. A total of 363 breast cancer cases were matched to cancer-free controls. An increased risk of breast cancer was found to be associated with increasing levels of two common trans fats: palmitoleic acid and elaidic acid. Elaidic acid is the main trans fat produced during partial hydrogenation of vegetable oils. The authors concluded that a high serum levels of trans fats, presumably reflecting a high intake of industrially processed foods, is probably one factor contributing to increased risk of invasive breast cancer in women.

The new Norwegian prospective study referenced at the beginning of this news article and described more fully below reported no association between intake of trans fats from partially hydrogenated oils and risk of breast cancer. Trans fat from animal sources were associated with increased risk of postmenopausal breast cancer, but the association did not reach statistical significance. This association might reflect an increased risk for saturated fat rather than ruminant trans fat.

The French study appears to be the most compelling of the three since blood tests rather than food questionnaires were used to determine levels of exposure to trans fat.

Trans fat and survival after breast cancer diagnosis

Only one prospective study has investigated the influence of trans fat intake on breast cancer prognosis. The 2011 U.S. study included 4,441 women who were diagnosed with invasive breast cancer between 1987 and 1999. A food frequency questionnaire was used to assess diet. A total of 137 of the women died from breast cancer during the first seven years after enrollment in the study.

Women with the highest fifth of trans fat intake were found to have 1.8 times the risk of dying from any cause compared to lowest fifth. By comparison, women with the highest saturated fat consumption had 1.4 times the risk of dying as those in the lowest. Both types of fat increase cholesterol and risk of heart disease and stroke, which were major causes of death in addition to breast cancer. Results were similar for risk of dying specifically from breast cancer, however they did not reach statistical significance.

Latest research on trans fat and cancer risk

The study referenced at the beginning of this news article was designed to investigate the associations between dietary trans fat and risks of several types of cancer. Trans fats have been linked to increased inflammation, insulin resistance, and obesity, but whether intake influences cancer risk has not been determined. To conduct the study, the authors used data from the Norwegian Counties Study. The analysis included 77,568 men and women who were followed through 2007. Trans fat consumption was determined three times between 1974 and 1988 using a food questionnaire. Three sources of trans fat were included in the assessment: (1) partially hydrogenated vegetable oils; (2) partially hydrogenated fish oils (which are used to manufacture margarine in Scandinavia); and (3) ruminant fat (animal fat, which is a natural source of trans fat).

A total of 12,004 cancers were diagnosed during the study period. The authors compared the highest to the lowest trans fat intake levels in each analysis. Intake of partially hydrogenated vegetable oil was found to be associated with reduced risks of pancreatic cancer in men and non-Hodgkin lymphoma in both men and women. Partially hydrogenated fish oil was associated with increased risks of rectal cancer and multiple myeloma and reduced risk of prostate cancer. Consumption of trans fat from ruminant animal sources was associated with increased overall risk of cancer, and cancers of the mouth/pharynx and non-Hodgkin lymphoma in particular, but reduced risk of multiple myeloma. Positive trends that did not reach statistical significance were found for partially hydrogenated fish oils and stomach cancer, as well as trans fat from animal sources and postmenopausal breast cancer. Inverse trends were found for partially hydrogenated vegetable oils and overall risk of cancer, as well as cancer of the central nervous system in women. The authors comment that partially hydrogenated fish oils, but not partially hydrogenated vegetable oils, appear to increase cancer risk. The increased risks observed for trans fat from animal sources may be linked to saturated fat, according to the authors.