Conjugated linoleic acid (CLA) is anticarcinogenic in experimental animal studies. We studied dietary and serum CLA in Finnish patients with
breast cancer in 1992-1995. Participants were consecutive women with
breast cancer (68 premenopausal and 127 postmenopausal) and population-based control women (75 premenopausal and 133 postmenopausal), matched for age and area of residence. Diet was assessed by a validated food frequency questionnaire and the
fatty acid composition of serum by gas-liquid chromatography. In postmenopausal women, dietary CLA, serum CLA,
myristic acid, and
trans-vaccenic acid were significantly lower in cases than in controls. The odds ratio for
breast cancer in the highest quintile vs. the lowest was 0.4 [95% confidence interval (CI) = 0.2-0.9]for CLA, 0.3 (95% CI = 0.1-0.7) for
myristic acid, and 0.3 (95% CI = 0.1-0.7) for
trans-vaccenic acid in serum. The odds ratios remained similar after adjustment for known risk factors of
breast cancer. A diet composed of CLA-rich foods, particularly cheese, may protect against
breast cancer in postmenopausal women, but it is impossible to assess the independent effects of CLA in this study. The findings may be of relevance for food production, inasmuch as it is possible to increase CLA and its precursor
trans-vaccenic acid in foods by modifying the feeding of ruminants.