Prospective study of plasma fatty acids and risk of prostate cancer.

BACKGROUND: Although some evidence suggests that dietary fat intake is related to prostate cancer, epidemiologic studies have been inconsistent.
PURPOSE: Our purpose was to assess the association between plasma lipid levels, particularly linoleic and alpha-linolenic acids, and the development of prostate cancer.
METHODS: In 1982, at the start of the Physicians' Health Study, 14916 U.S. male physicians provided plasma samples, which were frozen at -82 degrees C. Data accumulated from a series of questionnaires were used to assess the intake of various foods. We used a nested case-control design to compare the fatty acid compositions in plasma from 120 men who later developed prostate cancer with 120 matched controls who did not. Individual fatty acids were measured in plasma as a percentage of total fatty acids, using capillary gas chromatography. Conditional logistic regression models were used to obtain odds ratio estimates while adjusting simultaneously for the effects of one or more potential confounders.
RESULTS: The relative risks (RRs) of prostate cancer for men in successively higher quartiles of plasma alpha-linolenic acid level were 3.0 (95% confidence interval [CI] = 1.2-7.3), 3.4 (95% CI = 1.6-7.5), and 2.1 (95% CI = 0.9-4.9), compared with those with levels below the detection threshold (P trend = .03). For linoleic acid, RRs in successively higher quartiles were 0.7 (95% CI = 0.4-1.5), 0.8 (95% CI = 0.4-1.6), and 0.6 (95% CI = 0.3-1.3), with the lowest quartile as referent (P trend = .24). The effect estimates were not notably altered by adjustment for exercise, body mass, meat and dairy consumption, or other fatty acid levels in the plasma. The RR for eating red meat at least five times per week compared with less than once a week was 2.5 (95% CI = 0.9-6.7) and was little changed by adjustment for alpha-linolenic acid, although alpha-linolenic acid levels were correlated with intake of red meat and butter. The association of alpha-linolenic acid levels with prostate cancer was greater among men with low linoleic acid and reduced meat intake.
CONCLUSIONS: These results suggest that low plasma levels of alpha-linolenic acid might be associated with reduced risk of prostate cancer, independently of high meat intake. High linoleic acid and low marine fatty oils were not associated with increased risk, as previously hypothesized. IMPLICATIONS: The effects of dietary alpha-linolenic acid, particularly from vegetable sources, warrant further study. The effects of dietary linoleic acid and marine fatty acids seen in animal bioassays might not apply to human prostate cancer.