Twenty-five men with prostate cancer who were awaiting prostatectomy were prescribed a low-fat diet (maximum fat content, 20% of total energy) supplemented with 30 g/day of ground flaxseed, with added stabilizers to prevent rancidity. The mean duration of diet therapy was 33.7 days (range, 2177 days). Compared with baseline values, diet therapy was associated with a significant 15% decrease in serum total testosterone (p <0.05) and a 19% decrease in the free androgen index (p < 0.05). No significant change was seen in serum prostate-specific antigen (PSA) levels. The mean tumor proliferation index was 5.0, compared with 7.4 in a group of historic controls matched for age, race, PSA level, and Gleason sum (p = 0.05).

Comment: Epidemiological studies have suggested that ingestion of large amounts of alpha-linolenic acid (the main fatty acid present in flaxseed and flaxseed oil) is associated with an increased risk of developing prostate cancer. However, epidemiological studies do not indicate cause-and-effect, and no long-term intervention trials have been done. Although the present study does not demonstrate a clear benefit of flaxseed supplementation in men with prostate cancer, neither is there any hint of an adverse effect. Thus, there is no strong evidence that consuming moderate amounts of non-rancid flaxseed or flaxseed oil would increase the risk of developing prostate cancer. However, alpha-linolenic acid is an unstable molecule that readily undergoes spontaneous oxidation, and it is conceivable that oxidative byproducts of this fatty acid could be carcinogenic. Therefore, foods that are rich in alpha-linolenic acid should be kept airtight and refrigerated, and should not be cooked at high temperatures.