Alpha-linolenic acid (ALA), an omega-3 fatty acid found in plants, appears unlikely to be a cause of prostate cancer, contrary to some previous findings, according to a research review and meta-analysis by investigators at the San Francisco VA Medical Center and the University of California, San Francisco.
Consumption of omega-3 fatty acids, which are found in fish oils and vegetable oils, is associated with lowered risk of cardiovascular disease. The type found in fish oils is not associated with any known health risks, but some studies have implicated ALA, a type found in soybeans, canola oil, flax seed oil, and walnuts, as a possible risk factor for prostate cancer.
“This is significant because almost 90 percent of the omega-3 acids consumed by Americans is ALA from plants,” says lead author Joel A. Simon, MD, MPH, a staff physician at SFVAMC and a professor of clinical medicine and epidemiology & biostatistics at UCSF. “Fortunately, we found only a very modest risk that was no longer evident once we controlled for publication bias.”
Publication bias, a long-observed phenomenon in scientific literature, is the tendency of researchers to not submit findings for publication that are neutral or negative – which in this context would be findings that do not indicate an association between ALA and prostate cancer risk, explains Simon. The authors of meta-analyses such as Simon’s use well-established statistical techniques to account for these missing studies in order to keep their own results from being skewed.
The study appears in the May 1, 2009 supplement of the American Journal of Clinical Nutrition.
Simon and his team pooled the results from 16 studies examining a potential association between ALA and prostate cancer conducted from 1963 to 2007. Initially, they found a 20 percent increased risk of prostate cancer when comparing the highest with the lowest levels of ALA, as measured by dietary intake, blood levels, and adipose (fat) tissue levels among study participants. “This difference was just barely statistically significant,” says Simon.
However, he says, there was significant heterogeneity among the studies – that is, large inconsistencies in the findings that tended to undermine any conclusion of increased risk.
When the researchers adjusted for publication bias, the risk disappeared.
Yet another factor gives the authors confidence that ALA is probably not linked with prostate cancer risk, according to Simon: Recent large prospective studies that were specifically designed to look for prostate cancer risk factors found no such link.
The authors caution that, at the “highest levels” of consumption, there may be a “small increased risk” of prostate cancer; however, they characterize this conclusion as “highly qualified” because of heterogeneity and publication bias.
“This makes me feel considerably better about eating plant foods rich in alpha-linolenic acid,” concludes Simon.
Co-authors of the study are Yea-Hung Chen, MS, and Stephen Bent, MD, of SFVAMC and UCSF.
SFVAMC has the largest medical research program in the national VA system, with more than 200 research scientists, all of whom are faculty members at UCSF.
UCSF is a leading university dedicated to promoting health worldwide through advanced biomedical research, graduate-level education in the life sciences and health professions, and excellence in patient care.