A large-scale prospective cohort study of 253,138 UK Biobank participants examined the associations of plasma omega-3 and omega-6 polyunsaturated fatty acids with cancer risk, finding that both fatty acid types provided some measure of cancer protection to different population groups.

With an average follow-up of 12.9 years, 29,838 incident cancer cases were documented among the 90.9% White cohort with a mean age of 56.4 years. Results showed a 2% reduction in overall cancer risk per standard deviation (SD) increase in plasma omega-6 and a 1% reduction per SD increase in omega-3 levels. Each SD increase corresponded to a 3.63 percentage-point increase for omega-6 and 1.55 for omega-3.

Omega-6 was inversely associated with 14 types, while omega-3 showed inverse associations with five types for site-specific cancers. Protective associations ranged from a 3-7% reduced risk per SD for omega-6 and 5-8% for omega-3. However, omega-3 was associated with a 3% increased risk of prostate cancer.

Subgroup analysis indicated that omega-6’s protective effects were stronger in younger participants, who were less than 58 years old and women. While omega-3’s protective effects were more pronounced in older participants, men, and current smokers. No significant associations were observed for ovarian, breast, uterine, or lymphoid cancers.

The correlation between plasma omega-6% and omega-3% was low. In a subset with dietary data, correlations between plasma and dietary polyunsaturated fatty acid (PUFA) levels were 0.133 for omega-6%, 0.271 for omega-3%, and 0.051 for their ratio.

While both omega-3 and omega-6 individually showed protective associations, a higher omega-6/omega-3 ratio was linked to increased cancer risk, particularly for rectal, colon, and lung cancers.

This study, published in International Journal of Cancer, used objective plasma PUFA biomarkers instead of estimated dietary intakes. Cancer incidence data were sourced from cancer registries, aiming to minimize selection bias. Limitations included the predominantly White European cohort, potential unmeasured confounders, and limited statistical power for rare cancers. Also, omega-3 and omega-6 measurements were restricted to docosahexaenoic acid (DHA) and linoleic acid (LA), respectively.

The authors disclosed no conflicts of interest.