A nationwide trial of vitamin D and fish oil has shown that the popular supplements have an effect on some cancer and heart disease outcomes. But the results are mixed, according to the researchers.

The five-year VITamin D and OmegA-3 Trial followed nearly 26,000 participants treated with the supplements. While Omega-3 fatty acids (fish oil) were found to be associated with significant reductions in heart attacks, they did not significantly lower the risk of major cardiovascular events. The greatest treatment benefit was seen in people who ate less than 1.5 servings of fish per week, and not in those with higher intake. Additionally, African-American participants experienced the greatest risk reductions with fish oil intake. 

Similarly, vitamin D supplementation did not reduce major cardiovascular events or total cancer incidence. But it was linked to a significant reduction in total cancer mortality among people who participated in the trial for at least two years.

The heart-health benefits of fish oil and the cancer mortality-lowering effects of vitamin D were subsequently confirmed by data meta-analysis, according to JoAnn Manson, Ph.D., Brigham and Women’s Hospital, and colleagues.

“The pattern of findings suggests a complex balance of benefits and risks for each intervention and points to the need for additional research to determine which individuals may be most likely to derive a net benefit from these supplements,” Manson concluded.

The latest results from VITAL will be presented this week during The North American Menopause Society Annual Meeting in Chicago.