Results have been mixed but show promise for some outcomes and now confirmed by updated meta analyses, according to the researchers.
"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," said study lead author JoAnn Manson from Harvard University.
Nearly 26,000 US men and women participated in the nationwide VITAL clinical trial.
After more than five years of study and treatment, the results show promising signals for certain outcomes.
For example, while Omega-3 fatty acids (fish oil) showed only a small, but non-significant, reduction in the primary cardiovascular endpoint of major CVD events, they were associated with significant reductions in heart attacks.
The greatest treatment benefit was seen in people with dietary fish intake below the cohort median of 1.5 servings per week but not in those whose intake was above that level.
The heart health benefits are now confirmed by recent meta-analyses of omega-3 randomised trials.
Similarly, vitamin D supplementation did not reduce major CVD events or total cancer incidence but was associated with a statistically significant reduction in total cancer mortality among those in the trial at least two years.
The effect of vitamin D in reducing cancer death is also confirmed by updated meta-analyses of vitamin D trials to date.
The study is scheduled to be presented during The North American Menopause Society (NAMS) Annual Meeting in Chicago, September 25-28.
( With inputs from IANS )