In a study from the USA recently published in the New England Journal of Medicine, researchers used randomized, placebo-controlled trial, to assess the effects of cholecalciferol at a dose of 2000 IU per day and fish oil (omega-3 fatty acids) 1000 mg per day for the prevention of cancer and cardiovascular disease among men (> 50 years old) and women (> 55 years old). The primary end points were invasive cancer of any type and major cardiovascular events (a composite of myocardial infarction, stroke, or death from cardiovascular causes). Secondary end points included site-specific cancers, death from cancer, and additional cardiovascular events. Neither Vitamin D nor fish oil use was not associated with a lower risk of either of the primary end points. Analysis of death from any cause (978 deaths), revealed that the hazard ratio was was not significantly elevated for either group, and no excess hypercalcaemia or bleeding was observed in either group.

Read the papers summarising the results for each treatment here and here.