The National Institutes of Health funded the study, which recruited 25,871 healthy American men and women aged 50+, including 5,106 African Americans. The study participants were divided into four groups and randomly assigned to take supplements or placebos. They were observed on average for 5.3 years.
One group took 2,000 IU (international units) daily of vitamin D3 and 1 gram of omega-3 fatty acids. A second group received vitamin D and a dummy pill instead of omega-3. A third group received omega-3 fatty acids and a vitamin D placebo. And the last group received two placebos.
Pharmavite LLC of Northridge, California, donated the vitamin D supplements and their placebos, and Pronova BioPharma of Norway and BASF donated Omacor, a fish oil sold under the Lovaza brand in the United States.
The results, published in the New England Journal of Medicine, will be presented on Saturday at a conference of the American Heart Association in Chicago.
In many ways, the results are not surprising. The public has been shut down in recent years by a steady flow of information about the health benefits of vitamin D, as studies on low vitamin levels have led to conditions as diverse as diabetes, high blood pressure, cancer, heart disease, and depression. Many GPs are now routinely testing patients' vitamin D levels and stating that they are inadequate, and the sale of supplements has increased dramatically in recent years.
Time and again, however, critics have asked whether vitamin D is just an indicator of overall health and whether the threshold for deficiency has been set too high. The so-called sunshine vitamin is synthesized in the body when the skin is exposed to sunlight and is exhausted by smoking, obesity, poor diet and other factors. Certain foods, such as fatty fish, eggs and milk, also contain vitamin D.
The Institute of Medicine concluded in 2011 that most Americans were getting enough vitamin D and that the deficiencies were overstated. The group also noted that reports of potential benefits were inconsistent with higher blood levels.