Researchers at Harvard Medical School and Harvard-affiliated Brigham and Women’s Hospital are leading a five-year nationwide trial to find out whether the dietary supplements vitamin D and fish oil can boost the immune system and fight cancer, heart disease, and a host of other ills.

The “Vitamin D and Omega-3 Trial,” or VITAL, aims to sort out inconclusive and conflicting evidence from earlier research on the effects of the two compounds on human health.

Previous studies have turned up tantalizing clues that the two nutrients can have considerable protective effects. But JoAnn Manson, the VITAL study’s principal investigator, said those trials — and others showing no protective effect — either involved specialized populations, such as those suffering heart disease, or used low dosages, which may have prevented finding a conclusive answer.

The VITAL study is a large-scale, randomized trial involving 20,000 people across the country with no previous history of cancer, heart disease, or stroke, and is designed to test whether vitamin D and the omega-3 fatty acids in fish oil can help to prevent cancer and heart disease. Though cancer and heart disease are the study’s primary therapeutic targets, Manson said the study will also provide information on other ailments, such as diabetes, cognitive decline, depression, and respiratory diseases.

Scientists already know quite a bit about how these nutrients work in the body. Both have powerful anti-inflammatory effects. Vitamin D appears to benefit blood pressure and glucose tolerance, while working to prevent blood vessel growth that allows tumors to enlarge and spread. Omega-3 fatty acids have anti-clotting effects and have been shown to protect against irregular heart rhythms.

Manson, the Elizabeth Fay Brigham Professor of Women’s Health at Harvard Medical School and chief of Brigham and Women’s Hospital’s Division of Preventive Medicine, said the trial will enroll men age 60 or older and women age 65 and up. The older study population was selected because people of those ages are more commonly afflicted with the ailments the study seeks to test.

Researchers began seeking participants in January and will eventually send mailings to more than 1.2 million Americans, including health professionals and members of AARP. Potential participants will undergo a three-month screening before enrolling in the full trial. Participants will be divided into four groups and receive blister packs of daily supplements, along with questionnaires to complete and mail back to researchers. Though some participants may opt to visit nearby clinical centers for more-detailed assessments and to provide blood samples, most can participate entirely by mail.

The groups will receive supplements containing vitamin D, omega-3s, both, or placebos, allowing researchers to examine the effects of vitamin D and omega-3s independently as well as together.

The study’s vitamin D supplements will contain 2,000 international units (IUs) per day, five times the 400 IUs that the U.S. government currently recommends. Manson said most Americans get only about 300 IUs of vitamin D per day through their diet, and even with supplements few get more than 500 or 600 IUs. The human body can manufacture vitamin D when exposed to sunlight — more than 2,000 IUs for someone working lightly clothed in the sun all day — but the increase in people wearing sunblock to ward off skin cancer and the decreased prevalence of children playing outdoors have reduced the amount of vitamin D that many people get from sunlight.

Several other factors are working to further reduce the amount of vitamin D that people get. The increase in children drinking sugar-sweetened beverages instead of milk cuts vitamin D intake. Also, because vitamin D is fat soluble, the obesity epidemic is increasing the amount that is stored in fats in our bodies instead of being freely available.

The supplements will contain about one gram of omega-3s, Manson said, or about twice the amount people would get if they followed the government’s recommendation of two fish meals a week, and about five to 10 times what the typical American usually eats. It’s also about equal to the level in a typical diet in Japan, where heart disease rates are lower.

Manson said it would be unwise for the public to start taking megadoses of the two compounds before the study’s results come out, citing the examples of earlier large-scale trials of vitamins E and C and beta-carotene that showed little benefit of those vitamins in large doses and even suggested some risks. Should the trial turn up protective benefits to vitamin D and omega-3 fatty acids, it would open the door to greater therapeutic use of the compounds, which are easily accessible, unlike a new exotic drug that would require extensive testing.

Manson also plans to explore the role of vitamin D in reducing racial health disparities. The study will seek to enroll enough African Americans to make up a quarter of the study population in an effort to see whether low levels of vitamin D in African Americans are linked to higher incidence of diabetes, heart disease, and other chronic diseases and whether treatment with vitamin D can reduce these risks.

“It’s exciting to get started with this trial,” Manson said. “We’re really hoping it will provide important answers.”

To learn more about VITAL, visit VitalStudy.org.

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