• Health
  • Drugs

Fish Oil and Vitamin D Supplements May Not Help Prevent Heart Attacks and Cancer, Study Says

5 minute read

There’s good evidence that fish oil supplements may lower the risk of second heart events — like a heart attack or stroke — in people with heart disease, but few rigorous studies have investigated whether the supplement can help people to lower their risk of having a heart event in the first place. And while some data suggests that people with lower levels of vitamin D tend to have higher rates of heart disease and cancer, the evidence isn’t solid.

Now, a new study published in the New England Journal of Medicine and presented at the annual meeting of the American Heart Association offers more findings about the effects of these two supplements on heart disease and cancer. While they don’t appear to do much overall in this study, fish oil seems to help specific groups of people and the researchers believe that vitamin D may simply take longer to have an effect on cancer rates and deaths.

The VITAL study (VITamin D and omegA-3 triaL) set out to provide some more answers on what, if any, effect the two nutrients have on health. It is the first large-scale, rigorous controlled study to investigate how omega-3 supplements may affect heart disease risk in a people without a history of heart problems.

More than 25,800 people were randomly assigned to one of four groups: one received 2,000 IU of vitamin D a day and a placebo, another took 1g of omega-3 fatty acids a day with a placebo, a third took both 2,000 IU of vitamin D and 1g of omega-3 a day, and the last group received two daily placebos. All volunteers took their assigned nutrients for approximately five years, and researchers logged everything from cancer diagnoses to major heart events including heart attack and stroke, as well as deaths from cancer and heart disease.

The researchers, led by Dr. JoAnn Manson, from the Brigham and Women’s Hospital and professor of women’s health at Harvard Medical School, combined the studies of vitamin D and omega-3 fatty acids because they are not known to interact with each other in significant ways that would affect health outcomes. VITAL allowed her group to study both nutrients in an efficient way for their effects on heart disease and cancer, two chronic diseases that affect a large proportion of the population.

The people taking omega-3 fatty acids did not have a lower rate of heart events compared to the people taking placebo.

But the results don’t necessarily suggest that fish oil can only help people with a history of heart problems. The researchers in VITAL looked at overall heart outcomes together, combining heart attack, stroke and heart-related deaths; taken together, they found no difference between people taking omega-3 supplements and placebo. When they pulled out individual heart events, however, they found some interesting patterns. People taking the omega-3 supplements showed a 28% lower risk of having a heart attack compared to people taking placebo. In addition, people who reported eating less fish seemed to show a greater benefit in lowering their risk of heart problems compared to people who ate more fish. Manson and her team also found that African-American volunteers in the omega-3 group lowered their risk of heart problems more than whites taking the fish oil supplements. That finding, she says, remained regardless of how much fish the African-American volunteers ate, which suggests some possible genetic reasons for why African-Americans may benefit more from omega-3 fatty acids.

“We don’t know if this is a chance finding, or if it may in fact point to an important way to reduce heart risk in African-Americans,” says Manson.

When they looked at the effects of vitamin D, they also found little difference among those taking the nutrient and those taking placebo when it came to overall heart disease and cancer measures. But again, Manson says that doesn’t mean vitamin D is not playing a role in cancer. Some studies suggest that the vitamin can inhibit the progression of tumors, so while it may not lower the rate of new cancers, it could reduce the risk of dying from cancer. And because cancer takes longer to develop, longer term studies would need to verify if that’s indeed the case. “If someone has a cancer that hasn’t yet been diagnosed, but is already present, vitamin D may make that cancer less invasive and less likely to spread,” she says. Manson is planning on doing more research to delve further into the question of whether vitamin D supplements have differing effects on cancer at various stages of the disease.

In about six months, she says she also plans on reporting additional outcomes from VITAL, including whether vitamin D and omega-3 supplements have any effect on diabetes, cognitive function, mood disorders like depression, infections and autoimmune diseases.

More Must-Reads from TIME

Contact us at letters@time.com