Don’t count on vitamin D or fish oil supplements to reduce your risk of cancer or heart disease

Neither vitamin D nor fish oil supplements prevented cancer or serious heart-related problems in a widely anticipated clinical trial involving thousands of healthy people.
(Kirk McKoy / Los Angeles Times)
Kaiser Health News

A widely anticipated study has concluded that neither vitamin D nor fish oil supplements prevent cancer or serious heart-related problems — such as heart attacks and strokes — in healthy older people.

Although hundreds of studies of these supplements have been published over the years, the new clinical trial — a federally funded project involving nearly 26,000 people — is the strongest and most definitive examination yet, said Dr. Clifford Rosen, a senior scientist at the Maine Medical Center Research Institute who was not involved in the research.

The findings were presented this weekend at the American Heart Assn. Scientific Sessions and published in the New England Journal of Medicine.

Doctors have been keenly interested in learning the true value of these supplements, given their tremendous popularity with patients. A 2017 study in the Journal of Nutrition found that 26% of Americans age 60 and older take vitamin D supplements, while 22% take pills containing omega-3 fatty acids, a key ingredient in fish oil.

The new study also suggests there’s no reason for people to undergo routine blood tests for vitamin D, because patients’ vitamin D levels made no difference in their risk of cancer or serious heart issues, said Rosen, who co-wrote an editorial that was also published in the New England Journal of Medicine. Even people who began the study with clear vitamin D deficiency got no benefit from taking the supplements, which provided 2,000 international units a day, an amount equal to one or two of the vitamin D pills typically sold in stores.


“It’s time to stop it,” said Rosen of vitamin D testing. “There’s no justification.”

Dr. JoAnn Manson, the study’s lead author, agrees that her results don’t support screening healthy people for vitamin D deficiency.

But she doesn’t see her study as entirely negative. Her team found no serious side effects from taking either fish oil or vitamin D supplements.

“If you’re already taking fish oil or vitamin D, our results would not provide a clear reason to stop,” she said.

Manson notes that a deeper look into the data suggested possible benefits.

When researchers singled out heart attacks — rather than the rate of all serious heart problems combined — they saw that fish oil appeared to reduce them by 28% percent, Manson said. As for vitamin D, it appeared to reduce cancer deaths — although not cancer diagnoses — by 25%.

But slicing the data into smaller segments with fewer patients in each group can produce unreliable results, said Dr. Barnett Kramer, director of the cancer prevention division at the National Cancer Institute. The links between fish oil and heart attacks, and between vitamin D and cancer death, could be due to chance, he said.

Experts agree that vitamin D is important for bone health. But this study only looked at areas where vitamin D’s benefits haven’t been definitely proved, such as cancer and heart disease. Although preliminary research has suggested vitamin D can prevent heart disease and cancer, more rigorous studies have disputed those findings.

Manson and her colleagues plan to publish data on the supplements’ effects on other areas of health in coming months, including diabetes, memory and mental functioning, autoimmune disease, respiratory infections and depression.

Meanwhile, those who want to reduce their risk of cancer and heart disease can follow other strategies besides taking vitamin D and fish oil supplements.

“People should continue to focus on known factors to reduce cancer and heart disease: Eat right, exercise, don’t smoke, control high blood pressure, take a statin if you are high risk,” said Dr. Alex Krist, a professor of family medicine and population health at Virginia Commonwealth University.

Szabo is a senior correspondent for Kaiser Health News, an editorially independent publication of the Kaiser Family Foundation.