fish oil and vitamin D

Recent findings of a large study revealed the effects of fish oil and vitamin D supplements on the risk of cancer, stroke, and heart attack.

Many people take nutritional supplements such as fish oil and vitamin D with a hope of reducing their risk of heart attack, stroke and cancer. Some trials have suggested that higher vitamin D levels are associated with lower risk of cancer. Likewise, many studies show that consumption of fish oil is associated with reduced risk of heart attack and it also helps in preventing stroke.

However, there have not been any large-scale, randomized, controlled trials investigating the effects of high doses of vitamin D or omega-3 fatty acids on cancer, cardiovascular disease, and stroke prevention as the main outcome of the study. Consequently, the trials conducted so far have shown either null or mixed results.

Clear evidence about the effectiveness of fish oil and vitamin D supplements is needed

The results of past studies on the effectiveness of fish oil and vitamin D supplements to reduce the risk of heart attack and cancer have been inconclusive. Despite several studies conducted on this subject, many public health agencies are concerned about the lack of evidence to support the use of fish oil and vitamin D supplements.

For instance, the Institute of Medicine of the US, an organization that makes recommendations for public health based on evidence-based research, has suggested that effectiveness of vitamin D supplements for prevention of cancer and cardiovascular disease needs to be further evaluated.

Largest, five-year-long trial provides the much-needed answers

Brigham and Women’s Hospital in Boston, US, conducted the largest randomized clinical trial to evaluate the effects of both omega-3 fatty acids and vitamin D supplements on the risk of cancer, heart attack and stroke. The study was recently presented at the American Heart Association’s annual conference and published in the New England Journal of Medicine. A team of researchers conducted a rigorous placebo-controlled, over five years long, Vitamin D and Omega-3 trial (VITAL) that revealed what researchers refer to as a “treasure trove” of information. This information clears most doubts associated with fish oil and vitamin D supplements.

What is unique about the VITAL study?

The VITAL study included 25,871 men and women aged 50 and above from across the US, who had no history of cancer, heart disease or stroke. The study population was racially and ethnically diverse with 20% of the participants being African-American. This is especially important since there has not been enough research conducted on the effects of omega-3 fatty acids on the risk of cancer in the African-American population.

Furthermore, VITAL is the first large trial that evaluated the effects of omega-3 fatty acids in preventing the first occurrence of heart disease in a general population while other trials in the past have examined the effect of fish oil in reducing the risk of heart attack in patients with a history of heart disease.

Study evaluated the independent effects of both supplements

The design of VITAL allowed researchers to evaluate the independent effects of fish oil and vitamin D supplements. Participants were randomly assigned to one of the four groups. The first group received 2000 IU of vitamin D and a placebo, the second group received 1 gram of omega-3 fatty acids with a placebo, the third group took both 2000 IU of vitamin D and 1 gram of omega-3, and the last group received two placebos.

All participants took these assigned supplements once every day for over five years. The researchers recorded all events such as cancer diagnosis, major heart events, or deaths from cancer and heart disease. They compared the results of those who received omega-3 fatty acids with those who took the placebo and similarly for those who received vitamin D supplements with those who took the placebo.

Effect of omega-3 fatty acids on risk of heart attack and stroke

A total of 805 participants suffered from a heart attack or stroke. Notably, there was not a significant difference in the number of cardiovascular events in the omega-3 supplement group compared with the placebo. However, when researchers compared the individual heart events, they observed a 28% lower risk of heart attack in the participants taking omega-3 supplements compared with the placebo. Furthermore, the people who ate less fish showed a greater benefit of omega-3 supplements compared with the people who ate more fish. Lastly, the results showed no association of omega-3 fatty acids and the risk of cancer.

Effects of vitamin D levels on risk of cancer

A total of 1,617 participants were diagnosed with cancer during the five years of study duration.  There was no significant difference in the number of cancer diagnoses in the group taking vitamin D supplements compared with the participants taking a placebo. However, the researchers observed reduced deaths due to cancer when participants had been taking vitamin D supplements for at least 2 years. These results imply that although vitamin D may not lower the rate of new cancers, it can inhibit the progression of cancer and thus reduce the risk of cancer death.

Fish oil may be more beneficial for African-American individuals

The results of this study showed that the effects of fish oil were more pronounced in the African-American participants. The researchers observed a lower risk of heart attack among these participants in the omega-3 group compared with the white population taking the same supplement. This effect of omega-3 supplements in African-American participants was observed regardless of the amount of fish they consumed.

Neither fish oil nor vitamin D supplements lowered the risk of heart attack, stroke, or cancer

This study has brought forward some surprising results about fish oil and vitamin D supplementation. The study concluded that supplementation with fish-oil-derived omega-3 fatty acids did not lower the risk of cardiovascular events such as heart attack, stroke or death, nor any type of cancer.

Furthermore, high dose vitamin D supplements for five years did not lower the incidence of cancer or major cardiovascular events in initially healthy adults. Although researchers observed no serious side effects such as bleeding and gastrointestinal symptoms, they recommend talking to the health care providers before discontinuing or starting the fish oil or vitamin D supplements.

Written by Preeti Paul, MS Biochemistry


  1. JoAnne E. Manson et al., Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. DOI: 10.1056/NEJMoa1809944
  2. JoAnn E. Manson et al., Marine n-3 Fatty Acids and Prevention of cardiovascular Disease and Cancer. DOI: 10.1056/NEJMoa1811403
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