Dietary supplements containing vitamin D and fish oil with omega-3 fatty acids do not reduce signs of systemic inflammation linked to many chronic diseases, according to a recent study. This study was part of a larger trial that found these supplements did have some benefit in healthy adults, including decreased risk of heart attack and death from cancer.

Inflammation is how the body tells itself to heal and repair damaged tissue or defend itself against infections. Chronic, systemic inflammation occurs when the body constantly releases signals meant to activate the immune system, even when there is no injury or infection. Systemic inflammation is a significant factor in cardiovascular disease, diabetes, autoimmune disorders, and other chronic conditions.

Some advertisements for vitamin D and fish oil supplements claim they reduce systemic inflammation, potentially leading to benefits such as reduced risk of chronic diseases. However, evidence of vitamin D and fish oil supplements’ ability to reduce inflammation has been mixed and based mostly on small studies in which researchers observed the effects of supplements rather than comparing them directly to a placebo (inactive pill).

“People commonly think that these supplements can prevent inflammatory diseases, but when a patient asks their doctor, ‘Should I take this supplement?’ doctors often don’t know what to advise because there haven’t been large scale clinical trials,” said Karen Costenbader, MD, MPH, a researcher in the VITamin D and OmegA-3 TriaL (VITAL) study.

Unlike earlier studies, VITAL is a large, randomized controlled trial. Randomized controlled trials are thought to provide more reliable evidence than studies based solely on observation. Randomized means that participants are distributed between multiple treatment groups, and controlled means that treatments are compared to a placebo, which has no effect.

VITAL researchers studied 1,561 healthy participants—including men age 50 and older and women age 55 and older—who they assigned to four different treatments. One group took just vitamin D, the second took only fish oil, the third took a combination of the two, and the fourth group took a placebo. The researchers measured participants’ vitamin D and omega-3 fatty acids blood levels at the beginning of the study and compared them to the values measured at the end of the study. They also compared participants’ blood levels of proteins that rise with systemic inflammation. These proteins were interleukin-6 (IL-6), tumor necrosis factor-receptor 2 (TNFR 2), and high-sensitivity C-reactive protein (hsCRP).

After a year, the researchers found that vitamin D blood levels rose by 39% in participants who took vitamin D and omega-3 fatty acid blood levels rose by 55% in participants who took fish oil supplements. However, the levels of the inflammatory proteins stayed relatively stable or had only minor changes after a year. In contrast, if the supplements were beneficial for inflammation, the expected result would be a significant drop in the level of the inflammatory proteins.

VITAL researchers noted that although many vitamin D and omega-3 fatty acid supplements are available, they only tested one formulation and dose of each. Participants also were generally healthy, which means the study was not designed to evaluate whether these supplements can benefit people diagnosed with conditions that cause acute or chronic inflammation (e.g., autoimmune disorders, cancer).

“Vitamin D and marine omega-3 fatty acids are widely consumed supplements advertised to prevent disease and reduce systemic inflammation. Their purported health benefits have received enormous attention in the medical and popular presses,” Costenbader said. However, “in this study … neither vitamin D 2000 IU per day nor [omega-3 fatty acid supplements] reduced systemic inflammation biomarkers over one year.” As a result, “it is unlikely that these supplements, taken widely in the general population, have major anti-inflammatory effects,” she added.

The recent findings come out of a larger study—involving over 25,000 participants—of whether vitamin D and fish oil can help prevent disease in generally healthy men and women. Overall, the larger study suggests that vitamin D does not lower the risk of cancer, heart attack, and stroke, but does appear to be associated with fewer deaths related to cancer. The study also indicated that fish oil supplements are associated with a reduced risk of heart attack, especially for African Americans, and reduced risk of cancer in people who do not eat much fish.

Results from studies like VITAL provide solid evidence for consumers wanting to more clearly understand what supplements will and will not do for their health. Likewise, healthcare practitioners have reliable data they can point to when asked by their patients about the benefits, or lack of benefits, of taking supplements. In this case, vitamin D and fish oil are not associated with decreased systemic inflammation in healthy adults, but they may have other health benefits that consumers can consider.

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