Clinical Context

Regular fish consumption is a routine part of dietary recommendations for adults, but not all prospective studies have demonstrated improvements in outcomes among adults who eat fish. Jayedi and Shab-Bidar addressed this issue by completing an umbrella review of meta-analyses examining fish consumption. Their results were published in the September 2020 issue of Advances in Nutrition.^[1]

The authors found 34 meta-analyses of prospective observational studies, which featured a total of 40 different outcomes. Greater fish consumption of 100 g/day was associated with significant improvements in multiple health outcomes, including the risks for myocardial infarction, stroke, and heart failure. Increased fish consumption also improved rates of cardiovascular and all-cause mortality. However, fish consumption had no significant effects on rates of hypertension, atrial fibrillation, or type 2 diabetes.

There is a lack of large studies that have differentiated the effects of fish consumption among adults with and without a history of cardiovascular disease (CVD). The current study addresses this gap.

Study Synopsis and Perspective

People with CVD who regularly ate fish had significantly fewer major CVD events and there were fewer total deaths compared with similar individuals who did not eat fish, but there was no beneficial link from eating fish among the general population in prospective data collected from more than 191,000 people from 58 countries.

Despite the neutral finding among people without CVD, the finding that eating fish was associated with significant benefit for those with CVD or who were at high risk for CVD confirms the public health importance of regular fish or fish oil consumption, says one expert.

A little more than a quarter of those included in the new study had a history of CVD or were at high risk for CVD. In this subgroup of more than 51,000 people, those who consumed on average at least 2 servings of fish weekly (at least 175 g, or about 6.2 ounces per week) had a significant 16% lower rate of major CVD events during a median follow-up of about 7.5 years.

The rate of all-cause death was a significant 18% lower among people who ate 2 or more fish portions weekly compared with those who did not, say Deepa Mohan, PhD, and associates in their report published in JAMA Internal Medicine.^[2]

The researchers saw no additional benefit when people regularly ate greater amounts of fish.

"There is a significant protective benefit of fish consumption in people with cardiovascular disease," summed up Andrew Mente, PhD, a senior investigator on the study and an epidemiologist at McMaster University in Hamilton, Canada.

"This study has important implications for guidelines on fish intake globally. It indicates that increasing fish consumption and particularly oily fish in vascular patients may produce a modest cardiovascular benefit," he said in a statement released by McMaster.

The neutral finding of no significant benefit (as well as no harm) regarding either CVD events or total mortality among people without CVD "does not alter the large body of prior observational evidence supporting the cardiac benefits of fish intake in general populations," notes Dariush Mozaffarian, MD, DrPH, in a commentary that accompanies the report by Dr Mohan and colleagues.^[3]

Although the new analysis failed to show a significant association between regular fish consumption and fewer CVD events for people without established CVD or CVD risk, "based on the cumulative evidence from prospective observational studies, randomized clinical trials, and mechanistic and experimental studies, modest fish consumption appears to have some cardiac benefits," he adds.

"Adults should aim to consume about 2 servings of fish per week, and larger benefits may accrue from nonfried oily (dark meat) fish," writes Dr Mozaffarian, a professor of medicine and nutrition at Tufts University School of Medicine, Boston, Massachusetts.

Oily, dark fishes include salmon, tuna steak, mackerel, herring, and sardines. Species such as these contain the highest levels of long-chain omega 3 fatty acids, eicosapentanoic acid, and docosapentanoic acid; these nutrients likely underlie the CVD benefits from fish, Dr Mozaffarian says in an interview with JAMA Internal Medicine that accompanies his commentary.^[4] (Dr Mente also participated.)

"Fish oil lowers heart rate, blood pressure, and triglycerides (at high dosages), increases adiponectin, improves endothelial function, and in some studies improves oxygen consumption in myocardium. If there is benefit from fish it's from the omega 3s, and all in all the evidence supports this," but because the evidence is primarily observational, it can only show linkage and cannot prove causation, he explains.

Given the potential benefit and limited risk, "I think everyone should aim to eat 2 servings of fish each week, preferentially oily fish. That's very solid," says Dr Mozaffarian, who is also a cardiologist and dean of the Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy, Boston, Massachusetts.

The investigators did not have adequate data to compare the associations between outcomes and a diet with oily fish vs less oily fish.

For people who either can't consume 2 fish meals a week or want to ensure their omega 3 intake is adequate, "it's very reasonable for the average person to take 1 [over-the-counter] fish oil capsule a day," Dr Mozaffarian adds.

He acknowledges that several studies of fish oil supplements failed to show benefit, but several others have. "It's a confusing field, but the evidence supports benefit from omega 3s," he concludes.

He discounts the new finding that only people with established CVD or who are at high risk benefit. "I'm not sure we should make too much of this, because many prior studies showed a lower CVD risk in fish-eating people without prevalent CVD," he said. The new study "provides important information given its worldwide breadth," he added.

The new report used data regarding 191,558 people enrolled prospectively in any of 4 studies. The average age of the participants was 54 years, and 52% were women.

During follow-up, death from any cause occurred in 6% of those without CVD or CVD risk and in 13% of those with these factors. Major CVD events occurred in 5% and 17% of these 2 subgroups, respectively. To calculate the relative risks between those who ate fish and those who did not, the investigators used standard multivariate adjustment for potential confounders and adjusted for several dietary variables, Dr Mente says.

Dr Mohan and Dr Mente have disclosed no relevant financial relationships. Dr Mozaffarian has received personal fees from Acasti Pharma, Amarin, America's Test Kitchen, Barilla, Danone, GEOD, and Motif Food Works, and he has been an advisor to numerous companies.

JAMA Intern Med. Published online March 8, 2021.

Clinical Implications

• A previous umbrella study of meta-analyses found that increased fish consumption of 100 g/day was associated with significant improvements in multiple health outcomes, including the risks for myocardial infarction, stroke, and heart failure. Increased fish consumption also improved rates of cardiovascular and all-cause mortality. However, fish consumption had no significant effects on rates of hypertension, atrial fibrillation, or type 2 diabetes.
• In the current study, approximately 2 servings of fish per week were associated with improved CVD and mortality outcomes among adults with previous vascular disease, but not among adults without such a history.
• Implications for the healthcare team: The healthcare team should emphasize moderate fish consumption as part of a healthy diet, particularly for adults with a history of vascular disease.

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