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Previous research on the health benefits of eating fish, fish oil supplements, and other sources of omega-3 fatty acids has shown mixed results, with some studies revealing cardiac health benefits and others finding no benefit. However, when looking at recent studies separating eating foods rich in omega-3 fatty acids versus taking supplements, it appears that eating foods has various beneficial health effects, while taking a supplement may not find health benefits (here and here).

The latest research (reported in the journal JAMA Internal Medicine) looked at heart disease events (heart attack, cardiac related death) and actually measured the actual levels of omega-3 fatty acids in the participants' blood, as opposed to relying on questionnaires in which people report what they eat. The new study could not assess the usefulness of taking fish oil supplements, as opposed to eating fish, because so few people in the study took supplements. Thus the findings were generally from eating a diet rich in omega-3s from either fish or plant-based sources. (Note: By far the best source is fish. Some plant-based sources are: flaxseed, walnuts, edamame, black beans, kidney beans).

The new study — which combined 19 studies from 16 countries with more than 45,000 participants — found that higher circulating blood levels of omega-3 fatty acids were associated with a nearly 10 percent lower risk of a fatal heart attack, on average, compared with lower levels. The participants with the highest level of omega-3s in their blood had the greatest risk reduction — a more than 25 percent lower risk of having a fatal heart attack, the study found.

From Science Daily: Consumption of omega-3s linked to lower risk of fatal heart disease

A global consortium of researchers banded together to conduct an epidemiological study analyzing specific omega-3 fatty acid biomarkers and heart disease. They found that blood levels of omega-3 fatty acids from seafood and plant-based foods are associated with a lower risk of fatal heart attack.

A total of 19 studies were involved from 16 countries and including 45,637 participants. Of these, 7,973 people developed a first heart attack over time, including 2,781 deaths and 7,157 nonfatal heart attacks.

Overall, both plant-based and seafood-based omega-3s were associated with about a 10 percent lower risk of fatal heart attacks. In contrast, these fatty acids biomarkers were generally not associated with a risk of nonfatal heart attacks, suggesting a more specific mechanism for benefits of omega-3s related to death."At a time when some but not other trials of fish oil supplementation have shown benefits, there is uncertainty about cardiovascular effects of omega-3s," said Mozaffarian. "Our results lend support to the importance of fish and omega-3 consumption as part of a healthy diet."

Fish is the major food source of omega-3 fatty acids, including eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA). According to the U.S. Department of Agriculture's National Nutrient Database, fatty fish such as salmon, trout, anchovies, sardines, and herring contain the highest amounts of omega-3 fatty acids, although all fish contain some levels. In addition to omega-3 fatty acids, fish provide specific proteins, vitamin D, selenium, and other minerals and elements. Alpha-linolenic acid (ALA) is the plant-based omega-3 fatty acid found in walnuts, flaxseed oil, and canola oil and some other seed and nuts and their oils.

Once again, a study finds that a supplement has no benefit - here omega-3 supplements did not slow cognitive decline in older adults. On the other hand, studies find that eating foods with omega-3 fatty acids (such as fish) has beneficial health effects. Regular consumption of fish is associated with lower rates of age-related macular degeneration (AMD), cardiovascular disease, and possibly dementia.  From Science Daily:

No benefit of omega-3 supplements for cognitive decline, study shows

While some research suggests that a diet high in omega-3 fatty acids can protect brain health, a large clinical trial by researchers at the National Institutes of Health found that omega-3 supplements did not slow cognitive decline in older persons. With 4,000 patients followed over a five-year period, the study is one of the largest and longest of its kind.

Dr. Chew leads the Age-Related Eye Disease Study (AREDS), which was designed to investigate a combination of nutritional supplements for slowing age-related macular degeneration (AMD), a major cause of vision loss among older Americans. That study established that daily high doses of certain antioxidants and minerals -- called the AREDS formulation -- can help slow the progression to advanced AMD.

A later study, called AREDS2, tested the addition of omega-3 fatty acids to the AREDS formula. But the omega-3's made no difference. Omega-3 fatty acids are made by marine algae and are concentrated in fish oils; they are believed to be responsible for the health benefits associated with regularly eating fish, such as salmon, tuna, and halibut.* Where studies have surveyed people on their dietary habits and health, they've found that regular consumption of fish is associated with lower rates of AMD, cardiovascular disease, and possibly dementia. "We've seen data that eating foods with omega-3 may have a benefit for eye, brain, and heart health," Dr. Chew explained.

Omega-3 supplements are available over the counter and often labeled as supporting brain health. A large 2011 study found that omega-3 supplements did not improve the brain health of older patients with preexisting heart disease.

With AREDS2, Dr. Chew and her team saw another opportunity to investigate the possible cognitive benefits of omega-3 supplements, she said. All participants had early or intermediate AMD. They were 72 years old on average and 58 percent were female....Participants were given cognitive function tests at the beginning of the study to establish a baseline, then at two and four years later.... The cognition scores of each subgroup decreased to a similar extent over time, indicating that no combination of nutritional supplements made a difference.

* Other omega-3 fatty acids are found in plant foods such as flaxseed, walnuts, soy products, and canola and soybean oils. Specific omega-3 fatty acids from these sources were not studied.

Another piece of research that shows that eating more fish and less meat plus the Mediterranean diet is beneficial - this time it's linked to larger brain volume and "delaying age-related atrophy of the brain". Perhaps some other beneficial health-related behaviors are also occurring in these groups, but the link with better brain health and more fish, less meat, and Mediterranean diet is consistently occurring in research. From Medscape:

Mediterranean Diet Linked to Larger Brain Volume

Adherence to the Mediterranean diet (MeDi) may prevent brain atrophy in old age, new research suggests.A large cross-sectional study by investigators at Columbia University in New York linked adherence to the MeDI to larger brain volume in an elderly population, suggesting this type of diet has the potential to prevent brain atrophy and, by extension, preserve cognition in the elderly.

"Our study suggests that adhering to MeDi may prevent cognitive decline or AD [Alzheimer's disease] by maintaining the brain structure or delaying aging-related atrophy," said study investigator Yian Gu, PhD. Previous research has linked the MeDi to a reduced risk for AD and slower rates of cognitive decline in the elderly. "

The aim of the current study was to investigate the association between adherence to the MeDi and structural MRI-based measures of both brain volume and cortical thickness among elderly individuals participating in the Washington Heights-Inwood Columbia Aging Project (WHICAP).The study cohort included 674 multiethnic Medicare beneficiaries aged 65 years or older living in an uptown area of New York City andfish who had no signs of dementia.

All participants underwent high-resolution structural MRI as well as assessment of MeDi adherence based on a 61-item food-frequency questionnaire....A higher score (ranging from 0 to 9) indicated better MeDi adherence. The investigators assessed intracranial volume (ICV), total brain volume (TBV), total gray matter volume (TGMV), total white matter volume (TWMV), and cortical thickness (CT).... the investigators found that those with higher MeDi adherence scores (5 to 9) had larger TBV , TGMV, and TWMV compared with those with lower MeDI scores (0 to 4).

Among the 9 food components of MeDi, higher fish intake was associated with larger TGMV, and lower meat intake was associated with both larger TBV and TGMV . Higher fish intake was also associated with higher CT .

Participants who adhered more to a MeDI had larger brain volumes both in gray matter and white matter, said Dr Gu. She also noted that each additional higher MeDi adherence and total brain volume is equivalent to more than 1 year of aging. Dr Gu noted that most of the association was driven by higher intake of fish and lower intake of meat. Potential mechanisms, she said, include anti-inflammatory and/or antioxidative effects, as well as potential slowing of the accumulation of β-amyloid or tau.

Fish consumption was beneficial for the brain, but brain differences among the groups not correlating with blood omega-3 levels was a surprise. From  Science Daily:

Eating baked, broiled fish weekly boosts brain health, study says

Eating baked or broiled fish once a week is good for the brain, regardless of how much omega-3 fatty acid it contains, according to researchers. The findings add to growing evidence that lifestyle factors contribute to brain health later in life. Scientists estimate that more than 80 million people will have dementia by 2040, which could become a substantial burden to families and drive up health care costs.

"Our study shows that people who ate a diet that included baked or broiled, but not fried, fish have larger brain volumes in regions associated with memory and cognition," Dr. Becker said. "We did not find a relationship between omega-3 levels and these brain changes, which surprised us a little. It led us to conclude that we were tapping into a more general set of lifestyle factors that were affecting brain health of which diet is just one part."

Lead investigator Cyrus Raji, M.D., Ph.D., who now is in radiology residency training at UCLA, and the research team analyzed data from 260 people who provided information on their dietary intake, had high-resolution brain MRI scans, and were cognitively normal at two time points during their participation in the Cardiovascular Health Study (CHS), a 10-year multicenter effort that began in 1989 to identify risk factors for heart disease in people over 65.

"The subset of CHS participants answered questionnaires about their eating habits, such as how much fish did they eat and how was it prepared," Dr. Raji said. "Baked or broiled fish contains higher levels of omega-3s than fried fish because the fatty acids are destroyed in the high heat of frying, so we took that into consideration when we examined their brain scans."

People who ate baked or broiled fish at least once a week had greater grey matter brain volumes in areas of the brain responsible for memory (4.3 percent) and cognition (14 percent) and were more likely to have a college education than those who didn't eat fish regularly, the researchers found. But no association was found between the brain differences and blood levels of omega-3s.

"This suggests that lifestyle factors, in this case eating fish, rather than biological factors contribute to structural changes in the brain," Dr. Becker noted. "A confluence of lifestyle factors likely are responsible for better brain health, and this reserve might prevent or delay cognitive problems that can develop later in life."