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Food, Health, and Eco-news
Fish May Reduce Rates of “Silent” Stroke
Brain scans find fewer brain injuries related to undiagnosed strokes in people who eat fish three or more times a week 08/11/2008 by Craig Weatherby

You’ve probably heard that the omega-3 fatty acids in fish and fish oil may reduce the risk of stroke.

Current evidence suggests that eating baked or broiled (not fried) fish at least once a week appears to reduce the risk of stroke by about 25 percent. Eating fish more than four times a week seems to reduce stroke risk a bit more.

Key Points

People who eat fish frequently were 26 percent less likely have the brain lesions left by “silent”, undiagnosed strokes.

Fried fish eaters did not have lower rates of brain lesions, probably due the high levels of omega-6 fats in deep-frying oils.

Eating fried fish frequently may raise the risk of stroke by 44 percent.

And these positive findings have now been bolstered by the intriguing outcomes of a new brain-scan study.

Scientists from Harvard and Finland recruited 3,660 people age 65 and older, who agreed to complete diet questionnaires and undergo two brain scans, taken five years apart (Virtanen JK et al. 2008).

According to lead author Jyrki Virtanen, Ph.D., R.D., “Previous findings have shown that fish and fish oil can help prevent stroke, but this is one of the only studies that looks at fish’s effect on silent brain infarcts in healthy, older people.”

By “silent brain infarcts”, brain researchers mean minuscule areas of scarred or dead brain tissue caused by clots from damaged blood vessels. These tiny brain wounds are termed silent infarcts because they produce no apparent symptoms.

The same vascular disease process that leads to heart attacks also causes the most common kind of stroke (ischemic), which some doctors call a “brain attack”.

Prior research showed that one in five generally healthy elderly people have silent brain infarcts, which can only be found by brain scans using an MRI.

And by projecting these results on the entire population, it’s estimated that about 11 million Americans suffer 22 million silent, undiagnosed strokes annually (Leary MC et al. 2003).

In fact, the annual number of silent strokes in America is about 30 times higher than the incidence of diagnosed strokes.

Silent infarcts can lead to more strokes, to gradual loss of thinking skills, and to dementia.

Scans show healthier brains in fish eaters
In addition to receiving brain scans, the study participants also completed questionnaires about the amounts and types of fish in their diets

Five years later, the brain scans were repeated in the 2,313 volunteers who were still available to participate in the study.

The findings were striking:
  • Compared to people who did not eat fish regularly, the brain scans of participants who ate broiled or baked tuna or other fatty fish three times per week or more were 26 percent less likely to show brain lesions.
  • Eating just one serving of fatty fish per week cut the rate of scan-detected brain lesions by 13 percent.
  • People who ate fatty fish regularly had healthy white matter in their brains, and no signs of the kinds of white matter associated with brain atrophy.
As Dr. Virtanen noted in a press release, high levels of omega-3s probably explain the association between eating fatty fish frequently and having healthier-looking brains:

“More research is needed as to why these types of fish may have protective effects, but the omega-3 fatty acids EPA and DHA would seem to have a major role.”

Fish that provide high levels of long-chain “marine” omega-3s include Salmon, Sardines, Tuna, Sablefish, Mackerel, Herring, and Anchovies.

Fried fish seen to raise brain-lesion rates
As we reported last year, not all fish meals yield the same brain-protection benefits.

And that distinction was affirmed by the results of the new study,

According to Dr. Virtanen, “While eating tuna and other types of [fatty] fish seems to help protect against memory loss and stroke, these results were not found in people who regularly ate fried fish.”

In 2005, Harvard researchers reported their finding that, compared with eating fried fish less than once a month, eating fried fish more than once per week was associated with a 44 percent higher risk of stroke (Mozaffarian D et al. 2005).

(The term “fried fish” encompasses breaded, deep-fried fish products such as fish sticks and fish sandwich fillets.)

The Harvard researchers who reported this finding hypothesized that the greater stroke risk linked to eating fried fish stems from consequent overconsumption of omega-6 fatty acids and trans omega-6 fats, which constitute the vast majority of fatty acids in standard deep-frying oils (e.g., soy, corn, cottonseed, and “low-oleic” safflower or sunflower oils).

Americans’ diets typically contain amounts of omega-6s considered enormous in relation to human history until the 20th century, and this overload is undesirable for three reasons:
  • Unlike omega-3s, which moderate or reduce inflammation, omega-6s typically promote chronic inflammation and thereby raise the risk that arterial plaque will rupture and block blood flow, causing a heart attack or stroke.
  • Unlike omega-3s, omega-6s do not confer the specific cardiovascular benefits that would reduce the risk of ischemic stroke (“brain attack”) – by far the most common kind, resulting from blood clots or flow constrictions.
  • A large proportion of the omega-6s in fried foods occur in “trans” or other undesirable forms that raise the risk of atherosclerosis and heart attack, hence the risk of ischemic stroke.
  • Omega-3s and omega-6s compete for places in the post-digestive enzymatic processes that get dietary fatty acids into cell membranes, from where they exert all of their metabolic and immune system influences.
So, when you eat a food that’s high in omega-6s relative to omega-3s – such as fried fish – fewer of its omega-3s will make it to your cell membranes.
Strokes: Silent, whispering, or worse
Silent strokes are diagnosed with brain imaging that detects damage in people who did not have any apparent stroke symptoms.

However, some silent strokes may be better described as “whispering” because the symptoms are so minor that they don’t alarm the patient.

The warning signs of stroke are:
  • Sudden numbness or weakness of the face, arm or leg, especially on one side of the body;
  • Sudden confusion, trouble speaking or understanding;
  • Sudden trouble seeing in one or both eyes;
  • Sudden trouble walking, dizziness, loss of balance or coordination;
  • Sudden, severe headache with no known cause.
People need to take these symptoms seriously and see a doctor about them, and physicians should take the symptoms seriously when patients report them.

Stroke rates remain high
Stroke rates in America remain high, although they have fallen some over the past 50 years. One in seven men and one in six women aged 65 and older will suffer a diagnosed stroke.

One in three Americans aged 70 to 79 suffer a silent stroke each year, and the incidence of silent strokes rises as they age (Vermeer SE et al. 2003; Leary MC et al. 2003).

A stroke is an interruption of the blood supply to any part of the brain. If blood flow is stopped for more than a few seconds, brain cells die, causing permanent damage.

The most common type is called ischemic stroke, which accounts for about 85 percent of diagnosed strokes. The term “ischemia” means a deficient supply of blood to a body part (such as the heart or brain), due to obstruction of arterial blood flow.

For more on this topic, see “Stroke: An Update on Fish and other Factors”.

  • Mozaffarian D, Longstreth WT Jr, Lemaitre RN, Manolio TA, Kuller LH, Burke GL, Siscovick DS. Fish consumption and stroke risk in elderly individuals: the cardiovascular health study. Arch Intern Med. 2005 Jan 24;165(2):200-6. Erratum in: Arch Intern Med. 2005 Mar 28;165(6):683.
  • Prins ND, van Dijk EJ, den Heijer T, Vermeer SE, Jolles J, Koudstaal PJ, Hofman A, Breteler MM. Cerebral small-vessel disease and decline in information processing speed, executive function and memory. Brain. 2005 Sep;128(Pt 9):2034-41. Epub 2005 Jun 9.
  • Vermeer SE, Hollander M, van Dijk EJ, Hofman A, Koudstaal PJ, Breteler MM; Rotterdam Scan Study. Silent brain infarcts and white matter lesions increase stroke risk in the general population: the Rotterdam Scan Study. Stroke. 2003 May;34(5):1126-9. Epub 2003 Apr 10.
  • Vermeer SE, Prins ND, den Heijer T, Hofman A, Koudstaal PJ, Breteler MM. Silent brain infarcts and the risk of dementia and cognitive decline. N Engl J Med. 2003 Mar 27;348(13):1215-22.
  • Virtanen JK, Siscovick DS, Longstreth WT Jr, Kuller LH, Mozaffarian D. Fish consumption and risk of subclinical brain abnormalities on MRI in older adults. Neurology. 2008 Aug 5;71(6):439-46.
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