Omega-3s probably help prevent “brain attacks”; ER doctors don't endorse widely touted stroke treatment (TPA) 11/26/2007
As much as any medical matter can be determined with certainty,eating fish is believed to help prevent strokes.
In fact, the preponderance of evidence supports the hypothesis that fish and fish oil reduce the risk of stroke by about 25 percent.
And the credibility of positive results reported from the great majority of epidemiological studies is supported by the proven cardiovascular benefits of omega-3s from fish, which explain why fish should reduce stroke risks.
(A recent Swedish study yielded rare negative results, which we've addressed below: see “Stroke-study headlines prove highly misleading”.)
Naturally, there's more to the story, since there are different kinds of stroke, and different kinds and preparations of fish, with differing impacts on risks.
Before diving into the evidence concerning fish, let's take a closer look at the potentially disabling or deadly phenomenon known as stroke.
Strokes often go unnoticed, and come in two distinct forms
The risk of stroke is raised by the same factors associated with increased risk of heart attacks and sudden cardiac death:
- High blood pressure
- Chronic inflammation.
- Excessive abdominal fat
- Unhealthful cholesterol profiles
- High triglycerides and homocysteine levels
Stroke rates in America have fallen some over the past 50 years, but remain high. One in seven men and one in six women aged 65 and older will suffer a diagnosed stroke.
And stroke is a much bigger problem than these statistics suggest, because many of the strokes people suffer are undetected, “silent” strokes.
Hospitals and doctors report some 770,000 strokes in the US annually. However, researchers have conducted MRI brain scans on older people, and by projecting the results on the American population, they estimate that about 11 million of us suffer some 22 million asymptomatic, undiagnosed strokes annually (Leary MC et al 2003).
In other words, the annual number of “silent” strokes in America is about 30 times higher than the incidence of diagnosed strokes. Dutch researchers obtained similar results when they used MRI brain scans to detect undiagnosed strokes (Vermeer SE et al 2003).
Thus, one in three Americans and Northern Europeans 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).
What is stroke?
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.
Like heart attacks, ischemic strokes usually result from the condition called atherosclerosis, in which fatty, inflamed plaque in an artery wall ruptures and blocks or severely constricts blood flow.
Doctors sometimes call ischemic strokes “brain attacks” or brain infarctions because they result from the same cause that leads to most heart attacks (myocardial infarctions).
The term “infarct” means an area of dead tissue resulting from obstruction of blood flow, so a heart or brain “infarction” is any event that causes an infarct in one of these organs.
The main difference is that in a heart attack, the damage occurs to heart muscle (i.e., the myocardium) instead of brain tissue.
Hemorrhagic stroke is much less common, accounting for only about 15 percent of all diagnosed strokes, and very few “silent” strokes. It occurs when a blood vessel bursts inside the brain, often because of high blood pressure.
Fish consumption is proven to reduce the risk of ischemic stroke – which account for more than 80 percent of all strokes – but may offer less protection against hemorrhagic stroke.
What role does fish play?
In 2005, and again in 2006, researchers reviewed the available evidence, and concluded that higher intake of omega-3 fatty acids reduces the risk of ischemic stroke, which is the most common kind, by far (Bouzan C et al 2005; Psota TL et al 2006).
Compared with eating fish less than once a month, eating fish one to four times a week reduces the risk of ischemic stroke by 27 percent, while eating fish five or more times per week reduces the risk of stroke by 30 percent (Mozaffarian D et al 2005).
These findings related specifically to long-chain, “marine” omega-3s from fish oil and fish (EPA and DHA).
As has been the case in studies of heart attack and sudden cardiac death, the short-chain omega-3s in green vegetables and certain plant oils (canola, soy, walnut) do not appear to offer protection comparable to that provided by marine omega-3s.
There is no evidence that diets high in fish or fish oil either promote or prevent hemorrhagic strokes, despite the ability of omega-3s to reduce blood pressure slightly (Meyers VH, Champagne CM 2007; Yang H, Kenny A 2007).
Fried fish found counterproductive
Significantly, the anti-stroke benefits of fish do not apply to deep-fried fish (e.g., breaded fish, such as fish sticks and “fish-wiches”).
In fact, eating fried fish raises the risk of stroke. Compared with eating fried fish less than once a month, eating fried fish more than once per week raises the risk of ischemic stroke by 44 percent (Mozaffarian D et al 2005).
The Harvard researchers who conducted this study hypothesize that the distinct lack of protection from fried fish stems from the omega-6 fatty acids and “trans” omega-6 fats that predominate in the vegetable oils used to prepare deep-fried fish (e.g., soy, canola, safflower, sunflower, and cottonseed oils).
Omega-3s and omega-6s compete for space in our cell membranes, so fewer of the omega-3s in fried fish end up there.
In addition, there are three negatives associated with the omega-6s that constitute the vast majority of fatty acids in frying oil:
- Unlike omega-3s, which reduce inflammation, omega-6s promote chronic inflammation and thereby raise the risk that arterial plaque will rupture and block blood flow to the brain (and/or heart).
- Unlike omega-3s, omega-6s do not confer the specific cardiovascular benefits that would reduce the risk of stroke … especially the risk of ischemic stroke.
- 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
After analyzing medical records of 5,073 older adults, Harvard researchers found that people who consumed tuna or other broiled or baked fish regularly had better results on tests of cardiovascular health factors, while those who ate fried fish regularly showed structural abnormalities associated with high blood pressure and increased risk of coronary atherosclerosis.
As they wrote, “These findings suggest potential specific physiologic mechanisms that may, in part, account for the effects of fish intake on cardiovascular health.”
Stroke-study headlines prove highly misleading
As we said, not all evidence shows that fish reduces stroke risk, with two recent reports having made headlines that overstated their findings.
First, an epidemiological study published last year found no risk reduction among British fish eaters (Myint PK et al 2006) … nor did it find that eating fish raises the risk of stroke.
However, this study's failure to affirm the positive findings from most other studies raised few eyebrows, because uneven findings are expected from studies conducted in various countries, with differing diets, lifestyles, and genetic patterns.
More surprisingly, the results of a Swedish study reported this year seemed to suggest that eating fish might actually raise the risk of stroke in men, although the researchers found no such link in women (Wennberg M et al 2007).
However, there's little doubt, thanks to mountains of laboratory evidence, that the protective factor in fish is their omega-3 content. And most news stories failed to note that the risk of stroke was not greater for the men with the highest blood levels of marine omega-3s, which they could only have accumulated in such abundant quantities by eating ample amounts of fish and fish oil (EPA and DHA).
The fact is that, compared with people's answers to diet surveys, blood tests are much more reliable measure of omega-3 intake and absorption: blood tests are critical since, as the biomedical researchers we know like to stress, “The tissue is the issue.” That is, tissue tests that measure levels of food factors like omega-3s provide a far sounder basis for studying associations between nutrients and the risk of various diseases than do surveys.
So while the Swedish findings did not fit with the positive results reported from most fish-and-stroke studies, neither did they provide evidence that eating fish raises stroke risk.
Stroke signs and treatment
The American Heart Association lists these warning signs of stroke:
- 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
If you or someone with you has one or more of these signs, call 911 or an ambulance, and note the time when the first symptoms appeared.
But one part of the Heart Association's advice is more controversial: “It's very important to take immediate action. If given within three hours of the start of symptoms, a clot-busting drug can reduce long-term disability for the most common type of stroke”.
They refer here to a drug called TPA (tissue plasminogen activator), which can dissolve the clots that cause the most common kind of brain attack (ischemic stroke).
However, TPA will make a hemorrhagic stroke—which causes bleeding in the brain—even worse.
Clinicians read CT scans to diagnose strokes, and frequently misidentify hemorrhagic stroke as ischemic stroke, or diagnose other conditions as stroke.
And the major study upon which the hopes for TPA rest contains methodological flaws that make it unreliable (AAEM, 2007).
In fact, a recent study found higher death rates among patients receiving TPA, versus those who did not get the drug (Dubinsky R, Lai SM, 2006).
Many emergency room physicians believe that the risks of TPA outweigh the potential benefits, and the American Academy of Emergency Medicine does not consider use of TPA the current “standard of care” for treating stroke (AAEM, 2007).
- American Academy of Emergency Medicine (AAEM). Accessed online November 15, 2007 at http://www.aaem.org/positionstatements/thrombolytictherapy.php
- American Heart Association, Inc. Heart Attack, Stroke and Cardiac Arrest Warning Signs. Accessed online November 15, 2007 at http://www.americanheart.org/presenter.jhtml?identifier=3053
- Bouzan C, Cohen JT, Connor WE, Kris-Etherton PM, Gray GM, König A, Lawrence RS, Savitz DA, Teutsch SM. A quantitative analysis of fish consumption and stroke risk. Am J Prev Med. 2005 Nov;29(4):347-52.
- Bravata DM, Wells CK, Brass LM, Morgan T, Lichtman JH, Concato J. Dietary fish or seafood consumption is not related to cerebrovascular disease risk in twin veterans. Neuroepidemiology. 2007;28(3):186-90. Epub 2007 Aug 16.
- Ding EL, Mozaffarian D. Optimal dietary habits for the prevention of stroke. Semin Neurol. 2006 Feb;26(1):11-23. Review.