Omega-3 Fatty Acids (cont.)
What is the scientific evidence behind omega-3 fatty acids?
Animal studies
In studies involving animals (dogs, rats, and marmosets), omega-3 fatty acids
were found to prevent ventricular fibrillation (see MedicineNet article on heart
attack) when given to animals just prior to experimentally induced heart
attacks. Omega-3 fatty acids were also found to terminate ventricular
fibrillation in animals undergoing experimentally induced heart attacks.
Therefore, scientists suspect that omega-3 fatty acids may prevent ventricular
fibrillation of the heart in the event of a heart attack in humans. Since
ventricular fibrillation is the most important cause of sudden death among heart
attack victims, omega-3-fatty acids is believed to prevent sudden death.
Evidence from observational studies
Two large, long-term observational studies
have been published on the relationship between dietary intake of fish oil and
omega-3 fatty acids and risk of heart disease and sudden cardiac death; The
Nurses' Health Study and the Physician's Health Study.
The Physician's Health
Study began in 1982 when more than 20,000 healthy male physicians were followed
for 11 years. Lifestyle, coronary risk factors, and diet data were collected at
entry, and life style and diet data were collected via questionnaires at 12
months and 18 months. The results of the study were published in JAMA 1998, vol.
279, p. 23. The title of the article is "Fish consumption and risk of sudden
cardiac death." The study found that men who consumed one or more fish meals per
week had a 50% lower risk of developing sudden cardiac death than men who rarely
ate fish (less than one fish meal per month).
In a separate article, scientists
compared blood levels of omega-3 fatty acids in 94 of these men who died of
sudden cardiac death against living men matched for age and smoking habits. They
found that high levels of omega-3 fatty acids in the blood were associated with
a low risk of cardiac sudden death. Men with the highest blood levels of omega-3
fatty acids had an 80% lower risk of sudden cardiac death than men with the
lowest blood levels. High omega-3 fatty acids in the blood are usually due to a
high consumption of fish.
The Nurses' Health Study began in 1976 when more than
80,000 female nurses completed life style and diet questionnaires. They were
followed for 16 years for the development of coronary heart disease. Omega-3
fatty acid consumption was calculated from the questionnaires. The result of the
study was published in JAMA vol 287. No.14, p. 1815. The title of the article is
"Fish and omega-3-fatty acid intake and risk of coronary heart disease in
women." The study found that compared to women who rarely ate fish (less than
one fish meal per month), women who ate fish once a week had a 29% lower risk of
developing coronary heart disease. Those who ate fish five times a week had a
34% reduction in the risk of coronary heart disease and a 45% reduction in the
risk of death from heart disease (usually sudden cardiac death).
Controlled Studies
Controlled studies are prospective studies that randomly (by chance, e.g., by
a flip of a coin) assign subjects to two groups, a treatment group and control
group. Subjects in the treatment group are given the medication (or diet) being
tested, while the subjects in the control group are usually given an inert
substance (placebo).
Two controlled studies on omega-3 fatty acids and fish oil
have recently been published. The GISSI-Prevention trial studied the effect of
omega-3 fatty acids on sudden death and the recurrence of coronary heart disease
in patients who recently survived a heart attack. The Lyon Diet Heart Study
studied the effect of a Mediterranean diet (see discussion below) on the
recurrence of coronary heart disease in patients who recently survived a heart
attack.
The GISSI-Prevention trial randomly assigned more than 11,000 patients
with recent heart attacks to four treatment groups; omega-3 fatty acids (850 mg
capsule daily), vitamin E, both, or neither (control). The study subjects in all
four groups were followed for 3.5 years. Study results were published in
Circulation, 2002; 105:1897-1903. Subjects given omega-3 fatty acids had a 20%
lower death rate from coronary heart disease than subjects in the vitamin E and
control groups. Omega-3 fatty acids were particularly effective in preventing
sudden cardiac death (45% reduction in sudden cardiac death). The scientists
believed that the benefits of omega-3 fatty acids are in the prevention of
ventricular fibrillation. Vitamin E was found to have no benefit compared to
controls.
The Lyon Diet Heart Study randomly assigned 600 subjects who survived
recent heart attacks to either the Mediterranean diet or a prudent Western diet
(diet low in saturated fat and cholesterol). The Mediterranean diet is rich in
omega-3 fatty acids, as well as monounsaturated fats, fruits, vegetables, and
nuts. The study subjects were followed for 4 years. The results of the study
were published in Circulation, 1999; 99:779-785. The subjects eating the
Mediterranean diet had more than a 50% reduction in sudden cardiac death and in
repeat fatal or nonfatal heart attacks as compared to the Western diet group.
The GISSI-Prevention trial found that omega-3 fatty acids prevented sudden
cardiac death, but did not prevent repeat heart attacks. The Lyon Diet Heart
Study found that the Mediterranean diet not only prevented sudden cardiac death,
but also prevented the recurrence of both fatal and nonfatal heart attacks. The
reasons for the difference in the two studies are not clear. Perhaps there are
additional cardiac protective factors in the Mediterranean diet.
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