Fats, Fish Oil and Omega-3-Fatty Acids (cont.)

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. Life style, 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, p23. 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 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 is usually due to a high consumption of fish).

The Physician's health study found no association between fish and omega-3-fatty acid consumption and risk of developing nonfatal coronary heart diseases such as heart attacks without sudden death. Thus the scientists suspected that the major benefit of fish oil and omega-3-fatty acid was in preventing ventricular fibrillation in event of a heart attack.

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". They 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).

What are the shortcomings of observational studies?

Even though carefully conducted observational studies like the Nurses' health study and Physician's health study can provide useful scientific information, observational studies cannot differentiate between cause and association. In other words, the Physician's heath study found only an association between eating fish and lower risk of sudden cardiac death. The Nurses' health study only found an association between eating fish and lower risk of coronary heart disease and death from heart disease. Neither study could conclusively prove that eating fish caused these health benefits. Therefore, carefully conducted interventional studies are necessary to confirm findings from observational studies.

Interventional studies

Interventional 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 will be given the medication (or diet) being tested, while the subjects in the control group will usually be given an inert substance (placebo). Subjects are assigned to each group randomly to prevent either group from having sicker (or older) subjects that might bias the study. Subjects in both groups are evaluated the same way before treatment and during treatment. This way the beneficial or harmful effects of treatment can be determined by comparing the results of treatment against the results of the placebo.

Two interventional 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 recurrence of coronary heart disease in patients who recently survived a heart attack. The Lyon Diet Heart Study studied the effect of 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-fatyy 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 50% reduction in sudden cardiac death and in repeat fatal or nonfatal heart attacks.

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 recurrence of both fatal and nonfatal heart attacks. The reasons for the difference in the two studies are not clear; maybe there are additional cardiac protective factors in the Mediterranean diet.



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