It’s no surprise that fish oil is in great demand, given the evidence that the polyunsaturated fatty acids it contains—particularly omega-3 fatty acids EPA and DHA—are associated with neurological development and reduced risk of coronary heart disease. The bulk of epidemiological and clinical evidence supports recommendations that dietary omega-3 fatty acids can reduce the incidence of heart disease, particularly among those with a medical history of the disease.
The American Heart Association (AHA) notes that an intake of 0.5–1.8 g of DHA + EPA—either from fatty fish or dietary supplements—may reduce the risk of mortality from cardiovascular events, and ingestion of 1.5–3.0 g of α-linolenic acid (n-3) may be beneficial. Interestingly, except for linoleic acid (n-6) and α-linolenic acid (n-3) in infant formula, none of the guidelines for essential fatty acids addresses an "optimal" ratio or appropriate balance of dietary n-6 and n-3 fatty acids.
Wijendran and Hayes (2004) note the importance of both n-6 and n-3 fatty acids and their influence on heart disease and other discrete biological effects. They indicate that the practical balance of these kinds of fatty acids vs their ratio should be considered in an effort to manage and reduce the risk of coronary heart disease. The authors’ review suggests the amount of these fatty acids is more important in maintaining heart health than their ratio. Several critiques of the n-6 and n-3 balance controversy (Lands, 2008; Harris, 2006) acknowledge the cardio-protective significance of unsaturated fatty acids while stressing the role of these fatty acids in the inflammatory processes and an array of maladies. The authors advocate the importance of providing practical intervention to reduce the incidence of these diseases.
An earlier publication (Lands, 2003) suggests that an imbalance of highly unsaturated fatty acids in our diet, particularly n-6 fatty acids, contributes to dyslipidemia, heart disease, and premature mortality. This assessment indicates individuals in the United States are among those with the highest risk of mortality for heart disease, primarily due to apparently excessive intake of n-6 fatty acids, including those that may be considered healthy fatty acids like linoleic acid (18:2), gamma-linolenic acid (18:3), and arachidonic acid (20:4). It is important to remember that n-6 and n-3 fatty acids often compete for the same metabolic enzymes. Considering the pharmacokinetics of fatty acid metabolism, it may be that increased n-6 fatty acid intake accentuates the need for greater n-3 consumption in order to achieve a healthful benefit and that this ratio may not be an appropriate metric for assessing a healthful diet.
Increasing dietary n-3 intake via food sources is possible. Many organizations, including the AHA and the American Dietetic Association, advise that the consumption of 2 servings/week of fatty fish can provide adequate amounts (~500 mg) of DHA + EPA. Interestingly, while a serving (3 oz) of wild Atlantic salmon provides approximately 1,400 mg of DHA + EPA, wild Pacific and Atlantic salmon also contain roughly 1,000 mg n-6 fatty acids per serving and present a natural n-6:n-3 ratio of 1:10 (Hamilton et al., 2005). This ratio may be modified to 1:3-1:4 in farmed salmon. As noted by Wijendran and Hayes, practical intakes of n-6 and n-3 fatty acids suggest a ratio of 1:6 is achievable for most healthy adults.
Clearly, not all fish are rich in n-3 fatty acids, and virtually all fish contain n-6 fatty acids. Weaver et al. (2008) recently examined the n-3 and n-6 fatty acid concentration of 30 commonly eaten species of wild and farmed fish. The data indicated that the n-6 fatty acid concentration of these fish samples was approximately 10-15%, and the average n-6:n-3 ratio varied from 1:1 (farmed Atlantic salmon) to 3:1 (farmed catfish).
As consumers seek omega-3–containing foods, nutrition epidemiologists will continue to assess n-6 and n-3 intake, and clinical investigators will identify and examine other endpoints or biomarkers. N-3 fatty acids in particular present functions beyond modulation of dyslipidemia and may decrease the risks associated with the continuum of heart diseases, enzyme systems, inflammatory responses, and platelet aggregation (Harris, 2006). These functions and their metabolic interrelationships command us to further examine the potential clinical relevance and possible public health significance of these fatty acids as part of a healthful diet.
References for the studies cited above are available from the authors.
by Roger Clemens, Dr.P.H.,
Scientific Advisor, ETHorn, La Mirada, Calif.
by Peter Pressman, M.D.,
LCDR, Medical Corps, U.S. Navy