By Dr. Artemis P. Simopoulos
Over the past 20 years many studies and clinical investigations have been carried out on the metabolism of polyunsaturated fats in general and on omega-3 fats in particular.
Today we know that omega-3 fats are essential for normal growth and development and may play an important role in the prevention and treatment of coronary artery disease, hypertension, diabetes, arthritis, other inflammatory and autoimmune disorders, and cancer (1-7).
Research has beendone in animal models, tissue cultures, and human beings.The original observational studies have given way to controlled clinical trials. Great progress has taken place in our knowledge of the physiologic and molecular mechanisms of the various fats in health and disease. Specifically, their beneficial effects have been shown in the prevention and management of:
- Coronary heart disease (8, 9),
- Hypertension (10-12)
- Type 2 diabetes (13, 14)
- Renal disease (15, 16)
- Rheumatoid arthritis (17)
- Ulcerative colitis (18)
- Crohn disease (19)
- Chronic obstructive pulmonary disease (20)
However, this review focuses on the evolutionary aspects of diet, the biological effects of omega-6 and omega-3 fats, and the effects of dietary-linolenic acid (ALA) compared with long-chain omega-3 derivatives on coronary heart disease and diabetes.
Essential Fats Such As Omega-6 And Omega-3 Have Been Part Of Our Diet Since The Beginning Of Human Life
Before the agricultural revolution 10,000 years ago humans consumed about equal amounts of both. Over the past 150 years this balance has been upset. Current estimates in Western cultures suggest a ratio of omega-6 to omega-3 fats of 10-20:1 instead of 1-4:1.
On the basis of estimates from studies in Paleolithic nutrition and modern-day hunter-gatherer populations, it appears that human beings evolved consuming a diet that was much lower in saturated fats than in today's diet (21).
Furthermore, the diet contained small and roughly equal amounts of omega-6 and omega-3 PUFAs (ratio of 1-2:1) and much lower amounts of trans fats than does today's diet (21, 22). The current Western diet is very high in omega-6 fats (the ratio of omega-6 to omega-3 fats is 20-30:1) because of the indiscriminate recommendation to substitute omega-6 fats for saturated fats to lower serum cholesterol concentrations (23).
Intake of omega-3 fats is much lower today because of the decrease in fish consumption and the industrial production of animal feeds rich in grains containing omega-3 fats, leading to production of meat rich in omega-6 and poor in omega-3 fats (24). The same is true for cultured fish (25) and eggs(26).
Even cultivated vegetables contain fewer omega-3 fats than do plants in the wild (27, 28). In summary, modern agriculture, with its emphasis on production, has decreased the omega-3 fat content in many foods: green leafy vegetables, animal meats, eggs, and even fish.
Biological Effects of Omega-6and Omega-3 Fats
Linoleic acid and alpha linolenic acid ALA and their long-chain derivatives are important components of animal and plant cell membranes. When you eat fish or fish oil, the EPA and DHA partially replace the omega-6 fats especially arachidonic acid in cell membranes.
As a result eating EPA and DHA from fish or fish oil leadsto:
1) Decreased concentrations of thromboxane A2, a potent platelet aggregator and vasoconstrictor;
2) Decreased formation of leukotriene B4, an inducer of inflammation and a powerful inducer of leukocyte chemotaxis and adherence;
3) Increased concentrations of thromboxane A3, a weak platelet aggregator and vasoconstrictor;
4) Increased concentrations of prostacyclin PGI3, leadingto an overall increase in total prostacyclin by increasing PGI3 without decreasing PGI2 (both PGI2 and PGI3 are active vasodilators and inhibitors of platelet aggregation); and
6) Increased concentrations of leukotriene B5, a weak inducer of inflammation and chemotactic agent (29, 30).
Because of the increased amounts of omega-6 fats in the Western diet, the eicosanoid metabolic products from arachadonic acid, specifically prostaglandins, thromboxanes, leukotrienes, hydroxy fats, and lipoxins, are formed in larger quantities than those formed from omega-3 fats, specifically EPA.
A diet rich in omega-6 fats shifts the physiologic state to one that is prothrombotic and proaggregatory, with increases in blood viscosity, vasospasm, and vasoconstriction and decreases in bleeding time.
The higher the ratio of omega-6 to omega-3 fats the higher is the death rate from cardiovascular disease (33). As the ratio of omega-6 to omega-3 increases, the prevalence of type 2 diabetes also increases (13).
Effects of Dietary Flax Compared With Fish Oil
ALA, found in flax seed is the precursor of omega-3 fats,can be converted to long-chain omega-3 fats and can therefore be substituted for fish oils.
However, ALA is not equivalent in its biological effects tothe long-chain omega-3 fats found in marine oils. EPA and DHA are more rapidly incorporated into plasma and membrane lipids and produce more rapid effects than does ALA.
Experimental studies suggest that intake of 3-4 grams of ALA per day is equivalent to 0.3 grams (300 mg) EPA per day.
Relatively large reserves of LA in body fat, as are found in vegans or in the diet of omnivores in Western societies, would tend to slow down the formation of long-chain omega-3 fats like EPA and DHA from ALA.
One advantage of the consumption of ALA over omega-3 fats from fish is that the problem of insufficient vitamin E intake does not exist with high intake of ALA from plant sources.
Benefits of Omega-3 Fats
Dietary intake of omega-3 fats from seafood was associated with reduced risk of primary cardiac arrest compared with no fish intake; 5.5g omega-3 fats per month or the equivalent of 1 fatty fish meal per week was associated with a 50% reduction in the risk of primary cardiac arrest.
A 5.0% increase in omega-3 fats was associated with a 70% reduction in the risk of primary cardiac arrest.
An increase in EPA and DHA also leads to increases in membrane fluidity, the number of insulin receptors, and insulin action.
Clinical interventions provide further support for the beneficial effects of omega-3 fats in the prevention and management of cardiovascular disease, hyperinsulinemia, and possibly type 2 diabetes.
Omega-3 fats affect coronary heart disease beneficially not by changing serum lipid concentrations, although EPA and DHA do lower triglycerides, by reducing blood clotting in vessel walls (72, 76) and ventricular arrhythmias (8, 9, 75, 77).
Am.J. Clinical Nutrition, September 1999; 70: 560 - 569