Importance of Omega Three Fats in Health and Disease
By Dr. William Connor
From American Journal of Clinical Nutrition, Vol. 71, No. 1, 171S-175S, January 2000
Interest in omega-3 fatty acids began some 30 years ago and there are now several thousand papers in the scientific literature supporting their benefits.
There is little doubt that omega-3 fatty acids are important in human nutrition. They are significant structural components of the cell membranes of tissues throughout the body and are especially rich in the retina, brain, and sperm, in which docosahexaenoic acid (DHA) constitutes 36.4% of total fatty acids
Membrane fluidity is essential for proper functioning of these tissues. In the retina, where omega-3 fatty acids are especially important, deficiency can result in decreased vision and abnormal electroretinogram results.
Omega-3 Fatty acids are essential fatty acids, necessary from conception through pregnancy and infancy and, undoubtedly, throughout life.
The ratio of omega-6 to omega-3 fatty acids has increased in industrialized societies because of increased consumption of vegetable oils rich in omega-6 fatty acids, ie, linoleic acid, and reduced consumption of foods rich in omega-3 fatty acids.
Another important feature of omega-3 fatty acids is their role in the prevention and modulation of certain diseases that are common in Western civilization.
The following is a partial list of diseases that may be prevented or ameliorated with omega-3 fatty acids, in descending order of the strength of the available evidence as perceived by this reviewer:
- Coronary heart disease and stroke;
- Essential fatty acid deficiency in infancy (retinal and brain development);
- Autoimmune disorders (e.g., lupus and nephropathy);
- Crohn disease;
- Cancers of the breast, colon, and prostate;
- Mild hypertension; and
- Rheumatoid arthritis.
Cardiovascular Benefits Of Omega-3 Fatty Acids
The strongest evidence of a relation between omega-3 fatty acids and disease is the inverse relation between the amount of omega-3 fatty acids in the diet and in blood and tissues and the occurrence of coronary heart disease and its many complications.
Effects of omega-3 fatty acids on coronary heart disease have been shown in hundreds of experiments in animals, humans, tissue culture studies, and clinical trials.
Omega-3 fatty acids from fish have been shown to be protective of heart disease and, by a variety of mechanisms, prevent deaths from coronary disease, particularly cardiac arrest.
The unique properties of these fatty acids in coronary heart disease first became apparent in the investigations of the health status of Greenland Eskimos who consumed diets very high in fat from seals, whales, and fish and yet had a low rate of coronary heart disease.
Further studies clarified this paradox. The fat the Eskimos consumed contained large quantities of the very-long-chain and highly polyunsaturated fatty acids of EPA and DHA, which are abundant in fish, shellfish, and sea mammals and are scarce or absent in land animals and plants. EPA and DHA are synthesized by phytoplankton, which are the plants of the waters and the base of the food chain for marine life.
Dietary omega-3 fatty acids act to prevent heart disease through a variety of actions. They:
- Prevent arrhythmias (ventricular tachycardia and fibrillation),
- Prostaglandin and leukotriene precursors,
- Have anti-inflammatory properties,
- Inhibit synthesis of cytokines and mitogens,
- Stimulate endothelial-derived nitric oxide,
- Have hypolipidemic properties with effects on triglycerides and VLDLs, and
- Inhibit atherosclerosis.
EPA and DHA have strong antiarrhythmic action on the heart. In experimental animals and tissue culture systems, EPA and DHA prevent the development of ventricular tachycardia and fibrillation.
Even total mortality has been improved in several studies in which the omega-3 fatty acid intake was increased. In one study, men who consumed salmon 1 time/wk had a 70% less likelihood of cardiac arrest.
In another study overall mortality was decreased by 29% in men with overt cardiovascular disease who consumed omega-3 fatty acids from fish or fish oil, probably because of the reduction in cardiac arrests.
The most recent data on fish consumption and risk of sudden cardiac death were from the Physician's Health Study in the United States in 20551 male physicians. Consumption of 1 fish meal/week was associated with a 52% lower risk of sudden cardiac death compared with consumption of < 1 fish meal/month.
Total Death Rate Was Also Lower In Those Who Ate Fish.
Thrombosis, or the tendency to form blood clots, is a major complication of coronary atherosclerosis that can lead to heart attacks.
The omega-3 fatty acids from fish oil have powerful antithrombotic actions. EPA inhibits the synthesis of thromboxane A2 from arachidonic acid in platelets. This prostaglandin causes platelet aggregation and vasoconstriction.
As a result, fish oil ingestion by humans increases the bleeding time and decreases the stickiness of the platelets for aggregation to glass beads. In addition, the administration of fish oil enhances the production of prostacyclin, a prostaglandin that produces vasodilation and less sticky platelets.
The EPA and DHA contained in fish oil fed to experimental animals actually inhibited development of atherosclerosis. There is evidence in both pigs and monkeys that dietary fish oil prevents atherosclerosis by actions other than reducing plasma cholesterol concentrations.
Atherosclerotic plaque formation may also be lessened by the reduction in growth factors after fish-oil consumption. Not only is platelet-derived growth factor diminished by fish oil consumption, but its messenger RNA is reduced. Because atherosclerosis begins with cellular proliferation in response to the influx of cholesterol-rich lipoproteins, the inhibition of this proliferation would greatly reduce the growth of the atherosclerotic plaque.
Reduced Cholesterol Levels
The pronounced effect of fish oil on high blood fats or elevated cholesterol levels is especially well documented and is supported by results of precise dietary studies in which the effects of a diet rich in salmon oil were compared with those of a vegetable oil and a diet high in saturated fat.
Fish oil in particular was shown to lower plasma cholesterol and triglyceride concentrations. Apolipoprotein B production is reduced by consumption of fish oil in comparison with vegetable oils such as safflower or olive oil.
Omega-3 Fatty Acids Essential Components Of Cell Membranes In Infancy
There are 2 critical periods for the acquisition of these essential omega-3 fatty acids: during fetal development and after birth until the biochemical development in the brain and retina is completed.
As already noted, the omega-3 fatty acid DHA is an important constituent of the cell membrane of these neural structures.
Omega-3 fatty acid deficiency is manifested in both the blood and in tissue biochemistry. Of note is a strikingly low concentration of DHA, which may fall to as much as one-fifth of the normal amount.
In addition, the body attempts to replace the deficient DHA with another highly polyunsaturated fatty acid of the omega-6 series. In rhesus monkeys, omega-3 fatty acid -- deficient diets fed to pregnant animals and then continued after birth induce profound functional changes such as reduced vision, abnormal electroretinograms, impaired visual evoked potential, more stereotypic behavior (e.g., pacing), and, perhaps, disturbances of cognition.
Some of these findings have been replicated in infants fed formulas deficient in omega-3 fatty acids. Most studies of premature infants have shown visual impairment and abnormal electroretinograms.
A recent study in full-term infants, in which a standard infant formula was compared with human milk and with formulas enriched with DHA, provided unequivocal evidence of considerable differences in visual evoked potential.
In all of the human studies, the biochemical evidence in plasma, red blood cells, and, occasionally, in tissues from autopsied infants has substantiated the omega-3 fatty acid deficiency state. The lower concentrations of DHA in plasma and erythrocytes are mirrored by lower concentrations in the brain and retina. Formula-fed infants have lower concentrations of brain DHA than do infants fed human milk. They also have lower intelligence quotients.
During pregnancy, both maternal stores and dietary intake of omega-3 fatty acids are of importance in insuring that the baby has adequate amounts of omega-3 fatty acids at the time of birth.
All the polyunsaturated fatty acids, including DHA, are transferred across the placenta into fetal blood. In addition, EPA and DHA in maternal adipose tissue can be mobilized as free fatty acids bound to albumin and be made available to the developing fetus via placenta transport.
Several studies in monkeys have indicated that when the maternal diet is deficient in omega-3 fatty acids, the infant at birth is likewise deficient as evidenced by low DHA concentrations in their plasma and red blood cells.
In humans, it was shown that the administration of fish oil or sardines to pregnant women led to higher DHA concentrations in both maternal plasma and red blood cells and in cord blood plasma and red blood cells at the time of birth.
Once membrane phospholipids have adequate concentrations of DHA, there is an avid retention of these fatty acids in the brain and the retina, even though the diet may subsequently be deficient. Several studies illustrate clearly the effects of omega-3 deficiency in both animals and humans.
American Journal of Clinical Nutrition, Vol. 71, No. 1, 171S-175S, January 2000