Omega-6 Fatty Acids
Omega-6 fatty acids are one of two groups of essential fatty acids (EFAs) that are required in human nutrition. (The other is the omega-3 fatty acid group.) Omega-6 fatty acids include linoleic acid and its derivatives. Essential means that these fatty acids must be consumed in the diet because humans cannot manufacture them from other dietary fats or nutrients, nor can they be stored in the body. They must be consumed daily to meet the body's requirements. They are macronutrients, required in amounts of grams per day (compared to micronutrients such as vitamins, which are required in milligrams per day). EFAs provide energy and are also components of nerve cells, cellular membranes, and are converted to hormone-like substances known as prostaglandins.
In the body, prostaglandins and EFAs are necessary for normal physiology, including:
- producing steroids and synthesizing hormones
- regulating pressure in the eye, joints, and blood vessels
- mediating immune response
- regulating bodily secretions and their viscosity
- dilating or constricting blood vessels
- regulating collateral circulation
- directing endocrine hormones to their target cells
- regulating smooth muscles and autonomic reflexes
- being primary constituents of cell membranes
- regulating the rate of cell division
- maintaining the fluidity and rigidity of cellular membranes
- regulating the inflow and out-flux of substances into and out of cells
- transporting oxygen from red blood cells to the tissues
- maintaining proper kidney function and fluid balance
- keeping saturated fats mobile in the blood stream
- preventing blood cells from clumping together (conglomeration, which is the cause of atherosclerotic plaque, and blood clots, can be a cause of stroke)
- mediating the release of inflammatory substances from cells that may trigger allergic conditions
- regulating nerve transmission and communication
- being the primary energy source for the heart muscle
EFAs are therapeutic, and clinical trials have shown that EFAs protect against such conditions as heart disease; cancer; autoimmune diseases, including rheumatoid arthritis and multiple sclerosis; skin diseases, including acne, atopic eczema, and psoriasis; and may protect against stroke. The prevalence of heart disease in populations has been shown to be inversely proportional to the relative concentration of linoleic acid in the diet.
Both linoleic acid and its derivatives are obtained from plant and animal sources. Plant sources include unprocessed, unheated vegetable oils such as corn, sunflower seed, safflower, soy, sesame, and cottonseed oils. They are also found in plant materials such as evening primrose, black currant seeds, and gooseberry oils as well as in raw nuts and seeds, legumes, and leafy greens. Animal sources of omega-6 fatty acids (although in smaller amounts than in plants) are lean meats, organ meats, and breast milk.
Linoleic acid is an 18-carbon long polyunsaturated fatty acid containing two double bonds. Its first double bond occurs at the sixth carbon from the omega end, classifying it as an omega-6 oil. As linoleic acid is absorbed and metabolized in the human body, it is converted into a derivative fatty acid, gamma linoleic acid (GLA), which is converted into di-homo-gamma linoleic acid (DGLA) and arachidonic acid (AA). The DGLA and AA are then converted into two types of prostaglandins by adding two carbon molecules and removing hydrogen molecules. There are three families of prostaglandins, PGE1, PGE2, and PGE3. DGLA is converted to PGE1, while AA is converted into PGE2. PGE3 is made by the conversion of omega-3 fatty acids. Both PGE1 and PGE3, anti-inflammatory agents, protect against coronary disease by keeping blood platelets slippery and flowing, thus preventing blood clotting. PGE2 is inflammatory and increases platelet stickiness and blood clotting. All three forms must be present to ensure a functioning clotting system. There must be enough PGE2 to ensure healthy clotting, but enough PGE1 and PGE3 to protect against too much clotting, which can lead to hardening of the arteries, heart attack, and stroke. Likewise, PGE1 appears to act as a diuretic, while PGE2 aids in the retention of water and salts in the kidneys. PGE2 also is required for healthy brain and synapse functioning. The three types of prostaglandins serve as a system of checks and balances within the body.
However, if AA and its derivative, PGE2, are over-produced or imbalanced with PGE1 and PGE3, they can
Daily consumption of omega-6 fatty acids by many people may be excessive, due to the presence of omega-6 fatty acids in common cooking vegetable oils and processed foods. The ratio of omega-6 to omega-3 fatty acid consumption can often reach 20:1. To achieve a more desirable ratio, an approach is to eliminate sources of omega-6 fatty acids, especially those hidden in processed foods and to increase the amount of omega-3 fatty acids consumed through fish oil or flaxseed supplements. In addition, to convert the omega-6 fatty acids present in oils (such as corn, safflower, or soybean) to GLA requires that the oils be unprocessed and unheated and in the natural form (cis form). In oils that have undergone processing (heating and/or hydrogenation) to prolong shelf life (e.g., many store bought oils) or to form a solid at room temperature (e.g., shortening and margarine), the fatty acid structure has been changed to the trans form, and the conversion process of omega-6 fatty acids to GLA may be inhibited.
Most people receive sufficient amounts of omega-6 fatty acids in their diet. Deficiencies are considered to be rare and limited to people with severe malabsorption, short bowel syndrome, or for people on an extremely low fat diet. For those who are unable to convert LA to GLA, dietary supplements containing GLA can be taken to increase the production of prostaglandins. Evening primrose, black currant, and borage oil all contain GLA. For individuals with diabetes, GLA supplementation can improve nerve function and help prevent diabetic nerve disease. Long term exclusive or excessive use of flaxseed oil, which contains large amounts of omega-3 fatty acids, can result in omega-6 fatty acid deficiency and require the addition of oils containing omega-6 fatty acids to the diet.
Omega-6 fatty acids may be consumed either as linoleic acid in oils that contain high levels of linoleic acid, or in the converted form, GLA, in dietary supplements. Oils high in linoleic acid include soybean, peanut, corn, sunflower seed, cottonseed, soy, sesame, and safflower. There is no official recommended daily dose for omega-6 fatty acids. Taken in oil form, approximately 1–2% of the daily calorie intake, or 1 tsp (3–6 g per day), should prevent signs of deficiency in most adults. An optimal dose is around 3–6% of daily calorie intake, or 1 tbsp (9–18 g daily). Some researchers suggest that 10% of daily calorie intake as linoleic acid is required to maintain optimal function.
For GLA supplementation, primrose oil, borage oil, and black currant seed oil are available in capsule form.
Stress, alcohol consumption, and prescription medicines can interfere with the conversion of linoleic acid to its derivatives. Therefore, those with such conditions may benefit from the use of GLA supplementation to improve the production of prostaglandins. In addition, to improve prostaglandin production, the use of unprocessed, unheated omega-6 oils in the cis form is recommended.
Overconsumption of omega-6 oils in relation to consumption of omega-3 oils may lead to an overproduction of inflammation-producing prostagladins (PGE2s) and a scarcity of anti-inflammatory prostaglandins (PGE1s and PGE2s), which may lead to a variety of health problems.
Linoleic acid appears to have at least one negative effect on the human body. It appears to increase a person's risk of developing age-related macular degeneration (ARMD), a disease of the eye that leads to a progressive loss of vision and eventual blindness.
Gittleman, Ann Louise. Eat Fat, Lose Weight. Keats Publishing, 1999.
Murray, Michael T., and Jade Beutler. Understanding Fats and Oils: Your Guide to Healing with Essential Fatty Acids. Apple Publishing Co. Ltd., 2000.
"Harvard Study Outlines Role of Fats in Blinding Eye Disease." Angiogenesis Weekly (October 12, 2001).
Majumder, Barun, Klaus J. Wahle, Susan Moir, and Steven D. Heys. "Conjugated Linoleic Acid Reduces Breast Tumor Growth Both by P53-Dependent and P53-Independent Pathways." Journal of Nutrition 131 (November 2001): 3140S.
Rebecca J. Frey, PhD