Essential Fatty Acids in
Vegetarian Nutrition
by Brenda Davis, RD
http://www.andrews.edu/NUFS/essentialfat.htm
Mahatma Gandhi once said, "Where ever flaxseeds become a regular food item among the people, there will be better health". While this prediction was based on simple observation, scientific evidence would suggest there is more than a grain of truth to his words. Flaxseeds are an exceptional source of lignans, a potent anticarcinogen and the richest known source of the essential omega-3 fatty acid, alpha-linolenic acid.
It is generally assumed that North Americans need not worry about getting enough fat of any kind, but as research unfolds, a fatal flaw in this thinking is revealed. Not all fats are damaging to health. Some fats are protective, and two in particular are essential to life they are the essential fatty acids (EFA), linoleic acid (omega-6) and alpha-linolenic acid (omega-3).
Changes in our food supply since the industrial revolution have jeopardized both the quantity and balance of these nutrients. Our current dietary pattern provides excessive amounts of omega-6 fatty acids in relation to omega-3 fatty acid content. This imbalance of essential fatty acids affects vegetarians at least as much as omnivores. In addition, the trend towards very low fat vegetarian diets (10% or less of calories from fat) may further compromise essential fatty acid intake.
Function of EFAs
Essential fatty acids are necessary for the formation of healthy cell membranes, the proper development and functioning of the brain and nervous system, and for the production of hormone-like substances called eicosanoids (thromboxanes, leukotrienes, prostaglandins). These chemicals regulate numerous body functions including blood pressure, blood viscosity, vasoconstriction, immune and inflammatory responses (3). Humans have the ability to convert the linoleic and alpha-linolenic acid to longer chain fatty acids, which serve as precursors for eicosanoids.
Eicosanoids formed from arachidonic acid (AA) (omega-6 family) have the potential to increase blood pressure, inflammation, platelet aggregation, thrombosis, vasospasm, allergic reactions and cell proliferation. Those formed from eicosapentanoic acid (EPA)(omega-3 family) have opposing affects (4,5). Omega-6 and omega-3 fatty acids are not interchangeable; we must consume both.
These two families of essential fatty acids compete for enzymes involved in their desaturation, thus the excessive consumption of foods rich in omega-6 fatty acids may compromise the conversion of alpha-linolenic acid to EPA, with adverse affects for health and disease. Current research suggests that the levels of essential fatty acids and the balance between them may play a critical role not only in growth and development, but also in the prevention and treatment of chronic diseases including coronary artery disease, hypertension, type II diabetes, arthritis and other immune/inflammatory disorders, and cancer (6-11).
The primary sources of essential fatty acids are plants on land and in the sea. Linoleic acid is found primarily in seeds, nuts, grains and legumes. Alpha-linolenic acid is found in the green leaves of plants, including phytoplankton and algae, and in selected seeds, nuts and legumes (flax, canola, walnuts and soy). Arachidonic acid (AA) and docosahexaenoic (DHA) acid are obtained directly from animal foods AA from meat and poultry and DHA and EPA from fish.
How much omega-6 and omega-3 do vegetarians need?
There are two primary considerations when assessing the adequacy of these essential fatty acids: quantity and balance. The World Health Organization recommends that polyunsaturated fats make up 3 -7% of the energy in the diet (12), without any specification as to the amount needed from each family of fats. However, experts advise that one should consume a minimum of 3% of energy from omega-6 fatty acids and 0.5% from omega-3 fatty acids. Many now suggest that infants, and others who do not consume preformed EPA and DHA, should consume 1% of their energy needs as omega-3. This would include vegetarians and others who do not eat fish (13). While no definitive recommendations are in place for pregnant and lactating vegetarians, it may be appropriate to increase the intake of alpha-linolenic acid to 2% of calories due to the importance of long chain omega-3 fatty acids in the developing fetus and infant .
An adult consuming 2000 calories could achieve the recommended minimum 3% omega-6 fatty acids and 1% omega-3 fatty acids with 60 calories of linoleic acid (6.7 g) and 20 calories of alpha-linolenic acid (2.2 g). Obtaining 6.6 g of linoleic acid is easy on almost any diet, even those that are very low fat (10-15% fat). Omega-3 fatty acids are not as plentiful in our food supply, and the primary source for most North Americans is fish. Vegetarians and others not eating fish are well advised to include omega-3-rich plants in their diet on a regular basis.
Scientists use the ratio of omega-6 fatty acids to omega-3 fatty acids to assess the balance between essential fatty acids in the diet. Research scientists from around the world recommend ratios varying from 5:1 to 10:1, while some experts suggest a ratio of between 1:1 and 4:1 as being optimal. The current ratio in our diet is estimated to be 14:1 to 20:1 with some studies indicating higher ratios in vegetarian populations compared to omnivorous populations.
Are plant sources of omega-3 sufficient for human needs?
Vegetarians and vegans have no direct sources of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (long chain omega-3 fatty acids) in the diet, hence they must convert alpha-linolenic acid to EPA and DHA in the body. Researchers have questioned whether this conversion is adequate to meet human needs for long chain omega-3 fatty acids. Scientific studies suggest that although the conversion is slow and incomplete (perhaps only 10% of alpha-linolenic acid is converted), and although vegetarians tend to have lower blood levels of long chain omega-3 fatty acids , it is sufficient to meet the needs of most people .
It is important to note, however, that certain factors can depress the enzymes responsible for the desaturation of alpha-linolenic acid, thus adversely affecting this important conversion process. These factors include high intakes of saturated fat, trans fatty acids, cholesterol and alcohol, an inadequate intake of energy or protein, or a deficiency of certain nutrients, such as zinc or copper. There may also be conversion problems for people with diabetes or other metabolic disorders and for those who inherit a limited ability to produce conversion enzymes (possibly where fish has been a major component of the diet for centuries).
In addition, infants convert alpha-linolenic acid to DHA and EPA more slowly than adults. Studies have provided evidence that preterm infants do not have the capacity to form sufficient DHA, resulting in reduced visual acuity and brain function. Thus DHA must be considered an essential nutrient for these babies. Currently, infant formulas in the U.S. are not fortified with DHA, although several companies have patented DHA blends for this purpose and DHA-fortified formulas are expected to hit the U.S. market sometime this year. Several European countries, including the United Kingdom, Belgium, the Netherlands, Finland and Spain, presently produce DHA-fortified formulas.
Breastfed infants generally receive ample DHA from their mother's milk, although amounts vary considerably depending on maternal intake of omega-3 fatty acids. Vegetarian and vegan mothers have lower concentrations of DHA in their milk, although infant levels of DHA appear to be only slightly less than that of infants of omnivorous mothers. A DHA supplement based on cultured microalgae (under the trademark Neuromins) is now available from natural food stores nationwide.
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