by Brenda Davis, RD
Reproduced, with permission, from Issues in Vegetarian Dietetics, 1998;
Vol. 7(4):5-7
Vai alla traduzione in italiano
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 (1,2). 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 EFA's
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.
When addressing the issue of essential fatty acids in vegetarian diets, three key questions arise:
1. How much omega-6 and omega-3 fatty acids do vegetarians need?
2. Do vegetarians convert alpha-linolenic acid to EPA and DHA in sufficient quantity?
3. How can vegetarians insure an adequate intake of omega-3 fatty acids?
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 (14-16).
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 (1). 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 (17,18).
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)(8), and although vegetarians tend to have lower blood levels of long chain omega-3 fatty acids (19,20), it is sufficient to meet the needs of most people (21,22).
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 (5). 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 (23,24). 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 (25). 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 (26,27), although infant levels of DHA appear to be only slightly less than that of infants of omnivorous mothers (28). A DHA supplement based on cultured microalgae (under the trademark Neuromins) is now available from natural food stores nationwide.
How can vegetarians insure an adequate intake of omega-3?
The following four guidelines will help to insure an appropriate quantity and balance of essential fatty acids in vegetarian and vegan diets:
Limit intake of saturated fats and trans fatty acids. In vegetarian diets the primary sources of saturated fats are dairy products and eggs, while vegan diets are low in saturated fats, unless there is a heavy reliance on tropical oils. Trans fatty acids come primarily from hydrogenated vegetable oils. These fats have the potential to interfere with the conversion of alpha-linolenic acid to DHA, in addition to increasing risk for degenerative diseases. Trans fatty acids are plentiful in shortening, hydrogenated margarines, processed foods containing hydrogenated and partially hydrogenated vegetable oils (crackers, cookies, cakes, pastries, frozen convenience foods, snack foods) and fast foods (hydrogenated oils are used for deep frying).
Make monounsaturated fats the principal fat in the diet. Monounsaturated fats should make up the largest portion of fat in the diet, as they have proven to have neutral or positive affects on health. In addition, emphasizing monounsaturated fats will help to keep saturated fats, trans fatty acids and omega-6 fatty acids to a modest level. Olive oil (77% mono-fat) and/or canola oil (58% mono-fat) would be the recommended oils to use. Avocados, olives, hazelnuts, pistachios, almonds, macadamia nuts, peanuts and pecans are also excellent sources of monounsaturated fats. These foods also provide valuable vitamins, minerals, phytochemicals and fiber, and when used moderately, make an excellent addition to the vegetarian diet.
Limit the intake of omega-6-rich vegetable oils. Minimizing the use of oils rich in omega-6 fatty acids is the easiest way to keep linoleic acid levels at a reasonable level. Oils that contain predominately omega-6 fatty acids include corn oil, sunflower oil, safflower oil, and cottonseed oil. Commercial products such as margarine, salad dressing and mayonnaise that are made with these oils should also be limited. Soybean oil and walnut oil are also rich sources of omega-6 fatty acids, but their omega-6 content is partially compensated for by an omega-3 content of 5-8%.
Include a source of omega-3 fatty acids in the daily diet. In order to obtain sufficient omega-3 fatty acids in the vegetarian diet, good plant sources of this nutrient must be incorporated into the daily diet. If we assume an energy intake of 2000 kcal/day, 5% of calories as polyunsaturated fats and a omega-6: omega-3 ratio of 4:1, one would require 8.9 g of omega-6 and 2.2 g of omega-3 fatty acids. It would not be a challenge to consume the 9 grams of omega-6 fatty acids, even on a very low-fat vegetarian diet. The Reversal Diet (the Dean Ornish program that allows for no oils, nuts or seeds) provides an average 6 grams of linoleic acid primarily from whole grains and soy products. On the other hand, it would require considerable effort for vegetarians to get 2.2 grams of omega-3 fatty acids, by carefully selecting excellent plant sources of omega-3.
The best source of alpha-linolenic acid is flaxseed oil (57% omega-3 and 17% omega-6 fatty acids). Other omega-3-rich plant foods are much less concentrated sources of this nutrient, often coming packaged with much higher amounts of linoleic acid. Table 1 lists good plant sources of alpha-linolenic acid.
Table 1 - plant sources of alpha-linolenic acid
| Food (serving size) | Omega-3 (g) | Omega-6 (g) |
|---|---|---|
| Oils | ||
| Flax seed oil, 1 Tbsp. | 6.6 | 1.6 |
| Canola oil, 1 Tbsp. | 1.6 | 3.2 |
| Walnut oil, 1 Tbsp. | 1.4 | 7.6 |
| Soy oil, 1 Tbsp. | 1.0 | 7.0 |
| Nuts and Seeds | Flax seeds, ground, 2 Tbsp. | 3.2 | 0.8 |
| Walnuts (English), 2 Tbsp. | 1.0 | 5.4 |
| Vegetables, Fruits, and Legumes | ||
| Soybeans, cooked, 1 cup | 1.1 | 7.8 |
| Tofu, firm, ½ cup | 0.7 | 5.0 |
| Tofu, medium, ½ cup | 0.4 | 2.9 |
| Soy milk, 1 cup | 0.4 | 2.9 |
| Berries, 1 cup | 0.2 | 0.2 |
| Peas, ½ cup | 0.2 | 0.2 |
| Legumes, ½ cup | 0.05 | 0.05 |
| Green leafy vegetables | Broccoli, kale, Chinese greens, salad greens; 1 c. raw or ½ c. cooked | 0.1 | 0.03 |
| Grains | ||
| Oat germ, 2 Tbsp. | 0.2 | 1.6 | Wheat germ, 2 Tbsp. | 0.1 | 0.8 |
| NdT: Si fa presente che l'indicazione di grandezze quali "cucchiaino", "cucchiaio", "tazza" ecc., non va considerata in modo approssimativo, ma rappresenta quantità precise, descritte nelle tabelle di conversione. | ||
Practical Pointers for Using Flaxseeds and Flaxseed Oil
The oil from flaxseeds is highly unsaturated, thus will be easily damaged upon exposure to light, heat or air. Flaxseed oil can be purchased in the refrigerator section of natural food stores. It is packaged in black plastic bottles or dark brown glass bottles to protect it from light. The oil must be kept refrigerated and will stay fresh for up to 8 weeks after it is opened. One should check the expiration date. Flax oil that will not be used within this time should be frozen. It normally lasts for a year or more in the freezer. Rancid flax oil will smell "skunky" and should be discarded.
Flax oil should never be exposed to direct heat, thus it should not be used in cooking. It works well as a salad dressing base or as a tasty topping for pasta, potatoes, rice or vegetables (garlic/chili flavored flaxseed oil is a great choice). It can also be added to hot cereal, soup, sauces, dips or blender drinks. Mix flax oil with butter or a non-hydrogenated margarine for an omega-3-rich spread. The oil can also be taken as a supplement (1-2 tsp./day or in pill form for those who don't like the taste).
Whole flaxseeds are protected by a hard outer coat and will last for many months in your pantry. Unfortunately, because the seeds are so small, they generally enter the gastrointestinal system intact, and exit intact also. To enhance the digestibility of flaxseeds, simply grind them in a blender or a coffee grinder. Flaxseeds can be purchased in the bulk section of natural food stores and in most large grocery stores. Once flaxseeds are ground they go rancid quickly, thus should be stored in the refrigerator or freezer.
Ground flaxseeds can be sprinkled on salad or cereal. The soluble fiber in the seeds will make the liquid in your cereal very thick if it sits for too long. Ground flaxseed also makes an excellent egg replacer. One tablespoon of ground flax mixed with 3 tablespoons of liquid replaces one egg in baking. This works especially well in muffins, pancakes, cookies and cakes.
Flaxseed Cautions
Flaxseeds absorb 5 to 6 times their weight in water, so it is important to drink plenty of fluids when consuming ground seeds.
Raw flaxseeds, but not the flaxseed oil, contain cyanogenic glucosides, which are converted in the body to thiocyanates. These chemicals may interfere with iodine uptake by the thyroid gland and may increase the risk of goiter (especially when dietary iodine is limited). It has been suggested that intake of raw flaxseed should be restricted to 3-4 tablespoons a day. Cooking inactivates the cyanogenic glucosides, so there is less concern with flaxseed used in baking.
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