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.

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.