For over two decades, accepted dietary guidance has stressed the importance of choosing a diet that is low in fat, saturated fat, and cholesterol. Consumer research has shown that people interpret this advice to mean that they should eliminate all fat from the diet, in effect making "fat" a "four-letter word" and something to be avoided as much as possible.
Science is continually evolving, however. Two decades of research has improved our understanding of the health benefits of many foods and food components, including the essential roles of fats and individual fatty acids in the diet, "With continually emerging research we are unravelling the complex relationship between food and health," says Penny Kris-Etherton, Ph.D., R.D., professor of nutrition at Pennsylvania State University. The guidance on dietary fat today reflects this greater understanding of science and is best summarized by the U.S. Department of Agriculture (USDA) 2000 edition of Dietary Guidelines for Americans which recommends that people "choose a diet that is low in saturated fat and cholesterol and moderate in total fat."
There Are Fats, and Then There Are Fats
The three major categories of dietary fats-saturated, monounsaturated, and polyunsaturated-have various effects on low-density-lipoprotein (LDL) cholesterol ("bad" cholesterol) and high-density-lipoprotein (HDL) cholesterol ("good" cholesterol) levels. Saturated fats, in general, are shown to elevate LDL-cholesterol levels, and high levels of LDL cholesterol are considered a major risk factor for heart disease. In contrast, diets higher in monounsaturated and polyunsaturated fats are known to lead to lower LDL-cholesterol levels.
There are two subclasses of fatty acids within the polyunsaturated fat category: omega-6 (n-6) fatty acids and omega-3 (n-3) fatty acids. Vegetable oils such as corn, sunflower, safflower and soybean oils are rich in n-6 fatty acids. Soybean oil is also an excellent source of n-3 fatty acids, as are canola oil and deep-sea fish, or "fatty fish."
Beyond Basic Nutrition: The Functions of n-3 Fatty Acids
New research is showing that n-3 polyunsaturated fatty acids may have significant health benefits. Alpha-linolenic acid (ALA), a precursor to long chain polyunsaturated fatty acids (LCPUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are of particular interest. These n-3 fatty acids are thought to help reduce the risk for cardiovascular disease, to play major roles in promoting eye health, and possibly, to improve immune function.
Getting to the Heart of the Matter
Researchers suggest that EPA and DHA promote heart health by preventing blood platelets from clotting and sticking to the artery walls-effects that are similar to those observed with aspirin. Decreased clotting helps reduce the chances of blockages in an artery and thereby decreases the risk for heart attack or stroke.
EPA and DHA have been shown to reduce the levels of serum triglycerides, which, like cholesterol, are associated with an increased risk for heart disease.
Several clinical studies support the cardiovascular health benefits associated with n-3 fatty acids. For example, the results of a recent study were presented at the American Heart Association's (AHA's) 41st Annual Conference on Cardiovascular Disease Epidemiology and Prevention in March 2000. Its findings suggest that older Americans who consume fatty fish, a rich source of n-3 fatty acids, one or more times a week had a 44 percent lower risk of experiencing a fatal heart attack. The study also reports that n-3 fatty acids appear to have a positive effect on the rhythm of the heartbeat, an effect that specifically reduces the rate of occurrence of a major cause of fatal heart attacks.
In addition to this evidence, research published in the January 2001 issue of the Journal of the American Medical Association concluded that consumption of larger amounts of fish rich in n-3 fatty acids (at least one serving per week) was associated with a significant reduction in the incidence of certain types of stroke caused by blood clots. Furthermore, the consumption of larger amounts of n-3 fatty acids was not related to an increased risk of stroke caused by bleeding.
On the basis of a variety of studies similar to these, the U.S. Food and Drug Administration approved, for dietary supplements only, the use of a qualified health claim about n-3 fatty acids and their role in reducing the risk of heart disease. Although this health claim, approved for use in October 2000, offers some validation for the role of n-3 fatty acids in the diet and their relationship to heart health, it is important to recognize that science is still evolving in this area.
It is especially important to note that if n-3 fatty acids are only added to an individual's current diet without lowering the amounts of saturated fat in the diet, it is likely that LDL-cholesterol levels will not improve, and may actually increase. That is, n-3 fatty acids should be seen as a healthful substitute for saturated fat in the diet.
Eye See!
DHA is naturally concentrated in the retina of the eye and is thought to promote healthy retinal function. Results of a study published in the February 2001 issue of the American Journal of Clinical Nutrition (AJCN) indicates that the consumption of larger amounts of fish is associated with a decreased risk of development of age-related macular degeneration (AMD). AMD is an untreatable disease that causes fuzziness, shadows, or other distortions in the center of vision and is the leading cause of blindness in older adults.
Researchers at Bristol University in Bristol, United Kingdom, who also published an article in the February 2001 issue of AJCN, found that women who ate fatty fish while pregnant gave birth to children who experienced better visual development. In addition, babies whose mothers had significant levels of DHA in their diet while they were breast-feeding experienced faster-than-normal eyesight development. Longer-term studies have shown that at two and four years of age, these differences seem to have disappeared. In further studies, LCPUFA supplementation resulted in improved cognitive function and enhanced information processing in some infants.
n-3 Fatty Acids and Immunity
At a conference hosted by the National Institutes of Health in September 2000, investigators made several presentations on studies exploring the role of n-3 fatty acids on immune functions. Some of these studies suggested that the addition of vitamin E to a diet rich in n-3 fatty acids may further enhance the beneficial effects of those n-3 fatty acids in delaying autoimmune diseases such as rheumatoid arthritis.
Researchers have also found that n-3 fatty acid supplementation is associated with a reduced incidence of joint tenderness and morning stiffness in study participants with rheumatoid arthritis. The study participants consumed supplements of EPA and DHA, in addition to traditional anti-inflammatory drugs, and some clinical benefits were seen. The effects were not apparent until the compounds were consumed for 12 weeks or longer. In addition to its reported impact on arthritis, the potential effects of n-3 fatty acids are being studied in individuals with a wide range of immune-compromised conditions including asthma, lupus, kidney disease, and cancer.
Even with these intriguing results, some experts recommend a cautious approach to the use of fish oil supplements, one source of n-3 fatty acids. On the basis of the current scientific evidence, the AHA advises that fish oil capsules be used judiciously. Specifically, AHA recommends that fish oil supplements be used only by individuals with severely high triglyceride levels, individuals who have not responded well to conventional treatment for heart disease, and individuals who are not at risk for pancreatitis (inflammation of the pancreas). As with any healthcare decision, consumers should consult with their health professionals to determine the best individual approach.
Pass the n-3 Fatty Acids Please
ALA, the most abundant n-3 fatty acid in the U.S. diet is found in a variety of foods including tofu, soybean, flax, and canola oils, flaxseed, nuts, and fish. However, very small amounts of EPA and DHA, which is mostly found in fatty fish such as tuna as salmon, are present in the diet. Much of the current research shows health benefits from n-3 fatty acids found in fatty fish, particularly because of recent speculation on the health benefits of EPA and DHA in heart and eye health and the immune system. Even with this range of plant and animal sources of n-3 fatty acids, however, diets in the United States are considered to be low in n-3 fatty acids.
Current dietary guidance rightly continues to stress the importance of choosing a diet that helps reduce saturated fat intake. Ongoing research on a variety of individual polyunsaturated fatty acids, including n-3 fatty acids, supports the recommendations of USDA and others to choose a diet moderate in total fat.
Therefore, moderate. Don't eliminate. Implement the AHA recommendation to eat at least two servings of fish per week. With summer weather coming, have a tuna salad or salmon steak with a marinade consisting of oil-and-herbs and throw it on the grill for a flavorful and healthful entree.
