Over the last four decades, the widely publicized message that "fat is bad" has been taken to heart by many Americans. But as the fat content in the average diet has dropped, average weight has risen along with diabetes and heart disease, while cancer levels have changed little. Decades of research now indicate that this message has been oversimplified. Experts say a more accurate and effective message should convey that some fats are actually good for you and an important part of a healthy diet.

While most dietary recommendations refer to reducing your total fat intake, recent studies at the Harvard School of Public Health show that it’s not the total amount of fat that increases the risk of many types of cancer and heart disease, as long as weight and calories are appropriate, but the type of fat that may indeed play a more significant role.

Fat affects your health differently because it differs in its chemical make-up and structure. There are four main types: saturated, trans, monounsaturated and polyunsaturated. Research shows that eating less saturated and trans fats and more mono- and polyunsaturated fats, while getting a suitable amount of calories for your body type and activity level, improves cholesterol levels across the board and helps prevent heart disease in other ways as well.

Usually solid at room temperature, it is well known that saturated fat raises your level of low-density lipoprotein (LDL) "bad" cholesterol, which increases your risk of heart disease. Among saturated fats, those that have the most significant effect on cholesterol are found in dairy products, including butter, milk and cheese, sources that are also often high in cholesterol. Those from plant sources such as palm oil and coconut milk have less of an impact. Be aware that saturated fats are often added to snack foods such as potato chips, crackers and cookies.

Also typically solid at room temperature, trans fats are similar to saturated fats in their physical structure and their negative effect on health, but appear to be even worse. Not only do they raise LDL "bad" cholesterol levels, but they have also been shown to lower high-density lipoprotein (HDL) "good" cholesterol levels. They also appear to increase triglyceride levels in the blood and may form clots inside the blood vessels in the heart, brain and elsewhere, all leading to an increased risk of heart disease.

Trans fats are commonly found in margarine, vegetable shortening and some vegetable oils. Packaged snack foods, deep-fried foods and commercially baked goods often contain these fats, largely because they extend shelf life. In 2006, you’ll be able to see "trans fats" on the nutrition panel; but in the meantime, look for the words "hydrogenated" or "partially hydrogenated" in ingredient lists to avoid these. (For more information on trans fats, see the November Coop Scoop article, "The Latest on Trans Fat.")

Monounsaturated fats, which are liquid at room temperature, are one of two unsaturated fats that appear to protect against heart disease. They have been shown to reduce "bad" cholesterol levels without disturbing "good" levels, and are used by the body as a very efficient source of energy.

These fats are found in olive and canola oils along with many nuts including peanuts, almonds and cashews. Avocados are another good source.

Polyunsaturated fats, also liquid at room temperature, contain the unsaturated fats that may be most important to good health. While they are critical to normal body functioning, they are the only type of fat our body cannot make, and so must be obtained from the diet. They include the essential omega-3 and omega-6 fatty acids. Some researchers have recommended a higher proportion of omega-3s to omega-6s; but there is also strong evidence that the omega-6s, which make up the majority of the polyunsaturated fats in our diet, also contribute to healthy cholesterol levels and reduce heart disease.

Far less debatable is the ability of omega-3s to help prevent heart disease by their proven ability to decrease "bad" and increase "good" cholesterol levels, and by keeping the heart beating at a steady rhythm. Omega-3s have also been shown to drive normal brain and nervous system development and function, immune function, blood flow and healthy skin. Growing evidence also indicates they help to prevent complications of diabetes and soothe the inflamed joints of arthritis.

Polyunsaturated fats are found in both animal and plant sources. The most active of the omega-3 fatty acids, known as EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), are found in cold water fish including salmon, tuna mackerel and herring. Another type, alpha-linolenic acid, is found in flax and hemp seeds, canola oil, and walnuts. Omega-3s are also increasingly used to fortify bread, eggs, dairy products, margarine, baby food and cereal and are found in dozens of dietary supplements. Omega-6s are found in oils including corn, cottonseed, and soybean oils and many seeds and nuts including sesame seeds and peanuts.

To reduce saturated fats in your diet, and get more heart-healthy unsaturated fats, take advantage of the wide assortment of mono- and polyunsaturated oils, nuts and seeds mentioned above at the Coop, particularly in the bulk section. You can also special order sustainably sourced fish from the Coop. Just remember, all fats are loaded with calories, so the goal here isn’t to get more fat, but to get more of your "fat" calories from healthier fats. The Coop also offers a number of omega-3 supplements from both fish and plant sources and in liquid and capsule forms.

Stacy Kennedy, MPH, RD. 2004. "It’s A Good Thing: Omega-3 Fats" (January 23). Online at www.intelihealth.com.

Sally Squires. 2003. "The Omega Principle: Some Fish Fats Protect the Heart. What If They Could Also Treat Your Brain?" Washington Post (August 19): F1.

Andrew Weil, MD. 2001. "Eating Well for Optimum Health." HarperCollins.