Dietary recommendations to reduce the risk of heart disease and other chronic conditions emphasize the selection of foods rich in nutrients and low in components, such as saturated fat and salt, known to increase health risks. Until recently, recommendations focused mainly on ways of lowering blood cholesterol levels because high blood cholesterol increases the risk of heart disease. While controlling cholesterol remains important, other nutrients and foods also benefit heart health without affecting cholesterol levels. Examples are foods rich in dietary fiber and omega-3 fatty acids.
Recommendations from the Dietary Guidelines for Americans 2005 capture the big picture of a healthy lifestyle. These are shown in the box and described in detail on the U.S. Department of Agriculture’s web site. Here we describe current nutrition advice about food fats, emphasizing the most healthful fats. Deliberately choosing foods with the most healthful fats can significantly lower the chance of heart attack and mortality, independent of changes in cholesterol.
Food fats are characterized in terms of their saturated, monounsaturated and polyunsaturated fatty acid content. We now know that the type of fat a person usually eats is more important than the total amount of fat. However, because fat has more than twice the calories of other food components, eating lots of fatty foods can pack on the pounds. Being overweight works against heart health, blood pressure, diabetes, arthritis and other conditions better avoided. Control weight gain by watching meal sizes and exercising regularly. More vigorous and longer exercise times promote weight loss, so long as you don’t reward your virtue by eating!
Saturated fats: These are the main culprits in raising blood cholesterol levels. For that reason, have less of them than all other types of fat. They are prevalent in dairy products, e.g., butter, cheese, cream, whole milk, fatty meats, and tropical oils. Processed meats, shortenings, and many commercially prepared fried or baked foods have plenty of saturates too. Limit these to no more than 20 grams/day for an average 2,000-calorie diet. Women usually eat fewer calories, so they should have less saturated fat. Food labels tell how many grams of saturates are in one serving of that food.
Trans fatty acids: These fatty acids occur in foods largely from the hydrogenation of vegetable oils to make them more closely resemble solid food fats. Some shortening and stick margarines are the richest sources of these substances and small amounts occur in dairy fats. Many baked and processed foods contain them too. Their consumption is strongly discouraged because they raise damaging LDL cholesterol in blood and reduce the amounts of helpful cholesterol – HDL or “good” cholesterol – as well. They are to be avoided as much as possible.
Tropical oils: Oils from coconut, palm, and palm kernel were once widely used in manufactured foods, but because of their high content of saturated fatty acids, were largely replaced by hydrogenated vegetable oils. Tropical oils range from about 50% to 90% saturates. At the time, all saturates were considered “bad” because many of them raised blood cholesterol levels. But hydrogenated oils are rich in trans fatty acids and raise cholesterol levels too. They also reduce HDL or “good” cholesterol. For these reasons, they are being removed from processed foods. Now, tropical oils are being reappraised. It turns out that not all tropical oils have atherogenic effects. For example, palm oil raises both LDL and HDL cholesterol, but reduces the amount produced by the liver. It also contains carotenoids, vitamin E, and plant sterols, which are healthful. Coconut oil, with about 90% saturates, contains smaller fats (medium chain triglycerides), which are metabolized differently from triglycerides with a mixture of fatty acids.
The third tropical oil, palm kernel oil, raises cholesterol more than the other two, but much of that includes HDL or good cholesterol. The significance of these varying effects is difficult to gauge, especially in view of the cholesterol-lowering properties of some mixtures of tropical oils. Expect to hear more about these oils and see them in stores more often.
Monounsaturated oils: These oils contain predominately oleic acid, the major fatty acid in olive oil, and newly developed “high-oleic” acid oils from canola, sunflower, safflower, soybean, corn, and cottonseed. Substitution of the polyunsaturated fatty acids in these oils creates a product with over 80% oleic acid. Developed from selective plant breeding or gene technology, these oils are low in saturates and the polyunsaturated linoleic and alpha-linolenic acids. The majority of dietary fat should come from these oils.
Polyunsaturated oils: These oils are rich in linoleic acid and come from soybean, canola, corn, sunflower, and safflower. Linoleic acid is an essential fatty acid that the body cannot make, so small amounts must be obtained from food. Canola and soybean oils also have alpha-linolenic acid, an omega-3 fatty acid. Consumption of polyunsaturates has been encouraged because they lower blood cholesterol levels. However, the US diet has 10 times or more linoleic as alpha-linolenic acid, an imbalance that may not help heart disease. Because omega-3 fatty acids, especially those from fish and shellfish, are protective of heart health, their consumption is now strongly encouraged.
Oils Rich in Omega-3s: Only one type of omega-3, alpha-linolenic acid, is found in plants. It is most abundant in flaxseed oil, but occurs in smaller amounts in canola and soybean oils and walnuts. The body cannot make alpha-linolenic acid and must obtain it from food. We can convert alpha-linolenic acid to the omega-3s found in fish, but do so very inefficiently. Only about 5% or less is converted. Most dietary alpha-linolenic acid is oxidized or burned for energy.
Fish oils, found in fish and shellfish, fish oil capsules, and omega-3 eggs contain the omega-3s EPA and DHA. They differ from alpha-linolenic acid by being longer and much more unsaturated. EPA and DHA are called long-chain omega-3s or marine omega-3s. DHA is necessary for brain growth and development, and is concentrated in the retina and sperm.