As researchers have made significant advances in uncovering the link between diet and heart disease, it's become increasingly obvious that fats have gotten a somewhat undeserved bad reputation.
Some types of fat raise blood cholesterol levels. High cholesterol levels are an important risk factor for heart attacks and stroke. Other types of fat, however, may help fight heart disease and have other benefits.
Making healthy choices at the grocery store and at meal times is more complicated than choosing low-fat products, experts say. Instead, it involves understanding the differences among fats.
"All fats contain the same amount of calories, and from a weight standpoint, you need to watch your total intake," said Dr. Donald Hensrud, a nutrition specialist at the Mayo Clinic in Rochester, Minn. Current guidelines call for healthy adults to limit fats to less than 30 percent of daily calories.
"But individual fats have an advantage or disadvantage when it comes to heart health and other benefits," Hensrud said.
Fat is one of the three main nutrients, along with protein and carbohydrates, said Jennifer Nelson, head of clinical dietetics at Mayo Clinic. Fat is rich in energy and an important part of the diet, in moderation.
Subtle differences at a molecular level, such as the addition of a hydrogen atom, influence the effect fats have on the body. There are three main groups of dietary fats.
Nutrition experts are unanimous on this one, according to Robert Jeffery, a nutrition expert at the University of Minnesota. Saturated fat is mainly responsible for giving all fats a bad rap. The main reason is that the body converts it into low-density lipoprotein (LDL) cholesterol. LDL cholesterol is the "bad" cholesterol that clogs arteries. Saturated fat is so-named because its molecular structure has squeezed in the maximum number of hydrogen atoms. This saturation with hydrogen plays a role in the body's conversion of the fat to LDL cholesterol.
Saturated fat is found primarily in foods from animal sources, such as meat, butter, high-fat dairy foods and egg yolks. Coconut and palm oils are also high in saturated fat, and are often used in coffee creamers and commercially baked products.
While still considered a far better choice than saturated fat, polyunsaturated fat has seen its reputation change over the years, experts say. Its ability to lower blood cholesterol levels once led to it being touted as a heart-healthy fat.
Research has since found that polyunsaturated fat has some undesirable effects. It appears to lower high-density lipoprotein (HDL), which may help scour the arteries of fatty plaque. And polyunsaturated fats are believed to undergo changes in the body that contribute to atherosclerosis (hardening of the arteries). That change also may increase the risk of some cancers.
Sources of polyunsaturated fat in the diet include vegetable oils, such as safflower, sunflower, corn and soybean. Grains, usually low in fat, also contain primarily polyunsaturated fat.
One of the good fats, mono unsaturated fat appears to help lower bad cholesterol, Hensrud said, without affecting levels of good cholesterol. It also appears to resist the chemical process that may detract from polyunsaturated fat's heart-health benefits..
Foods rich in monounsaturated fat include olive oil, canola oil, avocados and most nuts, Hensrud said. Researchers believe that the widespread use of olive oil in Mediterranean countries may be one reason heart disease is less common there.
Consumers also should be aware of two other types of fat, experts said.
The first is trans fat, which is essentially polyunsaturated fat to which hydrogen atoms have been added at the molecular level. The process is known as hydrogenation, Nelson said, and it's done to help extend the fat's shelf life. It also makes polyunsaturated fat solid at room temperature.
Trans fat once was thought to help fight heart disease. Now, however, it's believed that it can significantly raise levels of bad cholesterol and lower good cholesterol, Hensrud said. Foods in which trans fats are used include margarine and commercially baked products.
The second type of fat receiving increased attention from heart specialists is omega-3 fat, sometimes known as omega-3 fatty acid. This time, the news from diet researchers is good.
Omega-3 fats are a type of polyunsaturated fat found mainly in fish. They don't lower bad cholesterol, but at high doses they appear to lower the amount of other types of fat linked with heart disease, according to Hensrud. They also may lower blood pressure, reduce the risk of blood clots and the risk of abnormal heart rhythms.
For that reason, nutrition experts recommend two meals of fish a week.
Although experts generally agree that less than 30 percent of calories in a healthy adult's diet should come from fat, there are different views about how much of each type of fat is needed, said the University of Minnesota's Jeffrey.
In general, though, experts say it's best for healthy adults to limit saturated and trans fats as much as possible. Getting most of your fat calories from monounsaturated fats is best, Hensrud said.
Choosing foods rich in this type of fat is the best way to do this, according to Kristine Kuhnert, a Michigan dietitian and managing editor of "The Good Food Encyclopedia," available in November from Academic Press.
Food labels can help determine how much of each fat is in foods, she said. However, not all fat types may be listed. Trans fats often are not, for example, but the Bush administration recently asked the U.S. Food and Drug Administration to require trans fats be listed on food labels.
Tips on making smart "fat" choices:
- Stick with whole grains, fruits and vegetables. The more processing food has undergone, the likelier it is to contain saturated fat or trans fat.
- Avoid foods labeled "hydrogenated" - a signal of trans fat in a product.
- Avoid foods containing palm oil, palm kernel oil and coconut oil. These oils are rich in saturated fat.
- Choose foods made with olive oil or canola oil. Switch to either of these for cooking.
- Use liquid or tub margarine instead of stick margarine, which may contain trans fat or saturated fat.
- Select reduced-fat and nonfat dairy foods. Whole milk and products made from it are high in saturated fat.
- Limit meat in the diet, and substitute fish several times a week.
- Bake cookies and pastries instead of buying them.