Dietary fat used to be portrayed as public enemy No. 1. That was because many studies linked a high-fat diet with heart disease, cancer and other health problems. More recently, however, studies have found that low-fat diets do not improve the odds of preventing disease or losing weight. Fats are needed for the proper functioning of the human body, and we need to consume some fat to remain healthy. It’s now clear that what matters most for good health is to eat the “right” fats, as explained below.
Technically, fats belong to a class of substances called lipids. Most of the fats in foods are triglycerides, which consist of three fatty acids attached to a glycerol molecule. Some of the fatty acids are called essential fatty acids, so named because the body cannot make them and must get them from foods.
Fats perform many important functions. Fatty acids are the raw material for several hormone-like compounds, including prostaglandins, that help control blood pressure, blood clotting, inflammation and other bodily functions. Fats also serve as the storage substance for the body’s excess calories, filling the balloon-like adipose cells that insulate the body. In addition, fats help maintain healthy skin and hair, transport the fat-soluble vitamins (A, D, E, and K) through the gastrointestinal tract and bloodstream and help regulate blood cholesterol levels.
But since all fats contain 9 calories per gram (versus 4 calories per gram for carbohydrates and protein, and 7 calories per gram for alcohol), too much of any kind of fat increases your risk of becoming overweight or obese, which is a risk factor for cardiovascular disease.
Different fats, different effects
Fatty acids vary in the length of their molecular chains and in the degree of saturation by hydrogen atoms—and these variations determine the properties of different fats. All fats in foods are combinations of saturated and unsaturated fatty acids, but they are often described with terms such as “highly saturated” or “highly unsaturated,” depending on which type predominates. For instance, about half the fatty acids in beef are saturated, which is a high proportion, while nearly all the fats in nuts are unsaturated.
- Saturated fatty acids are loaded with all the hydrogen atoms they can carry. Fats that are largely saturated are usually solid at room temperature. Such fats come mainly from animal sources—meats, poultry and whole-milk dairy products. Coconut, palm and palm kernel oils also are highly saturated. A diet high in saturated fats can raise your LDL (“bad”) cholesterol level, which increases the risk of heart disease. But the effect of saturated fats on blood cholesterol, like many other nutritional matters, actually varies from person to person, depending on genetic and other factors. It also depends on where the fat comes from. Chocolate, for instance, contains different saturated fatty acids than those in beef and appears to be more neutral in its effect on blood cholesterol.
- Unsaturated fatty acids, predominantly from plant foods and fish, do not have all the hydrogen atoms they can carry. Depending on the number of “missing” hydrogen atoms, these fatty acids are called monounsaturated or polyunsaturated. Olive and canola oils, as well as nuts and avocados, for example, are composed largely of monounsaturated fatty acids. Corn, sunflower and soybean oils, as well as the fats in fish, are largely polyunsaturated. In contrast to saturated fats, unsaturated fats have a beneficial effect on blood cholesterol levels, especially when they replace saturated fats in the diet.
- Trans fatty acids, found in some margarines, vegetable shortenings, baked goods and other processed foods, are created by adding hydrogen molecules to liquid vegetable oils to make them more solid and stable, a process called partial hydrogenation. Trans fats act much like saturated fats, which raise LDL (“bad”) cholesterol, but are even more unhealthful because they also lower HDL (“good”) cholesterol and have other harmful effects.
The good news is that since the Food and Drug Administration (FDA) began requiring packaged foods to list trans fat content on nutrition labels, many manufacturers have voluntarily reduced or eliminated partially hydrogenated oils—and thus trans fats—from their products. Some fast-food restaurants got rid of trans fats in French fries, while New York and California have banned synthetic trans fats in restaurants altogether. As a result, blood levels of trans fats have decreased in recent years, which should help lower the risk of cardiovascular disease, researchers say. Lots of foods still contain trans fats, however—sometimes at very high levels— so always check the nutrition labels. To avoid them, make sure that there are no “partially hydrogenated” oils in the ingredients list.
- Omega-3 fatty acids. These special polyunsaturated fats in fish have been linked to a variety of health benefits, particularly for the heart. They may, for instance, help prevent arrhythmias (irregular heartbeats) and blood clots, reduce chronic inflammation, and help lower triglycerides (fats in the blood) and blood pressure. Omega-3 fats are also vital for the brain development of infants and young children. The two main omega-3 fats in fish are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). Some plant foods—notably flaxseeds, walnuts and canola oil—contain a related omega-3 fat called alpha-linolenic acid, which may have some health benefits of its own, though it can’t replace the omega-3s from fish.
What about tropical oils? Tropical oils—coconut, palm and palm kernel oils— were once the favored fats for giving processed foods good texture and shelf stability. But starting in the late 1980s, they fell out of favor because most of the fats they contain are saturated. Partially hydrogenated oils, which largely replaced tropical oils at the time, were subsequently found to be even worse for your heart. Now, tropical oils are back again. You can find them in an increasing number of packaged foods, including crackers, cookies, pie crusts, energy bars and spreads, as well as in microwave and movie theater popcorn.
Tropical oils may not be as bad as once thought. Some studies have shown that palm oil (sometimes called palm fruit oil, from the pulp of the fruit) and coconut oil have a neutral effect on blood cholesterol. Palm kernel oil (from the fruit’s seed) is more saturated than palm oil, however. Plus, palm kernel oil is often further processed (“fractionated”) to remove the liquid portion, leaving behind even more saturated solids. It is not known if this highly processed form of palm kernel oil, in particular, is any less harmful for you than partially hydrogenated oils.