Alice H. Lichtenstein, a professor at the Friedman School of Nutrition Science and Policy at Tufts University, explains.
The most important factors in determining a person's risk of developing heart disease are body weight and the type of fat she consumes.
Body weight is important because people who are overweight are more likely to develop multiple risk factors for heart disease. These factors include elevated low density lipoprotein (LDL) cholesterol concentrations (so-called bad cholesterol), low high-density lipoprotein (HDL) cholesterol concentrations (so-called good cholesterol), high triglyceride concentrations, hyperglycemia (elevated blood glucose concentrations) and increased blood pressure.
A balance between the number of calories ingested and the amount of energy burned determines body weight. Eating more calories than you expend will result in weight gain, whereas eating fewer calories than you expend will result in weight loss. Carbohydrates and protein contain four calories per gram, but fat contains more than twice that, with nine calories per gram. For this reason a considerable amount of work has been focused on trying to determine whether diets high in fat make it more difficult to maintain a healthy weight as people age. At this point the jury is still out. Total caloric intake is determined by many factors, both biological and psychological, and people will lose weight if they consume either high- or low-fat diets as long as they restrict total calories. Because of differences in personal preference and environmental factors that influence food intake, what works for one person may not for another and vice versa.
The second major factor that determines heart disease risk is plasma lipoprotein concentrations. People who have elevated LDL cholesterol and/or low HDL cholesterol concentrations and elevated triglyceride concentrations are at increased risk of developing heart disease. The major determinant of LDL cholesterol concentrations is dietary consumption of saturated and trans fatty acids. The major sources of saturated fatty acids are animal fats, both meat and dairy, whereas trans fatty acids come from hydrogenated fat (which is found in foods such as solid vegetable shortening and products made thereof, as well as commercially fried and baked products) and, to a lesser extent, animal fat. The intake of both types is strongly associated with elevated LDL cholesterol concentrations and decreasing the intake of both has been associated with decreased LDL cholesterol concentrations and a lower risk of developing heart disease.
On the other hand, unsaturated fatty acids--both monounsaturated and polyunsaturated--have been associated with decreased LDL cholesterol concentrations relative to saturated fatty acids. The major sources of unsaturated fatty acids in the diet are vegetable oils in the natural state. For example, corn and sunflower oils are relatively high in polyunsaturated fatty acids, and canola and olive oils are relatively high in monounsaturated fatty acids.
In some people--especially those who have elevated body weights--very low-fat, high-carbohydrate diets can increase triglyceride levels and decrease HDL cholesterol concentrations, which raises the risk of developing heart disease. For this reason current recommendations suggest focusing more on total calories and the type of fat eaten, rather than your diet's total fat content.
It can be difficult to keep track of both the level and type of fat consumed, but there are certain dietary patterns that, if followed, will result in a diet low in saturated and trans fatty acids. Aim for a diet high in fruits and vegetables, whole grains, low-fat and non-fat dairy products, legumes (beans), fish, poultry and lean meat. In addition to being low in saturated and trans fatty acids, this diet is also rich in fiber and low in cholesterol, two dietary components also associated with decreased risk of developing heart disease.