Although consuming more than the recommended amount of fat is often associated with obesity, what many people fail to recognize is that fat is not in and of itself the cause of obesity. Fat doesn't necessarily make people fat; excess calories do. But fat is calorie-dense. One gram of fat contains 9 calories, whereas 1 gram of carbohydrate or protein contains only 4 calories. It is therefore quite easy to consume a great many calories in just a few bites when eating foods that are high in fat.
Particularly two particular types of fats have received a great deal of media coverage. The first of these is known as trans fat. When liquid oils are made into margarines or shortenings during a process known as hydrogenation, additional hydrogen atoms are forced to bond with the liquid unsaturated fats, effectively increasing their saturation levels and causing them to become more solid at room temperature. Their process results in the formation of trans fats (trace amounts of trans fats also occur naturally in some foods).
Until recently, trans fats were thought to be the lesser of two evils when compared to saturated fats in terms of their effect on serum cholesterol levels. The most current research, however, seems to indicate that trans fat is more detrimental than originally thought. It raises blood cholesterol levels and may be carcinogenic. However, American generally tend to consume much less trans fat than saturated fat, so current dietary advice places more of an emphasis on reducing saturated fat in the diet.
Commercially baked goods, margarines, and foods fried in or containing shortening that is solid at room temperature are the main sources of trans fats in the American diet ...
Omega-3 fatty acids have also been in the nutrition spotlight. These polyunsaturated fatty acids occur in fatty fish, dark green leafy vegetables such as spinach and broccoli, and certain nuts and oils such as walnuts and canola oil. They have been shown to be quite effective in reducing the risk of heart disease by lowering the amount of cholesterol manufactured in the liver and reducing the likelihood of blood clot formation around deposits of arterial plaque. Omega-3 fatty acids may also slow or prevent tumor growth, stimulate the immune system, and lower blood pressure.
The Professional Chef, 7th Edition by The Culinary Institute of America