Fats have been classified into three major categories – saturated fats, polyunsaturated fats, and monosaturated fats. This classification is based on the number of hydrogen atoms each has in its chemical structure.

Saturated fats, which are usually solid at room temperature, are found primarily in animal products, including fatty meats like beef, veal, lamb, pork, and ham; and in dairy items such as whole milk, cream, ice cream, and cheese. For example, the white marbling you can see in a piece of beef is saturated fat. Some types of vegetable products – including coconut oil, palm kernel oil, and vegetable shortening – are also high in saturates.

The liver uses saturated fats to manufacture cholestrol. Therefore, excessive dietary intake of saturated fats can significantly raise the blood cholestrol level, especially in people how have an inherited tendency toward high blood cholestrol.

Guidelines issued by the National Cholestrol Education Program (NCEP) and widely supported by most experts recommend that your intake of saturated fats should be kept below 10 percent of your total calorie intake. However, for people who have severe problems with high blood cholestrol, even that level may be too high.

Polyunsaturated fats are found in greatest abundance in corn, soybean, saffflower, and sunflower oils. Certain fish oils, particularly those containing the omega-3 fatty acids, are also high in polyunsaturates. Unlike the saturated fats, plyunsaturates may actually lower your total blood cholestrol level. In doing so, however, large amounts of polyunsaturates also have a tendency to reduce your HDLs – your “good cholestrol.” For this reason – and because, like all fats, polyunsaturates are high in calories for their weight and volume – the NCEP guidelines state that your intake of polyunsaturated fats should not exceed 10 percent of your total calorie intake.

Monosaturated fats are found mostly in vegetable and nut oils such as olive, peanut, and Canola (rapeseed). These fats appear to reduce blood levels of LDL cholestrol without affecting HDLs in any way. However, this positive impact upon LDL cholestrol is relatively modest. The NCEP guidelines recommend that your intake of monosaturated fats be kept between 10 and 15 percent of your total calorie intake.

Although most foods – including some plant derived foods – contain a combination of all three types of fats, one of the types usually predominates. Thus, a food is considered “saturated” or “high n saturates” when it is composed primarily of saturated fatty acids. Similarly, a food composed mostly of polyunsaturated fatty acids is called “polyunsaturated,” and a food composed mostly of monosaturated fatty acids is called “monosaturated.”

One other element, trans-fatty acids, might also play a role in blood cholestrol levels. Trans-fatty acids occur when polyunsaturated oils are altered through hydrogeneration, a process used to harden liquid vegetable oils into solid foods like margarine and shortening. One recent study found that trans-monosaturated fatty acids raise LDL cholestrol levels, behaving much like saturated fats. Simultaneously, these trans-fatty acids reduced HDL cholestrol readings. Much more research is necessary, since some studies have not produced clear-cut conclusions about these substances. But your dietary choices could become less matter-of-fact than they now appear. For now, however, it is clear that when your goal is to lower cholestrol, polyunsaturated and monosaturated are much more desirable than saturated fats, and are probably more desirable than any kind of hydrogenated fats.

Fat calories are different

Clearly, unlimtied dietary fat and cholestrol increases the risk of heart disease and other life-threatening conditions. This alone should be sufficient reason to limit dietary fat. Yet there is still another reason, especially for those of us who are wathcing our weight.

Carbohydrates, protein, and fat all provide the body with the energy it needs to function. This energy is measured in calories. But ounce for ounce and gram for gram, these nutrient do not supply the same number of calories, nor do the calories they provide all affect the body in the same way. Carbohydrates and protein each have approximately 4 calories in every gram, but fat has approximately 9 calories per gram. So on a gram-for-gram basis, fat is more than twice as fattening as carbohydrate or protein. It is no wonder, then, that when people want to lose weight, one of the smartest things they can do is to cut down on fatty foods.

In addition to having more calories than protein or carbohydrates, fat is metabolized differently by the body. Excess fat in your diet is more likely to be stored as body fat than is excess carbohydrate or protein. This is because dietary fat is similar in chemical composition to body fat, so it takes less energy to convert it into body fat. In fact, it takes only 3% of the calories in the fat you eat to turn that food into body fat, while it takes at least 25% of the carbohydrate and protein calories you eat to convert them into body fat.