FAT

Dietary fat is the most concentrated source of food energy. Like energy from dietary carbohydrate, energy derived from fat can support protein synthesis. Dietary fat also provides the essential fatty acid linoleic acid. Other than the need for adequate quantities of linoleic acid, there is no specific requirement for dietary fat as long as the diet provides adequate nutrients oxidizable for energy. Although the average American diet contains 35–40% of calories as fat, most current recommendations are to limit dietary fat to 30% or less of total calories. Diets containing as little as 5–10% of total calories as fat appear to be safe and well tolerated.
Dietary fats are composed primarily of fatty acids and dietary cholesterol. Fatty acids contain either no double bonds (saturated), one double bond (monounsaturated), or more than one double bond (polyunsaturated). Saturated fatty acids are associated with increased serum cholesterol, whereas polyunsaturated and monounsaturated fatty acids lower serum cholesterol. Trans-fatty acids, a particular form of unsaturated fat found in partially hydrogenated vegetable oils, also raise serum cholesterol levels. Saturated fats are solid at room temperature and in general are derived from animal foods; unsaturated fats are liquid at room temperature and in general are derived from plant foods.
The polyunsaturated fatty acid linoleic acid is an essential nutrient, required by the body for the synthesis of arachidonic acid, the major precursor of prostaglandins. Deficiency of linoleic acid results in dermatitis, hair loss, and impaired wound healing. For individuals with average energy requirements, approximately 5 g of linoleic acid per day—1–2% of total calories—is required to prevent essential fatty acid deficiency.
Cholesterol is a major constituent of cell membranes. It is synthesized by the body and is not an essential nutrient. Diets that contain large amounts of cholesterol partially inhibit endogenous cholesterol synthesis but result in a net increase in serum cholesterol concentrations because of suppression of synthesis of low-density lipoprotein receptors. Average American diets contain approximately 450 mg/d of cholesterol, but 200 mg or less per day is recommended.


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