264 OLIVE OILS AND HEALTH LIPIDS or FATS For a more complete understanding of this topic, see Chapter 1 and Figure 4 of Chapter 14. Index of this topic: • CONCEPTS • SATURATED LIPID/S • UNSATURATED LIPID/S MONOUNSATURATED LIPID/S POLYUNSATURATED LIPID/S omega-6 (ω-6) POLYUNSATURATED LIPID/S omega-3 (ω-3) The term lipids is synonymous with fats. Lipids are organic biomolecules composed mainly of carbon, hydrogen, and oxygen, and may also contain nitrogen, phosphorus, and sulphur. They form a highly heterogeneous group of compounds which, due to their chemical structure, are hydrophobic (insoluble in water) but soluble in so-called organic solvents (ether, acetone, chloroform, benzene, etc.). They are also characterised by their low density. The classification of lipids is very complex, but in a simpler and more intuitive way, they can be grouped into glycerides (mono-, di-, triglycerides), phospholipids, sterols, and steroids. Some also combine with sugars (glycolipids) or proteins (lipoproteins). Another basic form of classification refers to “lipids containing esterified fatty acids” (glycerides, phospholipids, and cholesterol esters) and “lipids without fatty acids” (cholesterol, steroids, eicosanoids, and vitamins A, D, E, and K). The former are known as saponifiable lipids, while the latter are unsaponifiable. In the human body, lipids are present in many areas and are a key component of all cell membranes. They are involved in cell signalling, steroid hormone synthesis, and the production of bile salts. Lipids also make up adipose tissue, or body fat, which is composed of specialised cells called adipocytes. These fats serve several purposes: energy storage (as the body’s energy reserve), thermal insulation, organ protection (e.g. the fat cushion surrounding the kidneys, known as perirenal fat), and the production of hormones involved in appetite regulation. Some vitamins are lipid-based in nature and play important physiological roles: Vitamin A (see carotenoids), Vitamin D, Vitamin E, and Vitamin K. In food, lipids are found in butter, lard, bacon, dairy products, fatty meat, lean meat (which may still contain hidden fat), nuts, oils, and some fruits such as avocado. A key factor regarding dietary lipids is that they provide essential fatty acids, those which the body requires but cannot synthesise, and which must therefore be obtained from the diet. Lipids or fats are the most energy-dense nutrients: each gram of fat provides an average of 9 kilocalories when oxidised by the body. In this sense, fats are a “concentrated source of energy”. Fat ingestion is appetising due to its contribution to palatability and to the dietary organoleptic characteristics (taste and sensory properties of the diet). From a health perspective, however, this type of food has a bad reputation. Nonetheless, fat intake cannot be avoided, as it provides essential fatty acids and other compounds essential for the body’s proper functioning. The only real issue is overconsumption. Even fats considered healthy, such as extra virgin olive oil, lose their beneficial properties if consumed excessively. Furthermore, excessive overall food intake can cause surplus proteins and carbohydrates to be converted into fatty acids and stored as triglycerides in adipose tissue,
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