LIPIDS
Lipids or fats are a heterogeneous group of complex substances that are insoluble in water but soluble in organic dissolvent such as eter.
These compounds constitute the main energetic nutrient, for a diet that lacks lipids, even if it contains a larger amount of carbohydrates, is insufficient to sustain life.
Recent years have demonstrated the relationship between diet-plasma lipids-cardiovascular diseases. It is thus necessary to understand the importance of fats in the diet so that excessive consumption is avoided, emphasising healthier diet habits.
Lipids are composed of carbon and hydrogen, and a relative lack of oxygen.
Classification
They are usually classified according to their chemical composition in terms of triglycerides, cholesterol and phospholipids.
a) Triglycerides
These compounds are the main components of natural fats in the diet. Due to their density and low solubility they are used to store energy in the form of adipose tissue. They are composed of a glycerol molecule and three fatty acids that dictate their activity.
Fatty acids are composed of carbon, oxygen and hydrogen. Depending on the links of the molecule, they are classified into saturated and unsaturated (monounsaturated and polyunsaturated). Unsaturated fats are of a vegetal origin mainly, with one of the most important being olive oil (monounsaturated) and corn and sunflower oil (polyunsaturated); saturated fats are of animal origin, although fish contains a high amount of unsaturated fats. At present, the positive role of unsaturated fats to prevent cardiovascular disease has been finally recognised.
There are two essential fatty acids (which are not synthesised by the body and need to be provided by external sources): linoleic acid and linolenic acid. Linoleic acid is found in corn, sunflower and soy oil and in fish; whilst linolenic acid is found mainly in soy and fish.
b) Phospholipids and composed lipids
These compounds are the second most important lipid component of the organism. They differ from triglycerides in that they only contain 2 fatty acids and a phosphorous compound.
They make up the structural materials of cell membranes in combination with proteins and in the blood transporting lipids. Other phospholipids are necessary for proper blood coagulation, to form the myelin that covers nerves, or form part of cells structures and structures of the nervous system.
Phospholipids and composed lipids are found in foods of animal origin (egg yolk and liver) and vegetal origin (soy). Nevertheless, these are not essential nutrients as the liver produces sufficient amounts.
c) Cholesterol and other esterols
These compounds have very complex molecules.
Cholesterol is only found in animal tissues, mainly in egg yolk and viscerae, whilst other esterols (such as ergosterol) is obtained from vegetal products.
Cholesterol is an essential substance of cell membranes and the main component of the brain and neurones. It is found in high concentrations in the liver (where it is stored) and in glands. It is produced in the liver in sufficient amounts (thus it is not essential). Biliary acids, hormones (suprarenal, estrogens and androgens) derive from cholesterol; being also a precursor of Vitamin D.
Excessive intake of cholesterol is directly related to the formation of atheroma plaques in blood vessels and the development of cardiovascular disease.
Digestion, Absorption And Metabolism
Digestion and absorption of lipids takes place in the duodenum and jejunum, where biliary salts breakup fat molecules into small particles (emulsion phenomena) and pancreatic enzymes breakup molecules that give rise to fatty acids, glycerol and cholesterol which are ultimately absorbed.
Inside intestinal cells, fatty acids, glycerol and cholesterol bind again to each other and also to special proteins to form large molecules called chylomicrons, which are transported to the liver via the lymphatic system. Once in the liver, these compounds are used depending on the current needs of the body, to produce energy, biliary salts or are used by different body tissues where they might be needed (through the formation of LDL and VDLA lipoproteins).
When there is excessive intake of fat, circulating triglycerides accumulate in adipose tissue and excess cholesterol accumulates in cells that are not capable of blocking its access, such as in the walls of blood vessels, giving rise to atheromatous or atherosclerotic plaques. In its turn, excess fat and cholesterol in the tissues can be picked up (through HDL proteins) and transported to the blood stream back to the liver.
Lipid Sources
The consumption of lipids in the world population is counter-faced. In Western industrialised countries, there is a tendency towards excessive consumption of fats; in the third world, a percentage of the population has a deficient intake.
In our population, the greatest supply of fat comes mainly from foods that are extensively consumed such as milk and its derivates, eggs, and meat, fish and vegetable oils. Fats from commercially available bakery products are also a source of unwelcome fats.
It is estimated that the proportion of fats in the diet is above the recommended daily allowances, accounting for 40% of total calories, with different proportions between saturated and unsaturated fats.
Unsaturated fats, such as olive oil, sunflower oil and corn oil, are from vegetal sources mainly. Saturated fats are from animal sources, even though special mention must be made to the high amount of unsaturated fats found in fish, and the large amount of saturated fat found in some vegetable oils such as coconut or palm oil.