What they are
Commonly called ‘fats’, lipids comprise of a large variety of molecules which have one main characteristic in common: they are insoluble in water. The most important lipids, from a nutritional point of view, are:
Our daily calorie intake should be made up of approximately 30% of lipids for children and 25% for teenagers and adults. These percentages, if applied to ‘normal’ diets, can be very useful for protecting our health. However, these guidelines become more complex and particular according to the quality of the lipids we consume, as well as the amount of saturated, unsaturated and polyunsaturated fatty acids we eat, along with essential fatty acids.
The amount of saturated fatty acids we consume must not be more than 10% of our total, daily calorie intake, however, the amount of cis-monounsaturates can be more (approximately 12%). Amongst monounsaturates, oleic acid should be favoured because it is readily oxidised or stored by the body and then easily disposed of when energy is required. Regarding polyunsaturates, the recommended quantity is less than 10% of our daily calorie intake because of their susceptibility to oxidisation. These oxidative changes actually encourage the production of derivatives (peroxides) that are potentially toxic and that can cause atherosclerotic processes and aging. An excess of polyunsaturates could also bring about the production of lithogenic bile and intestinal carcinogenesis. As far as our daily requirements of essential fatty acids goes, the recommended daily amount is 4.5-6g for adult males and 1-1.5g for adult females.
These are esters (chemical compounds) of glycerol and three fatty acids. Fatty acids are characterised by the difference of their chain’s length (long, medium or short) and by the presence, number and position of double bonds between the carbon and the hydrocarbon chains. Based on these chemical characteristics, fatty acids can be divided in to three categories:
The length of the chain of the fatty acids and the relationship between saturated and unsaturated fats present in a lipid influences the ‘physical state’ of them. The melting point is also important for distinguishing the type of fat, for example, fats that are solid at room temperature are usually characterised by a predominance of saturated fats, whilst oils, that are liquid at room temperature, are characterised by a predominance of unsaturated fats.
There are some essential fatty acids that must be introduced into our diets, such as polyunsaturated fatty acids (lionleic acid and alpha-linolenic acid). In the body, linoleic and linolenic essential fatty acids can be converted into other polyunsaturated fatty acids, which are crucial for bodily functions and indispensible for biosynthesis of eicosanoids (prostaglandins, prostacyclins, thromboxanes and leukotrienes), which are metabolites that are active in many, important bodily functions such as the contraction of smooth muscle, platelet aggregation, inflammatory response, etcetera. Derived fatty acids are essential (and therefore must be introduced through our diets) when the metabolism of the fatty acids from which they derive has been altered. For the synthesis of derivatives, certain enzymes are necessary which, as we get older, become absent. A good diet should compensate for these phenomena.
These are esters of glycerol with fatty acids in positions 1 and 2 and phosphoric acid in position 3. The latter is, in turn, connected to amine bases that have a low molecular weight. They are fundamental components of cell membranes and of complex lipoproteins that are involved in the absorption and transport of lipids.
This an alcohol with a complex and particular structure. As well as being introduced with other animal based foods (exogenous cholesterol), cholesterol is synthesised at a hepatic level (endogenous cholesterol). There is an inverse relationship between our dietary intake of cholesterol and hepatic, endogenous synthesis of it, which creates a control mechanism for the levels of cholesterolemia and which changes remarkably for each individual.
Cholesterol carries out many functions in the body: other than being an essential component of cell membranes, it is also necessary for the biosynthesis of various compounds that have steroidal structures (bile acids, adrenal hormones, androgen, oestrogen and progesterone) and, furthermore, it is a precursor of vitamin D.
The levels of cholesterolemia, in addition to the amount of cholesterol directly consumed in the diet, are sensitive to nutritional influences too, such as the amount of saturated fatty acids we eat (which increase cholesterolemia) and the quantity of monounsaturated and polyunsaturated fatty acids we consume (which decrease it). Fibre can also reduce cholesterolemia levels, in as much as it can reduce the absorption of bile acids and cholesterol, and it can modify bacterial flora, thus indirectly affecting the absorption of bile acids and cholesterol.
Additionally, some phytosterols - natural substances that are present in various plant based foods - have an aggressive effect on cholesterol at a receptor and intestinal absorption level. Beta-phytosterol is especially active in this function and, when there is a good amount of it, it can influence the level of cholesterol in the blood and thus reduce absorption. Extra virgin olive oil can sometimes contain a high content of phytosterols too, and so helps to control cholesterol.