Triglycerides
Most of the fat consumed by humans is in the form of neutral fats which are formed when three fatty acids combine with glycerol to form a triglyceride (Fig. 3.1).
Figure 3.1. Formation of a triglyceride (a neutral fat)
Saturated and unsaturated fatty acids
Fatty acids consist of hydrocarbon backbones which terminate in a carboxyl group (-COOH). Carbon has a valency of four, which means that it can combine with four other atoms, and in a saturated fatty acid all the additional bonds of the carbon chain are occupied by hydrogen atoms. There are thus no double bonds in the carbon chain of a saturated fatty acid. Animal fats are rich in saturated fatty acids and they contain mainly palmitic acid which contains sixteen carbon atoms, and stearic acid, which contains eighteen carbon atoms (figure 3.2)
Figure 3.2. The structure of palmitic and stearic acid.
Figure 3.3. The structure of oleic acid.
Unsaturated fatty acids have one or more double bonds between carbon atoms, and the carbon molecules are therefore “unsaturated” with hydrogen atoms. Unsaturated fatty acids are commonly found in plant foods, and can be either monounsaturated fatty acids or polyunsaturated fatty acids, depending on the number of carbon atoms which contain double bonds. The unsaturated fatty acids are classified according to the position of the double bond which they contain ie, omega-9 (n-9), omega-6 (n-6), or omega-3 (n-3), then they can also have different isometric configurations, namely cis or trans. The principle fatty acid found in olive oil, for example, is oleic acid, and because oleic acid contains only one double bond in its carbon chain, it is a monounsaturated fatty acid (fig. 3.3).
Polyunsaturated fatty acids contain more than one double bond in the carbon chain and they are more common in plant foods than in animal foods. The most important polyunsaturates are linoleic acid, linolenic acid and arachidonic acid, of which linoleic acid is the most abundant. A further fatty acid in this group is eicosopentanoic acid which is a long-chain fatty acid found in fish oil. The double bonds between the carbon atoms of polyunsaturated fatty acids make the fat more fluid and lower the melting point, which is the reason why plant oils are normally liquid and animal fats are solid.
Essential fatty acids
Essential fatty acids are fatty acids which we cannot manufacture ourselves and which must therefore be obtained from the diet. Animals and humans are capable of elongating fatty acid chains and they can also desaturate fatty acid chains but they cannot desaturate the fatty acid carbon chain at positions six and three. Because linoleic acid and a-linolenic acid are unsaturated at these positions, they cannot be manufactured from other fatty acids and must therefore be obtained from the diet. These two fatty acids are essential components of cell membranes and act as precursors to a group of molecules known as prostaglandins. They are not required in large quantities, and it has been found that if they comprise 1% of total calories, then this is sufficient to prevent deficiency manifestations. Arachidonic acid is similar in structure to linoleic acid and α-linolenic acid, and originally it was also considered to be an essential fatty acid, but since it can be manufactured from linoleic acid, it is no longer considered an essential fatty acid. The structures of linoleic and linolenic acid are given in figure 3.4 and in figure 3.5 the distribution of fatty acids in plant foods is presented. In table 3.1 the major fatty acids in Western diets are presented.
Figure 3.4. The structure of linoleic and linolenic acid.
Cholesterol
Cholesterol belongs to a group of fats known as steroids. Cholesterol has been blamed for the coronary havoc wreaked in the world today, but cholesterol is an essential compound in all animals, and it forms an important substrate for several biosynthetic pathways. Cholesterol occurs in almost all samples of animal fats as well as in blood and bile and is thus mainly derived from the consumption of animal foods. Another source of cholesterol is endogenous cholesterol, which is synthesized in the liver and intestine. Contrary to popular belief, plants do not produce cholesterol although they do produce phytosteroids. The consumption of plant foods per se, can therefore not increase cholesterol levels. In the absence of dietary cholesterol, the liver will synthesize enough cholesterol to meet all the needs of the body and a vegetarian diet will thus not lead to shortages in cholesterol.
Figure 3.5. Fatty acid distribution in vegetable oils. (Adapted from reference 5)
Cholesterol is usually bound to two kinds of protein carriers which are called high-density lipoprotein (HDL) and low-density lipoprotein (LDL). High levels of LDL are associated with vascular disease as these molecules tend to infiltrate arterial walls, whereas HDL seems to attract cholesterol out of the walls and transports it to the liver where it is metabolized. The consumption of animal fats will lead to increased levels of the harmful LDL-cholesterol, whereas plant foods tend to lower cholesterol levels. In table 3.2 the concentration of cholesterol in various foods is presented.
Table 3.1. The major fatty acids in the US diet. Besides these long-chain fatty acids the diets also include small quantities of short- and medium-chain fatty acids as well as other long-chain fatty acids. (From reference 6).
Table 3.2. The cholesterol content of selected foods. The amounts are for 100g edible