Structure of fat
The building blocks of fat (lipid) are fatty acids and glycerol (Figure 1). Fatty acids found in foods and fats stored in the body are mainly present in the form of triacylglycerols (TAG, sometimes referred to as triglycerides); a glycerol molecule backbone to which three, often different, fatty acids are attached (Figure 1). Individual fatty acids can be present at any of three positions on the glycerol molecule (termed sn-1, sn-2 and sn-3). The presence of specific fatty acids at different positions on the glycerol molecule will also influence its characteristics (i.e. melting point and digestibility).
The fatty acids (F1, F2, F3) in the sn-1, sn-2, sn-3 positions are often different from each other. Regardless of the type of fatty acid, at one end of the carbon chain is the acid group, show in Figure 2, that is able to combine with glycerol.
A fatty acid is made up of a chain of carbon atoms, with a methyl group at one end and an acid group at the other (Figure 2).
Attached to the carbon chain are hydrogen atoms, the number per carbon atom depending on whether the fatty acid is ‘saturated’ or ‘unsaturated’ (see Figure 3).
If the fatty acid has all the hydrogen atoms it can hold it is said to be saturated.
If some of the hydrogen atoms are missing and have been replaced by a double bond between the carbon atoms, then the fatty acid is said to be unsaturated.
If there is one double bond, the fatty acid is known as a monounsaturated fatty acid.
If there is more than one double bond, then the fatty acid is known as a polyunsaturated fatty acid.
In polyunsaturated fatty acids, the hydrogen atoms can be arranged in one of two ways. One arrangement is called cis, the other is called trans. Cis is the usual form found in nature. Positioning of the double bonds within the polyunsaturated fatty acid structure determines its type, the two main types being omega 3 (n-3) and omega 6 (n-6) fatty acids (some omega -9 (n-9) fatty acids also exist).
Fat is made up of different types of fatty acids, some of which are essential for health in small amounts. Fatty acids are usually classified as saturated, monounsaturated or polyunsaturated depending on their chemical structure. Among the polyunsaturates there are further structural differences which determine whether the fatty acid is known as an omega 3 (n-3) or omega 6 (n-6) fatty acid. These structural differences directly influence health effects, with mono- and polyunsaturates usually being associated with health benefits when consumed as part of a varied diet. The exception to this is trans fatty acids, which are unsaturated in terms of their structure but behave in the body like saturated fatty acids.
Fat provides energy; 1 gram provides 37 kJ (9 kcal). Foods that contain a lot of fat provide a lot of energy. Fat is a carrier of fat-soluble vitamins and is necessary for their absorption.
A high intake of saturated or trans fatty acids can have adverse effects on health.
In the UK, saturates currently contribute 12.7% of food energy in adults, which is above the recommendation of 11%, whereas average total fat intake is close to the 35% of food energy recommended for the population.
Intake of trans fatty acids is now well below the population recommendation of no more than 2% of food energy, at 0.7%.
In the UK, intakes of omega-6 polyunsaturated (principally linoleic acid) are close to the recommendation of 6.5% of dietary energy, but intakes of the long chain omega-3 fatty acids found in fish oils are low compared to recent recommendations.
Fat is the richest source of dietary energy available in the diet and so can readily contribute to weight gain. The structure of the building blocks of dietary fat – fatty acids – determines their health effects. Some fatty acids are essential components of the diet but others can be detrimental. As with most nutrients, recommendations exist to help establish dietary balance.
Trans fatty acids
Trans fatty acids occur seldom in nature but are produced naturally in the rumen of cows and sheep (ruminants) and so are present naturally in milk, beef and lamb. Trans double bonds can be produced during the processing of oils and trans fatty acids produced in this way have been targeted for reduction because they have been shown to have similar effects in the body to saturates (raise blood LDL cholesterol) but they also reduce HDL cholesterol, which means that their effect is more detrimental than saturates. They also influence other heart disease risk factors. Good progress has already been made by the food industry to remove trans fatty acids from the food chain and average intake in the UK is now thought to be 0.7% (and so below the target of less than 2% of energy intake).
This has largely come about by the use of alternative methods to process oils to make spreads and cooking fats. Nevertheless, intakes will be higher than this in those population subgroups who consume products that still contain partially hydrogenated (i.e. trans-containing) vegetable oils, so there is still room for more reformulation in some food categories.
Essential fatty acids
The body can make the fatty acids it needs except for two, known as alpha linolenic acid (omega-3) and linoleic acid (omega-6) fatty acids. Theseare called the Essential Fatty Acids (EFAs) and must be supplied in the diet. From these fatty acids we can make others which are important for health. From linoleic acid we make arachidonic acid and from ?-linolenic acid we make EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). These can also be obtained pre-formed from the diet. Oily fish and fish oil supplements are rich sources of EPA and DHA. They are also present in the meat or eggs of animals fed omega 3 (n-3) fatty acid-enriched diets, i.e. foods produced in this way could be considered alternative sources to oily fish, although the amounts of the so called ‘long chain’ omega-3 fatty acids present are typically much less than in oily fish. Arachidonic acid (a long chain omega-6 fatty acid) is found in small amounts in meat (especially pork and the dark meat from chicken and turkey) and in oily fish.
The EFAs and the fatty acids synthesised from them are important components of cell membranes. EPA and arachidonic acid are substrates for important signalling molecules, such as eicosanoids, that control many important functions at a cellular level.
Fatty acids in foods
The nature of a fat depends on the types of fatty acids which make up the triacylglycerol molecules. All dietary sources of fat contain both saturated and unsaturated fatty acids but are sometimes described as ‘saturated fat’ or ‘unsaturated fat’ according to the proportions of fatty acids present. For example, butter is often described as a ‘saturated fat’ because it has more saturated fatty acids than unsaturated fatty acids, while most vegetable oils are described as ‘unsaturated fats’ as they have more mono- and polyunsaturated fatty acids than saturated.
Generally, saturates are solid at room temperature and tend to be derived from animal sources. Most unsaturates are liquid at room temperature and are usually vegetable fats. But there are exceptions. Palm oil is a vegetable oil which contains a high percentage of saturated fatty acids. Also, vegetable and fish oils can also be hardened by a process which adds hydrogen atoms to some of the double bonds in the unsaturated fatty acids. This is known as hydrogenation. In this way some of the unsaturated fatty acids become fully saturated, and some trans fatty acids may be formed. Hydrogenated oils have traditionally been used in the manufacture of margarine and cooking fats. However, in recent years, because of concern about the health implications of high trans fatty acid intakes, manufacturing practices have changed considerably, resulting in a reduction in the trans fatty acid content of the diet.
Currently, intake in the UK is below 1% of energy intake; well below the population dietary reference value of no more than 2% of energy. This proactive approach on behalf of the food industry has been slower to take off in some parts of the world, and so trans intakes in the UK have fallen faster as a result.