Dietary fats largely consist of triglycerides, molecules with three fatty acids esterified to a glycerol backbone. Fatty acids are classified on the basis of their chain length, the number of double bonds in the molecule, the position of the first double bond from the methyl end and the configuration of the double bonds (trans or cis). Accordingly, fatty acids are categorised as saturated, (cis)-monounsaturated, trans and polyunsaturated (Fig. 1.1).
Saturated fatty acids (SAFAs) have no double bonds. They primarily come from animal products such as meat and dairy products, and from tropical oils such as palm oil, palm kernel oil, and coconut fat. In general, such fats are solid at room temperature. Stearic acid is a saturated fatty acid that may have different biological effects from other saturated fatty acids. Important food sources of stearic acid are beef, hydrogenated vegetable oils and chocolate. Monounsaturated fatty acids (MUFAs) have one double bond. Plant sources that are rich in MUFAs are liquid vegetable oils, such as rapeseed oil, olive oil, high-oleic sunflower oil, and nuts. Polyunsaturated fatty acids (PUFAs) have two or more double bonds. The large majority of PUFA in the diet (90% or more) is linoleic acid, an n-6 (or omega-6) fatty acid. Vegetable oils such as soybean, rapeseed and sunflower oils are important sources. PUFAs also occur as the n-3 (or omega-3) fatty acid alpha-linolenic acid in some vegetable oils and nuts, and as the very long chain n-3 fatty acids in fish and other seafood. Trans fatty acids (TFAs) are unsaturated fatty acids that contain at least one double bond in the trans configuration. TFAs are formed during partial hydrogenation of vegetable oils, and also by natural bio-hydrogenation of fats in the rumen of cattle and sheep. The partial hydrogenation of polyunsaturated oils with cis double bonds causes isomerisation of some of the remaining double bonds and migration of others, resulting in an increase in the trans fatty acid content and the hardening of the oil. Most TFAs are monounsaturated, with the trans double bond at different positions in the carbon chain. Processed fats thus contain a range of trans positional isomers (trans-C18:1n-6 to trans-C18:1n-14), with elaidic acid (trans-C18:1-n-9; Fig. 1.1) often in the largest amount. Dietary sources of trans fatty acids are foods made with partially hydrogenated vegetable oils, such as shortenings, commercially prepared baked goods, snack foods, fried foods and margarine. Trans fatty acids also are present in foods that come from ruminant animals (cattle and sheep); these include dairy products, beef and lamb. The predominant naturally occurring TFA is vaccenic acid (trans--C18:1n-7; Fig. 1.1).
The descriptors 'hydrogenated' and 'partially hydrogenated' on food labels are often used interchangeably but both indicate the presence of TFA in the processed vegetable oil used to prepare the food. For the sake of accuracy, in oil that is fully hydrogenated (i.e. the unsaturated fatty acids have all been converted to stearic acid), there are no trans unsaturated fatty acids. Thus, fats that are partially hydrogenated have variable amounts of TFA depending on the extent of hydrogenation.
Intakes of SAFAs are on average 5 to 10-fold higher than intakes of TFA. The average daily intake of SAFAs is about 11-13% of energy (18-32 g/day) in North America and ranges from 10 to 19% of energy (24 to 60 g/day) across European countries. Dietary SAFAs consist predominantly of lauric acid (C12:0), myristic (C14:0), palmitic acid (C16:0), and stearic acid (C18:0), with stearic acid providing about one-quarter of all SAFAs. The daily intake of total TFA is about 2-3% of energy (ca 4-7 g) in North America and ranges from 0.5 to 2.1% of energy (1.2 to 6.7 g/day) in Europe (Allison et al., 1999; Briefel & Johnson, 2004; Hulshof et al, 1999).
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