An unsaturated fat is a fat or fatty acid in which there is at least one double bond within the fatty acid chain. A fatty acid chain is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond.
A saturated fat has no carbon-to-carbon double bonds, so the maximum possible number of hydrogen is bonded to carbon, and thus, is considered to be "saturated" with hydrogen atoms. To form carbon-to-carbon double bonds, hydrogen atoms are removed from the carbon chain. In cellular metabolism, unsaturated fat molecules contain less energy (i.e., fewer calories) than an equivalent amount of saturated fat. The greater the degree of unsaturation in a fatty acid (i.e., the more double bonds in the fatty acid) the more susceptible it becomes to lipid peroxidation (rancidity). Antioxidants can protect unsaturated fat, from lipid, peroxidation.
Composition of common fats
editIn chemical analysis, fats are broken down to their constituent fatty acids, which can be analyzed in various ways. In one approach, fats undergo transesterification to give fatty acid methyl esters (FAMEs), which are amenable to separation and quantitation using gas chromatography.[1] Classically, unsaturated isomers were separated and identified by argentation thin-layer chromatography.[2]
| fat source | saturated | monounsaturated | doubly unsaturated | triunsaturated |
|---|---|---|---|---|
| Palm kernel | 60–65 | 10–18 | 1–3 | trace |
| Cottonseed oil | 23–30 | 14–21 | 45–58 | trace |
| Corn oil | 10–15 | 25–35 | 40–60 | trace |
| Linseed oil | 8–11 | 18–26 | 14–20 | 51–56 |
| Soybean oil | 11–17 | 18–25 | 49–57 | 6–11 |
| Peanut oil | 12–17 | 35–42 | 39–44 | trace |
| Lard | 36–48 | 36–52 | 10–12 | 1 |
| Beef tallow | 43–64 | 26–45 | 2–6 | 1 |
The saturated fatty acid components are almost exclusively stearic (C18) and palmitic acids (C16). Monounsaturated fats are almost exclusively oleic acid. Linolenic acid comprises most of the triunsaturated fatty acid component.
Chemistry and nutrition
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Although polyunsaturated fats are protective against cardiac arrhythmias, a study of post-menopausal women with a relatively low fat intake showed that polyunsaturated fat is positively associated with progression of coronary atherosclerosis, whereas monounsaturated fat is not.[4] This probably is an indication of the greater vulnerability of polyunsaturated fats to lipid peroxidation, against which vitamin E has been shown to be protective.[5]
Examples of unsaturated fatty acids are palmitoleic acid, oleic acid, myristoleic acid, linoleic acid, and arachidonic acid. Foods containing unsaturated fats include avocado, nuts, olive oils, and vegetable oils such as canola. Meat products contain both saturated and unsaturated fats.
Although unsaturated fats are conventionally regarded as 'healthier' than saturated fats,[6] the United States Food and Drug Administration (FDA) recommendation stated that the amount of unsaturated fat consumed should not exceed 30% of one's daily caloric intake.[7] Most foods contain both unsaturated and saturated fats. Marketers advertise only one or the other, depending on which one makes up the majority. Thus, various unsaturated fat vegetable oils, such as olive oils, also contain saturated fat.[8]
Membrane composition as a metabolic pacemaker
editStudies on the cell membranes of mammals and reptiles discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids (DHA, omega-3 fatty acid) than reptiles.[9] Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega-3 fatty acids as compared to omega-6 for a given body size.[10] This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions (H+ & Na+), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitant warm-bloodedness of mammals and birds.[9] However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In fish increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lower temperatures.[11][12]
See also
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- Iodine value – a chemical analysis method to determine the proportion of unsaturated fat.
- List of unsaturated fatty acids
References
edit- ^ Aizpurua-Olaizola O, Ormazabal M, Vallejo A, Olivares M, Navarro P, Etxebarria N, et al. (January 2015). "Optimization of supercritical fluid consecutive extractions of fatty acids and polyphenols from Vitis vinifera grape wastes". Journal of Food Science. 80 (1): E101-7. doi:10.1111/1750-3841.12715. PMID 25471637.
- ^ Breuer B, Stuhlfauth T, Fock HP (July 1987). "Separation of fatty acids or methyl esters including positional and geometric isomers by alumina argentation thin-layer chromatography". Journal of Chromatographic Science. 25 (7): 302–6. doi:10.1093/chromsci/25.7.302. PMID 3611285.
- ^ Thomas, Alfred (2000). "Fats and Fatty Oils". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a10_173. ISBN 9783527303854.
- ^ Mozaffarian D, Rimm EB, Herrington DM (November 2004). "Dietary fats, carbohydrate, and progression of coronary atherosclerosis in postmenopausal women". The American Journal of Clinical Nutrition. 80 (5): 1175–84. doi:10.1093/ajcn/80.5.1175. PMC 1270002. PMID 15531663.
- ^ Leibovitz B, Hu ML, Tappel AL (January 1990). "Dietary supplements of vitamin E, beta-carotene, coenzyme Q10 and selenium protect tissues against lipid peroxidation in rat tissue slices". The Journal of Nutrition. 120 (1): 97–104. doi:10.1093/jn/120.1.97. PMID 2303916.
- ^ Fats and sugars. BBC Health, retrieved 2013-04-07
- ^ "Fat Facts, the Right Amount for a Healthy Diet". extension.psu.edu. Retrieved 2024-06-03.
- ^ Storlien LH, Baur LA, Kriketos AD, Pan DA, Cooney GJ, Jenkins AB, et al. (June 1996). "Dietary fats and insulin action". Diabetologia. 39 (6): 621–31. doi:10.1007/BF00418533. PMID 8781757. S2CID 33171616.
- ^ a b Hulbert AJ, Else PL (August 1999). "Membranes as possible pacemakers of metabolism". Journal of Theoretical Biology. 199 (3): 257–74. Bibcode:1999JThBi.199..257H. doi:10.1006/jtbi.1999.0955. PMID 10433891.
- ^ Hulbert AJ, Faulks S, Buttemer WA, Else PL (November 2002). "Acyl composition of muscle membranes varies with body size in birds". The Journal of Experimental Biology. 205 (Pt 22): 3561–9. doi:10.1242/jeb.205.22.3561. PMID 12364409.
- ^ Hulbert AJ (July 2003). "Life, death and membrane bilayers". The Journal of Experimental Biology. 206 (Pt 14): 2303–11. doi:10.1242/jeb.00399. PMID 12796449.
- ^ Raynard RS, Cossins AR (May 1991). "Homeoviscous adaptation and thermal compensation of sodium pump of trout erythrocytes". The American Journal of Physiology. 260 (5 Pt 2): R916–24. doi:10.1152/ajpregu.1991.260.5.R916. PMID 2035703. S2CID 24441498.