Unsaturated fat
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An unsaturated fat is a fat or fatty acid in which there are one or more double bonds in the fatty acid chain. A fat molecule is monounsaturated if it contains one double bond, and polyunsaturated if it contains more than one double bond. Where double bonds are formed, hydrogen atoms are eliminated. Thus, a saturated fat is "saturated" with hydrogen atoms. The greater the degree of unsaturation in a fatty acid (ie, the more double bonds in the fatty acid), the more vulnerable it is to lipid peroxidation (rancidity). Antioxidants can protect unsaturated fat from lipid peroxidation. Unsaturated fats also have a more enlarged shape than saturated fats.
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[edit] Chemistry and Nutrition
Double bonds may be in either a cis or trans isomer, depending on the geometry of the double bond. In the cis conformation hydrogens are on the same side of the double bond, whereas in the trans conformation they are on opposite sides (see also Trans fat). Saturated fats are popular with manufacturers of processed foods because they are less vulnerable to rancidity and are generally more solid at room temperature than unsaturated fats. Unsaturated chains lower melting temperature, hence increasing fluidity of the cell membranes.
Both mono- and polyunsaturated fats can replace saturated fat in the diet; trans unsaturated fats should be avoided. Substituting(replacing) saturated fats with unsaturated fats helps to lower levels of total cholesterol and LDL cholesterol in the blood. This effect is attributed to the low melting point of unsaturated fats found in food. Trans unsaturated fats are particularly bad because the double bond stereochemistry allows the fat molecules to assume a linear conformation which leads to efficient packing (i.e., plaque formation). The geometry of the cis double bond introduces a bend in the molecule precluding stable formations (see specific fatty acid links above for drawings that illustrate this). Natural sources of fatty acids (see above) are rich in the cis isomer.
Although polyunsaturated fats are protective against cardiac arrhythmias a study of post-menopauseal women with a relatively low fat intake showed that polyunsaturated fat was positively associated with progression of coronary atherosclerosis, whereas monounsaturated fat was not [1]. 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 [2].
Examples of unsaturated fats are palmitoleic acid, oleic acid, linoleic acid, and arachidonic acid. Foods containing unsaturated fats include avocado, nuts, and soybean, canola, and olive oils. Meat products contain both saturated and unsaturated fats.
Although unsaturated fats are healthier than saturated fats[citation needed], the old Food and Drug Administration (FDA) recommendation stated that the amount of unsaturated fat consumed should not exceed 30% of one's daily caloric intake (or 67 grams given a 2000 calorie diet). The new dietary guidelines have eliminated this recommendation.
Insulin resistance correlates positively with monounsaturated fat (especially oleic acid) and negatively with polyunsaturated fat (especially arachidonic acid) in the phospholipids of human skeletal muscle [3].
[edit] Membrane composition as a metabolic pacemaker
Cell membranes of mammals have a higher composition of polyunsaturated fat (DHA, omega-3 fatty acid) and a lower composition of monounsaturated fat than do reptiles. Higher polyunsaturated membrane content gives greater membrane fluidity (and functionality), commensurate with the higher metabolic rate of the warm-blooded species. In fish, however, increasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, presumably to maintain greater membrane fluidity (and functionality) at the lower temperatures[4]
[edit] References
- ^ Dariush Mozaffarian (2004). "Dietary fats, carbohydrate, and progression of coronary atherosclerosis in postmenopausal women". American Journal of Clinical Nutrition 80 (5): 1175–1184. PMID 15531663.
- ^ B Leibovitz (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. PMID 2303916.
- ^ LH Storlien (1996). "Dietary fats and insulin action". Diabetologica 39 (6): 621–631. PMID 8781757.
- ^ AJ Hulbert (2003). "Life, death and membrane bilayers". The Journal of Experimental Biology 206: 2303–2311. PMID 12796449.
[edit] See also
- Saturated fat
- Monounsaturated fat
- Polyunsaturated fat
- Trans fat
- Iodine number - a chemical analysis method to determine the proportion of unsaturated fat.
