A fat is a triester of glycerol and fatty acids.
If the fatty acid side chains (R’,R’’and R”’)consist of no double-bonded carbons (C=C), then no more hydrogen can react with it, and the fat is said to be saturated. Such a compound tends to have a higher melting point and is more likely to be solid at room temperature. Fatty acids such as lauric acid, palmitatic acid and stearic acid are all saturated fatty acids. They have 12, 16 and 18 carbon atoms, respectively, with the following formulae:
(see Biochemistry 3rd Ed by Stryer p 470 for a more complete list. )
Unsaturated fats have one or more pairs of double-bonded carbons which can be potentially "saturated" with hydrogen. If only one pair of double-bonded carbons exists, the fat is said to be monounsaturated. An example is oleic acid, a main constituent of olive oil, show below. If more than one double-bonded carbons appears then the fat is polyunsaturated. An example of a polyunsaturated fatty acid is linoleic acid. Unsaturated fats make up the majority (80-90+%) of fats found in corn, cottonseed, safflower, soybean and olive oils.
Yes. Mammals cannot introduce double bonds at carbon atoms beyond C-9 in the
fatty acid chain.( Stryer, p
490). Linoleate (18 carbons ,
2 double bonds)
(Source: Composition of Foods, USDA Handbook No.8 ) Search the Data Base . Without these polyunsaturated fatty acids, the body would not, among other things, be able to produce a group of hormones known as prostoglandins, which are needed for regulation of blood flow to organs, ion transport, modulation of synaptic transmission and stimulation of inflammation. (Stryer, p992).
Either to raise the melting point of oils (thus rendering them solid at room temperature) and/or to prolong shelf life, the food industry often hydrogenates unsaturated fats and produces saturated ones. If they partially hydrogenate vegetable oils in order to create a softer product, they create an even greater percent of trans fats through isomerization. These, unlike cis fats, have hydrogen atoms positioned diagonally across a carbon double bond.
More importantly , a study published in Nov. 1997 in the New England Journal of Medicine tracked 80 000 nurses for 14 years, (also see Market Place, January 6th , 1998; and Trans Fat Fact Sheet) and by comparing dietary data, the researchers concluded that trans fats were more likely to lead to heart disease than saturated fats. Whereas saturated fats increase both HDL (high-density lipoproteins, which carry cholesterol back to cell membranes) and LDL (low-density lipoprotein which carry cholesterol to blood vessels), trans fats raise LDL and lower the beneficial HDL. LDL contributes to heart disease by laying down fatty deposits in the walls of arteries.
Even healthy oils can be converted to trans fats through deep frying, specifically when temperatures exceed 180 oC.
In the table below I selected foods from various food groups and I sorted them according to their ratio of the sum of saturated and trans-fats to their total fat content. In the last column of the spreadsheet, I actually multiplied the number of trans-fat grams by 5 since research suggests that they are worse than saturated fat. The lower the ratio, the healthier the food. Not surprisingly we see corn oil, walnuts and olive leading the way, and the worst fat sources are McDonald’s and Burger King French fries and foods rich in partially hydrogenated palm oil. (Note that some fats are counted twice since omega 3 and 6 are a type of polyunsaturated fat)
“In this large, prospective study of women, we found that a higher dietary intake of saturated fat and trans unsaturated fat was associated with an increased risk of coronary disease, whereas a higher intake of monounsaturated and polyunsaturated fats was associated with a decreased risk.”
We have already mentioned linolenate, the
essential fatty acid. If you look at its structure, the acidic H+1 is
missing from the carboxylate group (COO-1).
With the acidic H+1, linolenate becomes linolenic acid, which comes in two forms, the alpha(ALA)
and gamma form. The latter has double bonds in diffferent
positions. The location of the three double bonds of alpha linolenic
acid was shown in the structure of linolenate.
Anyhow, what's important is that ALA has been associated with a lower incidence of heart disease and stroke. (see Leaf A and Weber PC. N Engl J Med. 1988;318:549-557 and Dupont J, et al. Am J Clin Nutr. 1996;63:991S-993S.) A high concentration of ALA in the blood probably prevents blood clots from forming. Foods rich in ALA are not common but include walnuts, purslane (flaxseed), canola oil and soybean oil. Note that hens whose diets are supplemented with flaxseed lay eggs with higher concentrations of ALA.
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