180. ME TABOL ISH OF THE UNSA TURA TED AND SA TURA TED FA TS F. A. KUMMEROW No chemical distinction exists between plastic fats (shortening) and vegetable oils (salad oils); both contain glycerol connected to or "esterified" with three moles of fatty acid (Table 1). The saturated fatty acids in the comrnon edible fats vary from C4 to C22 in chain length and -7.9 to 79.9% in melting point. The unsaturated fatty acids in edible fats vary from C to C20 in chain length and -1 to -49OC in melting point (Markley '60). '$fe high melting points of the saturated and the low melt- ing points of the unsaturated fatty acids are important to the physical characteristics of fats and oils. If the glycerol is esterified with more than two unsaturated fatty acids, i.e., oleic or linoleic acid, the resulting triglyceride is a liquid or an ''oil" at room temperature. If, on the other hand, glycerol is esterified with only long chain saturated fatty acids or only one mole of oleic and two moles of palmitic or stearic acid, the resulting triglycer- ides is a solid or "fat" at room temperature (Table,2). A study of isolated triglycerides has shown that the substitution of one mle of oleic for stearic acid in an d -position in tristearin for example lowers the melting point from 73 to 38OC and the same substitution in tripalmitin from 66 to When one considers that the body temperature of a 35OC (Bailey '50). human being is 37.ZoC, the & substitution of linoleic or oleic acid for one mole of stearic or palmitic acid may change the physical character of the depot fat and its ability to act as EL "cushioning agent" to vital organs. These depot fats may become soft and oily. Natural fats and oils have been found to contain mixtures of triglycerides which are uniquely characteristic of a specific fat (Hilditch '56). As indicated by the melting points of isolated triglycerides, the physical properties of the mixture of triglycerides are governed by the physical properties of the particular fatty acid which is esterified with the glycerol (Table 3). In ''soft" fats such as corn or cottonseed oil, which contab the unsaturated oleic and linoleic acids as the predominant fatty acids, "the oils" are composed of a high proportion of di and tri unsaturated glycerides. In "hard" fats such as coconut oil, butterfat and beef tallow, which contain the saturated, myristic, palmitic and stearic acids as the predominant fatty acids, "the fats" are composed of a high proportion of di and tri saturated glycerides. Although coconut oil is a vegetable oil, it is classified as a fat as it contains 84% tri saturated glycerides and is a solid at room temperature. Human adipose tissue fat and human milk fat are semi solid fats with a high proportion of mono and di saturated glycerides. 181. The glycerides of human adipose tissue contain approximately 4% myristic, 25% palmitic, 7% stearic, 6% palmitoleic, 46$ oleic and 2% of fatty acids which are shorter than 14 or longer than 18 carbon atoms in chain length (Cramer and Brown '43). These fatty acids may be classified the "non essential fatty acids" as they can all be synthesized in the body from non fat precursors. They are also found in corn oil, beef tallow and lard but in different percentage composition in each case. In addition the adipose tissue fat is composed of approximately 9% linoleic and 1% arachidonic acid which contain two and four double bonds respectively. These two fatty acids have been classified the "essential fatty acids" as linoleic acid cannot be synthesized by animal tissue and serves as an essential precursor for the synthesis of arachidonic acid. The ability of unsaturated fatty acids to form "geometric isomers", plays an important role in the degree of hardness of a "hydrogen- ated" fat such as margarine (Table 4) and probably of human depot fats. Geometric isomers are important to the plasticity of fats because the -trans isomers of oleic and linoleic acid, which are produced during the commercial hydrogenation of an edible oil have melting points of 52OC and 29OC respectively and therefore tend to "harden" margarine. The natural cis isomers of oleic and linoleic acid have melting points of 14OC and mCrespectively and therefore tend to "soften" margarine at room temper- ature or 21OC. This property of oleic and linoleic acids to exist in either a solid or a liquid state at room temperature is important to the produc- tion of margarines of high linoleic acid content for two reasons. One, even though the trans isomers of oleic and linoleic acids influence the plasticity of margarine, these trans isomers have the same degree of un- saturation or iodine number as the natural cis isomers and therefore both contribute to the calculated tfpolyunsaturates" and both thus increase the polyunsaturated to saturated or P/S ratio of the fat. Two, the deliberate production of the high melting trans oleic and linoleic acid during com- mercial hydrogenation allows the margarine mnaufacturer to add a higher percentage of the low melting linoleic acid to a margarine fat without sacrificing the degree of desirable hardness. Thus mdern margarines, whether made from corn oil, cottonseed oil, or soybean oil, all contain two to three times more linoleic acid then a few years ago, but they also con- tain more trans oleic acid. We purchased four typical brands of both high and low priced margarine at a local supermarket and subjected them to fatty acid and infrared analysis (Table 5). The results (Kummerow '64) indicated that all of them contained more linoleic acid than the margarines which were avail- able a few years ago (Bailey '51). Furthemre, the cost of these margarines was independent of their linoleic acid content; the lowest priced margarine contained 15% more linoleic acid than the medium priced brand and only 54 less linoleic acid than the highest priced brand. The large arnount of "trans" fatty acid in all of the margarines indicated that both trans oleic and trans, trans linoleic acid may have been produced during the hydrogen- ation of soybean oil, which forms the base stock of most margarines. The trans fatty acids present in human tissue apparently arise solely from dietary fat, and as in rats, they do not normally appear in the tissues unless a source of trans fatty acids is included in the diet. Samples of fat from human placental, maternal., fetal, and baby tissue were 182. examined for the presence of trans fatty acids. While the maternal tissue contained considerable amounts of trans fatty acids, these lipids were not found to any measurable extent in placental, fetal, or baby fat (Johnston '58) a The percentage of trans fatty acids in rat fat decreased when trans fatty acids were removed from the diet (Johnston '58a). However, they did not completely disappear from the tissue even at the end of two months on a diet free of trans fatty acid. After one month on the diet free of trans fatty acid, the carcass fat of the rats which had received 10% of margarine stock had decreased from 18.6 to 6.5% and after two months to 4.4% of trans fatty acids. The carcass fat of the animals which had re- ceived margarine stock and olive oil contained approximately 11% of trans fatty acids. After one month on a diet free of trans fatty acids, the car- cass fat decreased to 4.9% and after two months to 2.8% of -trans fatty acids. It seems evident that the high trans fatty acid content of marga- rine fat could "harden" human depot fat and counteract the "softening" in- fluence of linoleic acid. The iodine value of such depot fat would indicate a higher P/S ratio. However, the melting point and other physical characteristics of the depot fat might not be changed significantly from a depot fat which contained stearic instead of trans oleic acid, that is someone eating butterfat instead of margarine. The palmitic and stearic acid which is found in tissue fat does not have to be consumed as a component of dietary fats. It has been shown with the aid of CI4 labeled acetic acid (two carbon atoms long in chain length) that fatty acids can be shortened or elongated --in vivo so that triglycerides specific to each species can be synthesized in animal tissue. For example (Table 6), it has been shown that stearic acid can be converted to palmitic acid through the collaboration of five different enzymes and the presence of the proper cofactors (Bloch '60). In the overall reaction coenzyme A adds to stearic acid and two carbon atoms are removed as acetyl Co A. The resulting palmityl Co A can add --in vivo to a dig1ycerj.de to pro- duce a triglyceride which contains one mole of pdmitic instead of stearic acid. When it is not needed for triglyceride synthesis, the palmityl Co A can be degraded until all of it is converted to acetyl Co A. The acetyl Co A, in the presence of bicarbonate, adenosine triphosphate and biotin enzyme, can be carboxylated to form malonyl Co A (Table 7). The malonyl Co A in the presence of reduced triphosphopyridine nucleotide (TFNH) and with the elimination of water can be converted back to palmitic acid (Lynen '61). In the process of synthesis, both palmitic and stearic acid can be dehydrogenated to palmitoleic or oleic acid respec- tively. Thus with the aid of a dietary source of essential fatty acids, animal tissue can produce fatty acids of proper chain length and the degree of unsaturation which is best suited for its needs.