Hydrogenated soybean oil as a major food fat in the American dietary. Food uses of hydrogenated soybean oil are in the production of margarine, shortening, and salad and cooking oils. These products are not formulated from completely hydrogenated vegetable oils; soybean oils for margarines and salad and cooking oils are only partially hydrogenated and the saturated fatty acid content is increased little by the process. The content of polyunsaturates in a soft tub margarine or salad and cooking oil may be two to three times that of the saturated fatty acids. Shortenings of the plastic type have a higher saturated fatty acids content than margarines but are no higher in this respect than the animal fats they have replaced, and generally have a higher content of polyunsaturated fatty acids.
Geometric and position isomers of the unsaturated fatty acids formed in hydrogenation have physical properties different from the isomers present in the natural oil. Presence of these unnatural isomers has led to extensive investigation of their nutritional and biological properties. The major geometric isomer formed is elaidic acid, the trans-isomer of oleic acid, although cis, trans-dienoic acids also are produced. It is estimated that the trans fatty acids content of the fat in the average American diet, largely contributed by hydrogenated vegetable oil products, is about 8 percent.
Digestibility of partially hydrogenated soybean oils is high and comparable to that of the unhydrogenated oil. No difference has been found in the intestinal absortion and rates of disappearance from the plasma of the trans-isomers of oleic and linoleic acid as compared to the cis-isomers. Rates of excretion as respiratory carbon dioxide are essentially the same for the trans- and cis-isomers.
The trans-isomers of linoleic acid do not have essential fatty acid activity, but the content of the cis- cis- isomer in hydrogenated soybean oil products is as high or higher than is found in butter fat, lard or tallow. Presence of either the trans-isomer of oleic acid or the trans-isomers of linoleic acid does not interfere with the utilization of linoleic acid as an essential fatty acid deficiency posed to consumers of commercial hydrogenated soybean oil products.
Many animal studies have demonstrated that the deposition of dietary trans-fatty acids in tissues and tissue lipids is selective. In vitro studies with liver mitochondrial and microsomal enzymes have demonstrated selectivity of action among fatty acids geometric and position isomers in the synthesis and hydrolysis of phospholipids and cholesterol esters. In a human subject labeled oleic and elaidic acids were rapidly exchanged with the fatty acids in the serum lipids. Analysis of human tissues at autopsy reflected the presence of trans- fatty acids in the diet.
In vitro experiments indicate that membrane functions can be affected by the incorporation of trans- fatty acids in experimental diets. Liver mitochondria isolated from rats fed elaidic acid exhibited two to threefold greater initial rates of swelling in hypotonic swelling media compared to those from control rats. Erythrocytes containing trans-fatty acids hemolyzed at a rate five times greater than that of control cells when incubated with ?- lecithinase. However, a long-term rat feeding test of a hydrogenated soybean-cottonseed oil containing 35 percent trans- acids showed no histopathology attributable to diet. Feeding hydrogenated oil of this composition to rats for 60 generations produced no adverse effects on fertility, litter, size, weight at weaning, growth at 90 days, and longevity. A two-year rat feeding test of a hydrogenated soybean cooking oil and a control soybean oil revealed no differences in histopathology attributable to diet.
Considerable experimental evidence shows that polyunsaturated fatty acids in dietary fat as triglycerids from natural sources have a hypocholesterolemic effect, monounsaturated fatty acids are neutral and saturated fatty acids, particularly those 12 to 16 carbons in chain lengh, are hypercholesterolemic when they replace mixed carbohydrates on an isocaloric basis. Soy bean oil, however, contains only traces of the 12 to 16 carbon unsaturated acids which can be converted to the corresponding saturated acids by hydrogenation, and it is only some of the industrial shortening that the stearic acid content is increases by hydrogenation. Although results of studies on the effect of trans-isomers on serum lipid levels in human are not definitive, the weight of evidence indicates that trans-monoenoic acids, the principal geometric isomers present in hydrogenated soybean oil, are not hypercholesterolemic. Similarly, the results of animal experimentation indicate that trans-acids of hydrogenated soybean oil are not atherogenic at normal dietary levels.
In the light of the foregoing, the Select Commmittee concludes that: There is no evidence in the available information on hydrogenated soybean oil that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when it is used as a direct or indirect food ingredient at levels that are now current or that might reasonably be expected in the future.