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Widespread use of partially hydrogenated vegetable oils in the United States during the past six or seven decades has raised questions about the health effects resulting from the consumption of trans fatty acids (TFAs) present in these products.
TFAs are formed during partial hydrogenation of fats and oils, a process used to impart desirable stability and physical properties to such food products as margarines, shortenings, frying fats, and specialty fats (e.g., for fillings, toppings, and candy). In addition, small amounts of TFAs occur naturally in foods such as milk, butter, and tallow as a result of biohydrogenation in ruminants.
Relatively high TFA intakes (greater than about 4 energy %) have been found to increase LDL-cholesterol levels compared to cis monounsaturated and polyunsaturated fatty acids. This relationship between TFAs and LDL-cholesterol suggests that TFAs may increase risk of coronary heart disease (CHD). On the other hand, the impact of TFAs appears to be reduced at high intakes of linoleic acid. Additionally, some but not all studies have shown decreased HDL-cholesterol levels after feeding high levels of TFAs compared to cis unsaturated fatty acids. Saturated fatty acids are well known to raise LDL-cholesterol levels (except for stearic acid which has a neutral effect on LDL-cholesterol), and the contribution of saturated fatty acids to U.S. and European diets is much greater than that of TFAs. Several epidemiologic studies have reported associations between intake of TFAs and mortality from CHD in the U.S. However, the mortality has continued to decrease during the past 25–30 years, while the availability of TFAs for consumption has remained relatively constant. Also, controlled studies with animals have shown that TFAs are not uniquely atherogenic.
TFAs have not been found to be tumor promoting in animals. A class of TFAs known as conjugated linoleic acid (CLA) appears to exhibit both anti-tumorigenic and anti-atherogenic properties. There appears to be little evidence that dietary TFAs influence growth, reproduction, or gross aspects of fetal development. One recent study reported an association of type 2 diabetes with dietary TFAs, whereas a followup study found no such association. There is no known functional or physiologic relation to connect TFAs to disease mechanisms involved with type 2 diabetes. Neither of two recent studies reported an association between dietary TFAs and risk of developing macular degeneration.
The current average level of intake of TFAs in the U.S. (2.6% of energy) is probably not a problem for most healthy individuals consuming a balanced diet. However, individuals in risk categories (e.g., those with a family history of CHD, high LDL-cholesterol levels, etc.) may need to reduce their intakes of both trans and saturated fatty acids. It is not clear at present what dietary level of TFAs may be sufficient to increase health risks and at what level and circumstances such concerns are unwarranted. A general consensus among health professional organizations in the U.S. and Europe is that the intake of cholesterol-raising fatty acids, including TFAs, should be reduced. The National Academy of Sciences’ Institute of Medicine has recommended that TFA consumption be as low as possible while consuming a nutritionally adequate diet.
Health professional organizations favor the reporting of trans fat content on food labels. The food industry generally supports such labeling. The Food and Drug Administration has proposed to require foods containing measurable levels (≥0.5 g/serving) to label trans fat content.
Efforts are ongoing to decrease TFAs in the food supply, both in the U.S. and globally. Trans-free margarines and spreads currently are marketed, largely in Europe and to a lesser degree in the U.S. The fats used in making such products have been produced primarily by either (1) blending fully hydrogenated hard fats having no TFAs (but a high content of stearic acid) with unhydrogenated oils or (2) interesterifying (molecularly rearranging) unhydrogenated oils with saturated-fat base oils. It is important to note that products containing fat blends with fully hydrogenated oil would need to include the term “hydrogenated fat” on their ingredient statements even though such products would contain no TFA.
For frying, an alternative to using partially hydrogenated oils containing TFAs would be to use stable vegetable oils relatively high in oleic acid (cis-18:1) derived through traditional plant breeding or biotechnological methods. High- and midrange-oleic sunflower oil, high-oleic safflower oil, and high-oleic soybean oil have been developed and are becoming more available. These high–oleic acid fats have increased oxidative stability for frying compared to their native unhydrogenated counterparts high in linoleic acid.
The challenge to the food industry in replacing TFAs in foods is to develop formulation options that are economically feasible and provide equivalent functionality in the product without increasing its saturated fatty acid content. In the meantime, as David Kritchevsky of the Wistar Institute has said, the best suggestion vis-à-vis diet is moderation, variety, and balance.
by J. Edward Hunter is an Adjunct Professor of Chemistry at the University of Cincinnati, Cincinnati, Ohio.