by Ben Best
|CIS ISOMER||TRANS ISOMER|
In a trans unsaturated fatty acid there is one or more double-bonds in a trans configuration (geometry) — meaning the hydrogens adjacent to the double-bonds are on the opposite sides of the molecule. Trans fats pack more tightly than cis fats, which can make fats be solid at room temperature that would be liquid if in the cis configuration. Industrial production of trans fats became popular, partly for this reason, and partly because trans fats are less vulnerable to rancidity.
(For more detailed background on the naming & configuration of essential fatty acids, see my essay Fats You Need — Essential Fatty Acids .)
A report by the Food and Drug Administration (FDA) in 1985 (HEALTH ASPECTS OF DIETARY TRANS FATTY ACIDS, F.R. Senti) concluded that there is no evidence of a harmful effect in humans at the existing dietary levels of trans fatty acid. Animal studies in that report showed no evidence of toxic effect. An Expert Panel supported this conclusion [AMERICAN JOURNAL OF CLINICAL NUTRITION 62:655S-708S (1995)], based on an estimate that trans fatty acids constitute 4-12% of American fat intake (2-4% of calories).
The Expert Panel noted the increased popularity of tub margarines (11-28% trans fat) over stick margarines (19-49% trans fat) since the 1960s. Vegetable shortenings have decreased trans fat content from 26% to 17% and fast food chains have decreased trans fat content of frying oil from 30% to 15%. (But trans fat from cheese corn chips increased from 33% in 1978 to 54% in 1988. The 3 foods which were most highly associated with an increase in trans fat in human body adipose tissue were brownies, pastry and margarine [AMERICAN JOURNAL OF CLINICAL NUTRITION 67:25-30 (1998)]. In the 1990s, salad oils were 8-17% trans, shortenings (cookies, pastry, donuts) 14-60% trans and margarines 16-70% trans (84% of margarine fat was hydrogenated soybean oil). Fat in dairy products (milk, butter, and cheese) as well as fat in beef naturally contains 2−9% trans fats [PLOS ONE; Brouwer,IA; 5(3):e9434 (2010)].
The Expert Panel report noted that trans fatty acids are less susceptible than cis fatty acids to free radical initiated autoxidation: "Trans bonds have lower pi electron density than do cis double bonds, which causes metal ions to form weaker complexes with trans fatty acids than with cis fatty acids." In simpler terms, trans fats reduce rancidity.
Later studies have shown that FDA reports that trans fatty acids are harmless were mistaken. A 2% increase in energy intake from trans fats is associated with a 23% increase in cardiovascular disease risk [JOURNAL OF THE AMERICAN DIETETIC ASSOCIATION; Remig,V; 110(4):585-592 (2010)]. The FDA does not limit the use of trans fats, but since 2006 has required that food products state the amount of trans fat, allowing that products containing less than 0.5 grams trans fat per serving can be listed as containing no trans fat [Ibid.].
Approximately 3-8% of the fatty acids in butter, cheese, milk, beef and mutton are trans fats (grass-fed cattle have higher trans-fat than grain-fed cattle). Trans fats from animal sources, but not those from hydrogenated oils have been linked to coronary heart disease [AMERICAN JOURNAL OF CLINICAL NUTRITION 62:522-523 (1995)]. Another study found that, despite the adverse effect on blood cholesterol levels, trans fatty acids were no more likely to cause atherosclerosis than their cis counterparts [PROSTAGLANDINS, LEUKOTRIENES AND ESSENTIAL FATTY ACIDS 57(4&5):399-402 (1997)].
Health professionals often recommend replacement of dietary saturated fat (such as animal fat) with unsaturated fat (such as fish or vegetable oil) because saturated fat increases blood LDL cholesterol without affecting HDL cholesterol). Trans fatty acids, however, not only elevate harmful LDL, but lower beneficial HDL cholesterol. In fact, net increase in LDL/HDL ratio due to trans fat is approximately double that of saturated fat [NEW ENGLAND JOURNAL OF MEDICINE 340(25):1994-1998 (1999)]. This adverse effect is greatest for elaidic (18-carbon) fatty acid [THE JOURNAL OF NUTRITION; Sundram,K; 127(3):514S-520S (1997)]. A prospective study in the Netherlands found a relative risk (increased probability compared to control) of 1.28 for coronary heart disease over a ten-year period for a 2% energy difference of increased trans fat in the diet [THE LANCET 357:746-751 (2001)]. A study of over 700 nurses showed that those in the highest quartile of trans fat consumption had blood levels of C-Reactive Protein (CRP, a pro-inflammatory cytokine which is a cardiovascular disease risk factor) that was 73% higher than those in the lowest quartile [THE JOURNAL OF NUTRITION; Lopez-Garcia,E; 135(3):562-566 (2005)]. CRP may only be a moderate risk factor for cardiovascular disease, however [NEW ENGLAND JOURNAL OF MEDICINE; Danesh,J; 350(14):1387-1397 (2004)].
Trans linoleates are devoid of essential fatty acid activity and they retard rat growth more than diets deficient in essential fatty acids through an adverse effect on enzymes that metabolize essential fatty acids. Trans 18:2-w6 decreases the conversion of linoleic to gamma-linolenic acid in rat liver microsomes [HANDBOOK OF LIPIDS IN HUMAN NUTRITION Gene Spiller, Ed. (1996) page 92]. I am also suspicious of the effect of trans fatty acids on the membranes of cardiac muscle and neurons. It seems to me that the trans fatty acids could more easily substitute for their cis counterparts than could saturated fatty acids and reduce membrane fluidity (decreasing functionality).
A developing foetus has a high demand for the fatty acids docosanhexaenoic acid (DHA) and arachidonic acid (AA) for growth & brain development. (For more detail about DHA — see my essay DHA for Hearts and Minds). Several studies have found that levels of trans fatty acids may be negatively correlated with both birth weight & infant head circumference [LIPIDS 36(9):997-1006 (2001)]. These results are not reputedly definitive, however, and have not been supported by animal experiments [EARLY HUMAN DEVELOPMENT 65(Suppl):S31-S41 (2001)]. Animal experiments have, however, confirmed the inhibition of delta-6-desaturase activity and decreased prostaglandin production with dietary trans fats [AMERICAN JOURNAL OF CLINICAL NUTRITION 34:2307-2318 (1981)].
A review of natural versus industrial trans fatty acids (TFAs) as a cardiovascular risk concluded that industrial TFAs are harmful, but natural TFAs are not [EUROPEAN JOURNAL OF CLINICAL NUTRITION; Bendsen,NT; 65(7):773-783 (2011)], whereas a similar review concluded that both forms of TFAs are harmful [PLOS ONE; Brouwer,IA; 5(3):e9434 (2010)]. A third review concluded that natural source TFAs are of less concern because larger quantities of industrial TFAs are consumed [JOURNAL OF THE AMERICAN DIETETIC ASSOCIATION; Remig,V; 110(4):585-592 (2010)].
Trans fats raise the plasma LDL/HDL ratio, while interfering with fat metabolism and contributing to inflammation. Trans fatty acids are an unnecessary source of calories with harmful effects greater than those found in other kinds of fat.