Trans fatty acids, sometimes shortened to trans fats or TFA, are unsaturated fatty acids with at least one double bond in the trans configuration. Unsaturated fatty acids are derived metabolically from saturated fatty acids by the abstraction of pairs of hydrogen atoms from adjacent methylene groups. The removal of a pair of hydrogen atoms gives rise to a double bond. The remaining hydrogen atoms can either be on the same side of the fatty acid molecule, in which case the double bond has the cis geometrical configuration, or on opposite sides, giving the trans configuration.
Trans fatty acids occur naturally in small amounts in a few foods, but the majority are formed during the partial hydrogenation of vegetable oils. This process converts vegetable oils into semisolid fats for use in margarines, commercial cooking, and manufacturing processes. There is strong evidence that the consumption of trans fatty acids from industrial sources increases the risk of coronary artery disease (CAD).
Partially hydrogenated vegetable oils were favored by the food industry because their high trans fatty acid content gave the oils a longer shelf life and an increased stability during deep frying (trans fatty acids have a higher melting point than some other types of fats). Their semi-solidity can be customized to enhance the palatability of baked goods and sweets. Because of consumer aversion to products containing trans fats, however, food producers have had to find trans fat–free alternatives.
The average consumption of industrially produced trans fatty acids in the United States is between 2%–3% of total calories consumed. The major sources of trans fatty acids in the American diet are deep-fried foods, bakery products, packaged snack foods, margarines, and crackers.
Trans fatty acids are produced by a process called partial hydrogenation. Within the process, hydrogen gas is added to heated liquid vegetable oils while in the presence of a nickel catalyst, and then pressure is applied. The process causes a change in the arrangement of the hydrogen atoms in the fat molecules. Specifically, one or more hydrogen atoms are moved from one side of the molecule to the other. This causes the polyunsaturated fat molecules to act more like saturated fat molecules. The liquid vegetable oils now become solid at room temperature. Upon completion, the artificially reformulated oil is less greasy to the touch, remains fresher longer, is more difficult for the body to digest, and is less likely to spoil. Food manufacturers like to prepare foods with trans fatty acids rather than saturated animal fats or tropical oils, such as coconut oil, because it is less expensive. Naturally occurring trans fatty acids are found in meats and dairy products from cows, sheep, and other ruminant animals; they are produced in the forestomach of the animal, where polyunsaturated fatty acids of plant origin, such as linoleic acid and linolenic acid, can undergo partial or complete hydrogenation by the action of symbiotic anaerobic bacteria present in the ruminant stomach. These naturally occurring trans fatty acids are consumed in much smaller amounts, approximately 0.5% of total energy intake. The predominant trans isomer in ruminant animals is vaccenic acid, from which conjugated linolenic acid (CLA) can be formed. It is possible to change the trans fatty acid content of ruminant products by altering the animals’ feed, although levels of trans fatty acids in meat and milk are already relatively low, between 1% and 8% of total fat content.
With respect to CLA, it is considered desirable to increase levels in foods rather than to decrease levels. This is due to the suggested health benefits of CLA in humans, such as reduced insulin sensitivity and improved immune function, although the evidence remains inconclusive. There is no association between intake of trans fatty acids from ruminant sources and risk of CAD; in fact, some studies have shown non-significant trends toward an inverse association. The absence of a positive association of trans fatty acids from ruminant sources compared with industrial sources may be due to lower levels of intake (less than 0.5% of total energy intake), different biological effects of different isomers, or the presence of other factors in meat and dairy products that outweigh any effects of the small amount of trans fatty acids they contain. Further research in these areas is needed, although studies suggest that trans fatty acids from ruminant sources do not pose a threat to public health (at average intakes).
The main effects of trans fatty acids are on serum lipid levels. Numerous controlled dietary trials have been conducted to evaluate the effect of isocaloric replacement of saturated or cis unsaturated fatty acids with trans fatty acids. The data from many of these studies has been used in a number of large meta-analyses, all of which strongly indicate that compared with saturated or cis unsaturated fatty acids, the consumption of trans fatty acids raises levels of low density lipoprotein (LDL) cholesterol (or ‘‘bad’’ cholesterol), reduces levels of high-density lipoprotein (HDL) cholesterol (or ‘‘good’’ cholesterol), and increases the ratio of total cholesterol to HDL cholesterol, all of which are powerful risk factors for CAD.
There is substantial evidence to show that trans fatty acids also promote systemic inflammation. In a large trial of women, greater intake of trans fatty acids was associated with increased activity of the tumor necrosis factor (TNF) system, a biomarker used to measure inflammation. Among those with a higher body mass index (BMI), a greater intake of trans fatty acids was also associated with other inflammatory substances. The presence of inflammation is an independent risk factor for atherosclerosis, sudden death from cardiac causes, diabetes mellitus, and heart failure. Thus, the inflammatory effects of trans fatty acids contribute further to overall CAD risk. To help ensure a reduction in trans fatty acid consumption, healthcare providers should advise consumers about how to minimize their trans fatty acid intake, consumers should learn to recognize and avoid products containing trans fatty acids, and restaurants and food manufacturers should use alternative fats in food production and preparation. All of these measures promote health benefits, particularly a reductionin the incidence of CAD.
On a per-calorie basis, trans fatty acids increase the risk of CAD more than any other macronutrient, conferring a substantially increased risk even at low levels of consumption (between 1%–3% of total energy intake). Even a small rise in energy intake from trans fatty acids can cause a large increase in risk.
A meta-analysis of four prospective cohort studies, which included data from 140,000 subjects, showed a 23% increase in CAD incidence when energy intake from trans fatty acids increased by just 2%. The potential benefits of reducing consumption of trans fatty acids from industrial sources on the incidence of CAD in the United States has been calculated. On the basis of predicted changes in total and HDL cholesterol, CAD events could be reduced by between 3%–6%.
If the influence of trans fatty acids on other risk factors, such as inflammatory effects, is considered, CAD events could be reduced by 10%–19% (equivalent to between 72,000 and 228,000 CAD events each year). This reduction could be even greater if healthier cis unsaturated fatty acids, including omega-3 fatty acids, are used to replace trans fatty acids. The association between risk of diabetes and trans fatty acid intake is less clear.
Parents should try to eliminate all sources of trans fatty acids from industrial sources from their children’s diets, as they have no intrinsic health value above their energy value. As adverse effects are seen at even low levels of intake—between 1%–3% of total energy, or 2 to 7 grams per day for a person consuming 2,000 calories—it seems complete or near complete avoidance of trans fatty acids should be advised in order to minimize health risks in both children and adults.