Trans fat


Trans fat is a type of unsaturated fat that occurs in foods. Small amounts of trans fats occur naturally in animal and dairy products, but large amounts are found in some processed foods made with partially hydrogenated oils. Because consumption of trans fats is associated with increased risk for cardiovascular diseases, artificial trans fats are highly regulated or banned in many countries. However, they are still widely consumed in developing nations where they are associated with increased risk of diabetes, cardiovascular diseases, and death.
In 2015, the US Food and Drug Administration determined that artificial trans fats made from partially hydrogenated oils were not generally recognized as safe. As a result, these oils were banned from foods sold in the US beginning June 18, 2018, with limited extensions for specific, approved uses until 2020 or 2021.
Following the US action, other governing bodies, including the European Union, Canada, Australia, and New Zealand introduced restrictions or bans on the use of partially hydrogenated oils and industrial trans fats in food manufacturing.
The World Health Organization set a global goal to eliminate industrially produced trans fat by the end of 2023. The goal was not fully achieved. In 2024, the WHO announced an updated plan in 2024 for accelerated action through 2025 to complete global elimination efforts.
Trans fatty acids are derived from trans fats, which are triglycerides. Trans fats are converted to trans fatty acids in the digestive tract prior to absorption.

Occurrence

Trans fats occur naturally in the fats of products made from ruminant animals, such as cheese or butter. Some trans fats are the result of food processing, especially when applied to cooking oils and margarine.

Naturally-occurring trans fats

Trans fats occur in meat and dairy products from ruminants. For example, butter contains about 3% trans fat by weight. These naturally occurring trans fats include conjugated linoleic acid and vaccenic acid. They arise from the action of bacteria in the rumen. Polyunsaturated fats are toxic to the rumen-based bacteria, which detoxify the fats by changing some cis-double bonds to trans-double bonds. In contrast to industrially produced trans fats, this bacterial process produces only a few specific isomers. As industrial sources of trans fats are eliminated, increased attention focuses on ruminant derived trans fats.
Small amounts of trans fats occur in meat and milk fat.

Hydrogenation

Trans fat can be an unintentional byproduct of the industrial processing of oils. Unlike naturally derived trans fats, the trans fats that result from hydrogenation consist of many isomers. In food production, liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce more saturated fats, which have desirable properties:
  • The shelf life of fats correlates with the degree of saturation: polyunsaturated fats are prone to autoxidation whereas saturated fats, being virtually inert in air, have very long shelf lives.
  • Saturated fats tend to be more solid at room temperature. This property is important for margarine, one of the original uses for fat hydrogenation.
However, an isomerization side reaction during fat hydrogenation can convert remaining unsaturated fats to the thermodynamically favored trans isomer.
A number of old and new ingredients are available to replace partially hydrogenated oil containing significant levels of trans fat. These include partially hydrogenated oil made with improved processes, plant oils rich in monounsaturated fats and saturated fats, and a mix of fats combined with interesterification. The technology has improved such that a 2021 review indicates that trans fat from hydrogenated fats is no longer a problem in modern countries.

Thermal isomerization

When heated, some unsaturated fats change from their normal geometry to trans. The rate of isomerization is accelerated by free radicals.

History

There were suggestions in the scientific literature as early as 1956 that trans fats could cause an increase in coronary artery disease. Studies in the early 1990s brought renewed scrutiny and confirmation of the negative health impact of trans fats. In 1994, it was estimated that trans fats caused at least 20,000 deaths annually in the U.S. from heart disease. In the 1990s, activist organization Center for Science in the Public Interest called for reducing trans fats in foods, and later lobbied for them to be disclosed on product labels and menus. Several lawsuits were launched against high-visibility restaurants and food manufacturers with the objective of supporting a broader phase-out of trans fats.
Mandatory food labeling was introduced in several countries and Denmark was first to mandate limits on industrially produced trans fats in 2004. In January 2007, faced with the prospect of an outright ban on the sale of their product, Crisco was reformulated to meet the U.S. Food and Drug Administration definition of "zero grams trans fats per serving" by boosting the saturation and then diluting the resulting solid fat with unsaturated vegetable oils. Noting that elimination of industrially produced trans fat is feasible and achievable, the World Health Organization has set a goal to make the world free from industrially produced trans fat by the end of 2023. By the end of 2021, the WHO announced that 40 countries had implemented industrial trans fat elimination policies that "are protecting 1.4 billion people from this deadly food compound" but that 10 of the 15 countries suffering the highest health impacts from trans fats had not yet adopted a policy.

Structure

A fatty acid is characterized as either saturated or unsaturated based on the respective absence or presence of C=C double bonds in its backbone. If the molecule contains no double C=C bonds, it is said to be saturated; otherwise, it is unsaturated to some degree.
The C=C double bond is rotationally rigid. If the hydrogen atoms bonded to each of the carbons in this double bond are on the same side, this is called cis, and leads to a bent molecular chain. If the two hydrogens are on opposite sides, this is called trans, and leads to a straight chain.
Because trans fats are more linear, they crystallize more easily, allowing them to be solid at room temperatures. This has several processing and storage advantages.
In nature, unsaturated fatty acids generally have cis configurations as opposed to trans configurations. Saturated fatty acids are abundant, but they also can be generated from unsaturated fats by the process of fat hydrogenation. In the course of hydrogenation, some cis double bonds convert into trans double bonds. Chemists call this conversion an isomerization reaction.
Any molecule with a C=C double bond can be either a trans or a cis fatty acid depending on the configuration of the double bond. For example, oleic acid and elaidic acid are both unsaturated fatty acids with the chemical formula C9H17C9H17O2. They both have a double bond located midway along the carbon chain. It is the geometry of this bond that sets oleic and elaidic acids apart. They have distinct physical-chemical properties of the molecule. For example, the melting point of elaidic acid is 45 °C, which is higher than that of oleic acid. This notably means that it is a solid at human body temperatures.

Hydrogenation as a source of trans fats

The hydrogenation process was widely adopted by the food industry in the early 1900s; first for the production of margarine, a replacement for butter and shortening, and eventually for various other fats used in snack food, packaged baked goods, and deep fried products.
Full hydrogenation of a fat or oil produces a fully saturated fat. For food purposes, hydrogenation generally is not allowed to go to completion. The main target is a specific melting point and hardness, and this fine-tuning requires that some unsaturation remain. This partial hydrogenation turns some of the cis double bonds into trans bonds by an isomerization reaction. This side reaction accounts for most of the trans fatty acids consumed today, by far.
The standard 140 kPa process of hydrogenation produces a product of about 40% trans fatty acid by weight, compared to about 17% using higher pressures of hydrogen. Blended with unhydrogenated liquid soybean oil, the high-pressure-processed oil produced margarine containing 5 to 6% trans fat. Based on current U.S. labeling requirements, the manufacturer could claim the product was free of trans fat. The level of trans fat may also be altered by modification of the temperature and the length of time during hydrogenation.
The trans fat levels can be quantified using various forms of chromatography.

Presence in food

Animal fats

Trans fatty acids occur in small amounts in meat and milk of ruminants, typically 2–5% of total fat. Natural TFAs, which include conjugated linoleic acid and vaccenic acid, originate in the rumen of these animals. CLA has two double bonds, one in the cis configuration and one in trans, which makes it simultaneously a cis- and a trans-fatty acid.
A type of trans fat occurs naturally in the milk and body fat of ruminants at a level of 2–5% of total fat.
The US National Dairy Council has asserted that the trans fats present in foods of animal origin are of a different type than those in partially hydrogenated oils, and do not appear to exhibit the same negative effects. A scientific review agrees with the conclusion but cautions that this may be due to the low consumption of trans fats from animal sources compared to artificial ones.
Despite this concern, the NAS dietary recommendations have not included eliminating trans fat from the diet. This is because trans fat is naturally present in many animal foods in trace quantities, and thus its removal from ordinary diets might introduce undesirable side effects and nutritional imbalances if proper nutritional planning is not undertaken. The NAS has, thus, "recommended that trans fatty acid consumption be as low as possible while consuming a nutritionally adequate diet". Like the NAS, the World Health Organization has tried to balance public health goals with a practical level of trans fat consumption, recommending in 2003 that trans fats be limited to less than 1% of overall energy intake.
A meta-analysis showed that all trans fats, regardless of natural or artificial origin equally raise LDL and lower HDL levels. Other studies though have shown different results when it comes to animal based trans fats like conjugated linoleic acid. Although CLA is known for its anticancer properties, researchers have also found that the cis-9, trans-11 form of CLA can reduce the risk for cardiovascular disease and help fight inflammation.