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Trans Fatty Acids And Hydrogenated Vegetable Oils

Q. What is hydrogenated vegetable oil?

Oils (such as vegetable, olive, sunflower) are liquids at room temperature. In the food industry, hydrogen is added to oils (in a process called hydrogenation) to make them more solid, or 'spreadable'. Hydrogenated oils can be sold directly as 'spreads', but are also used in the food industry in the manufacture of many foodstuffs such as biscuits and cakes. The use of hydrogenated helps to prolong the shelf-life of the food and maintain flavour stability.

Q. What are fatty acids?

Fatty acids are the chemical compounds that make up fats. They are abundant in animal tissues, as they are the major component of cellular membranes and have vital functions in nearly every metabolic action within the body. Most commonly they are found in carbon chains of 16-18 carbons (C) with several double bonds making them polyunsaturated. They have an acid (carboxyl) group at one end of the chain.

Q. What are saturated-, unsaturated- and polyunsaturated fatty acids?

Saturated fatty acids are chains of carbon atoms joined by single bonds, with a maximum number of hydrogen atoms attached to each carbon atom in the chain. Saturated fats are typically solid at room temperature.

Saturated Fatty AcidSaturated fatty acid 

Unsaturated fatty acids are chains of carbon atoms joined by single bonds and varying numbers of double bonds which do not have their full quota of hydrogen atoms attached. An unsaturated acid can exist in two forms, the more common cis form shown below and the trans form.

Unsaturated Fatty AcidUnsaturated fatty acid ( cis )

Monounsaturated fatty acids have two carbon atoms attached by one double bond as a pair of hydrogen atoms is missing (See: Basic Chemical Structure of an Unsaturated Fatty Acid ). Monounsaturated fats and oils are liquid at room temperature but start to solidify at refrigerator temperatures.

Polyunsaturated fatty acids have more than one double bond in the carbon chain and therefore more than one pair of hydrogen atoms. Polyunsaturated oils and fats are typically liquid at room temperature and in the refrigerator.

Q. What are trans fatty acids?

Trans fatty acids are unsaturated fatty acids that have at least one double bond in the trans configuration. While most unsaturated fatty acids in foods have the cis configuration, trans fatty acids may also be present. Trans fatty acids in foods originate from three main sources:

  • Bacterial transformation of unsaturated fatty acids in the rumen of ruminant animals. They can subsequently be present in the meat and milk of the animal
  • Hydrogenation and deodorization of unsaturated vegetable oils (or occasionally fish oils) high in polyunsaturated fatty acids
  • During the heating and frying of oils at high temperatures

Unsaturated Trans Fatty AcidUnsaturated fatty acid ( trans ) 

Q. How much trans fatty acid is in food?

Dairy and beef fat typically contains around 3-6% TFAs (% of total fat) and levels in mutton and lamb can be somewhat higher. TFA levels in vegetable oils and liquid margarines are around 1%. Soft yellow fat spreads typically have between 1% and 17% TFAs, whilst harder stick margarines have higher levels. The TFA content of bakery products (rusks, crackers, pies, biscuits, wafers etc.) vary from below 1% up to 30% of total fatty acids. Some breakfast cereal with added fat, French fries, soup powders and some sweet and snack products have been shown to contain high TFA levels (20-40% of total fatty acids). However, surveys have shown that levels of TFAs appear to be decreasing in these products as manufacturers reformulate to remove hydrogenated oils if present.

Q. Are trans fatty acids dangerous to eat?

Since the process of hydrogenation adds hydrogen atoms to oil, it will reduce the number of unsaturated fatty acids and increase the number of saturated fatty acids in the oil. Consumption of a high level of saturated fatty acids is associated with increasing the level of cholesterol in the blood and this may lead to coronary heart disease. Therefore, as part of a healthy diet, consumers are advised to try to lower their intake of saturated fatty acids.

Sometimes partial hydrogenation is carried out on oil as this will result in a lower level of saturated fatty acids formed in the product. However, partial hydrogenation does lead to the formation of TFAs, rather than cis fatty acids. TFAs, like saturated fats are also associated with increasing cholesterol in the blood. Although saturated fats also produce the 'good' cholesterol (HDL), trans fats increase levels of the 'harmful' cholesterol (LDL) and decrease the good cholesterol. TFAs also lead to increased levels of triglycerides in the blood. In these respects trans fat could be considered as more likely to promote heart disease than and equivalent level of saturated fat. However, to put this in context the intake of saturated fats in the European diet is approximately 10 times that of trans fats and therefore saturated fat in the diet is still considered to present the biggest risk with respect to heart disease.

The European Scientific Panel on Dietetic Products, Nutrition and Allergies (NDA) has reviewed the evidence of other harmful health effects attributed to trans fatty acids. They concluded that the scientific evidence with regards to a possible relationship of TFA intake and cancer, type 2 diabetes or allergies was weak or inconsistent.

Q. How much trans fat can I eat?

In the EU mean daily intakes of TFAs for 14 different countries ( Ireland not included) range from 0.5-2.1% and 0.8%-1.9% of total energy intake amongst men and women respectively. The major contributors to TFA in the diets of people in these 14 countries were edible fats and ruminant fat with bakery products and French fries being additional contributing foods in some countries. This intake level appears to be decreasing. However, as yet, there is no official guidance on the consumption of TFAs in the diet, other than we should not increase consumption of TFAs above the current level. The World Health Organisation recommended that "Food manufacturers should reduce the levels of trans isomers of fatty acids arising from hydrogenation".

Q. Are trans fatty acids labelled on food products?

In order to try and reduce our consumption, it is helpful to know where to look for relevant information on a food product.

At present, it is not mandatory for the presence of TFAs in a foodstuff to be mentioned on the label. However, the law does state that all pre-packaged foods must have their ingredients listed on the packaging. If 'partially hydrogenated' oil, or 'hydrogenated' oil is listed in the ingredients, this may suggest the presence of TFAs. Also, ingredients are listed in decreasing amount, therefore TFA levels are likely to be higher in a product where hydrogenated oil is listed as the first ingredient than a product where it is listed as the last ingredient.


Last reviewed: 18/3/2009

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