Industrial and natural sources of dietary trans fatty acids
Dietary trans fatty acids come from different sources:
- they can come from industrial processing, being the by-product of partial hydrogenation of unsaturated vegetable oils;
- they can be produced naturally by plants and animals.
Dietary trans fatty acids from partial hydrogenation of vegetable oils
In industrialized countries, greater part of the consumed trans fatty acids are produced industrially (in USA about 80%), in varying amounts, during partial hydrogenation of edible oils containing unsaturated fatty acids.
Hydrogenate means to add hydrogen atoms to unsaturated sites (that is, on a double bond) on the carbon chains of fatty acids by heating vegetable oils in presence of metal catalyst and hydrogen.
During the partial hydrogenation, an incomplete saturation of the unsaturated sites on the carbon chains of unsaturated fatty acids occurs: some double bonds remain, but they may be moved in their positions on the carbon chain, producing geometrical and positional isomers (double bonds are modified in both conformation and position).
Notably, with regard to fish oil, trans fatty acid content in non-hydrogenated oils and in highly hydrogenated oils is 0,5 and 3,6%, whereas in partially hydrogenated oils is 30%.
Hydrogenation converts vegetable oils into semisolid fats for use in:
- margarines and shortenings;
- commercial cooking;
- manufacturing processes.
- serum lipid levels (total and LDL-cholesterol);
- endothelial cells;
- systemic inflammation;
- other risk factors for cardiovascular disease;
- moreover, they are positively associated with the risk of coronary heart disease (CHD), and sudden death from cardiac causes and diabetes.
Industrial trans fatty acids are an independent cardiovascular risk factor.
Their adverse effects are seen at low level of intake: for a person consuming 2000 kcal/d, 20-60 kcal from industrial trans fatty acids, equivalent to about 2-7 g or 1-3% of the total energy intake, is enough.
So, avoidance of industrial trans fatty acids, or a consumption of less 0,5% of total daily energy intake is necessary to avoid their adverse effects (these are far stronger, on average, than those of food contaminants or pesticide residues!).
Dietary trans fatty acids from deodorization of vegetable oils
Very small amounts of trans fatty acids (less than 2 percent) are formed during deodorization of vegetable oils, a process unrelated to partial hydrogenation and necessary in the refining of edible oils. During this process trans fatty acids with more than one double bond are formed in small amounts (if the isomer contains 18 carbon atoms it is marked C18:2). These isomers are also present in fried foods and in considerable amounts in partially hydrogenated vegetable oils (e.g. soybean oil).
Dietary trans fatty acids from animals
A natural source comes from bacterial transformation of a proportion of the relatively small amounts of unsaturated fatty acids ingested by ruminants in their rumen.
They are present at low levels in meat and full fat dairy products from cows, sheep, and other ruminants (typically <5% of total fatty acids).
Dietary trans fatty acids from vegetables
Another natural source is represented by some plant species, and plant-derived foods as:
- leeks, peas, lettuce and spinach, that contain trans-3-hexadecenoic acid;
- rapeseed oil, that contains brassidic acid and gondoic acid.
In these sources trans fatty acids are present in small amounts.
“Homemade”dietary trans fatty acids
They are produced at home during frying with vegetable oils.
Isomers of dietary trans fatty acids
The most important cluster of trans fatty acids both animal and industrial origin is isomers containing 18 carbon atoms plus one double bond (C18:1) whose position varies between the Δ6 and Δ16 carbon atoms of molecule.
Even if the same trans fatty acids are largely present in industrial trans fatty acids and in trans fatty acids from ruminants, there is a considerable quantitative difference between individual molecules in the two different sources.
The most common isomers in both sources are those with double bond in position between Δ9 and Δ11, but Δ11-C18:1 or vaccenic acid represents over 60% of the trans C18:1 isomers in ruminant trans fatty acids, whereas in industrial ones Δ9-C18:1 or elaidic acid comprises 15-20%, and Δ10-C18:1 and Δ11-C18:1 over 20% each others.
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