As well as trans fats the typical Western diet also contains a group of oils called the oxidised fats. Like all biomolecules, fats can react with a number of substances to produce derivatives that change their chemical and physical properties. In particular, fats and oils readily react with oxygen to form a range of chemicals in a process called lipid peroxidation. The products of these reactions can be put under the umbrella heading of oxidised fats. Some fats react more readily with oxygen, and the reactivity with oxygen is generally dependent on the number of double bonds present in the carbon chain of the fat. Polyunsaturated fats have many double bonds and so readily react with oxygen. Monounsaturated fats possess only one double bond and so are more stable than polyunsaturated fats. Saturated fats have no double bonds in their carbon chains and so are the most stable form of fatty acid. Oxidation of fats causes them to become rancid, and rancid fats are now thought to possess a significant detrimental effect on health.
Vegetable oils are generally polyunsaturated oils, and this means that they are highly susceptible to reaction with oxygen. However, fish oils are even more unsaturated than vegetable oils. The vegetable oils linoleic acid and alpha linolenic acid possess two and three double bonds, respectively. However, the eicosapentaenoic acid and docosahexaenoic acid in fish oils possess five and six double bonds, respectively. Fish oils are therefore highly susceptible to oxidation and rancidity and most sources of fish oils possess some concentration of oxidised fats. Although vegetable oils are generally more stable than fish oils, they are just as likely to contain oxidised oils and go rancid. This is because vegetable oils are often extracted from the seed with extreme heat and pressure that can increase the risk of reactions with oxygen occurring. In addition, the oils are often processed further and this involves further adulteration of the delicate polyunsaturated fatty acids in their oils.
Lipid peroxidation produces a number of reactive aldehyde chemicals including malondialdehyde and 4-hydroxynonenal that are able to react with cellular structures when ingested. This process occurs because the lipid peroxides can initiate free radical chain reactions whereby they steal electrons from cellular components to stabilise their own structures. This creates an unstable biomolecule within the tissues that then does the same to another biomolecule, which in turn, reacts with another and so on in a chain reaction. Antioxidants are able to quench such reactions through donation of an electron to the free radical, thus preventing further damage. High intake of oxidised fats, as are present in the typical Western diet, therefore increases free radical chain reactions leading to the development of oxidative stress. Oxidative stress is thought to be responsible for a number of diseases and may contribute to the development of insulin resistance and weight gain.