cientific evidence now supports traditional nutritional wisdom with regard the healing properties of the essential fatty acids α-linolenic acid (ALA, C18:3 (n-3)) and linoleic acid (LA, C18:2 (n-6)). The ratio of LA to ALA in the diet must be around 3 to 1 for optimal cell function in humans, and divergence from this ratio can lead to the development of Western lifestyle diseases. This is because the ratio of LA to ALA determines the production of autocrine hormones that regulate cellular and tissue function. However, because the essential fatty acids and their metabolites are long chain unsaturated fatty acids, they are prone to oxidation during manufacture and storage. This can cause them to lose their healing properties and become drivers of disease through production of oxidative stress and inflammation. This explains the detrimental effects of refined, deodorised supermarket oils and products containing them.
Natural oils contain antioxidants produced by plants to protect the delicate unsaturated fatty acids from light and heat. However, during the manufacture of deodorised oils these antioxidants are removed, while the application of heat converts the healing fats to oxidative products. The fish oil eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)) are even more susceptible to oxidation than LA and ALA because they are more unsaturated, forming 4-hydroxy-2-alkenals such as 4-hydroxy-2-hexanal (4-HHE) and 4-hydroxy-2-nonenal (4-HNE) as oxidation end products. While consumption of fish oils may be beneficial to health, consumption of their rancid products may have negative effects. Few studies have been performed on humans, but animal studies involving mice have shown that the oxidised product of long chain unsaturated fatty acids, 4-HHE, builds up in the plasma of mice fed high fat diets containing oxidised and unoxidised n-3 fatty acids.
Mice fed such diets also show increased release of nuclear factor kappa β in the small intestine suggesting that pro-inflammatory oxidative reactions increase with ingestion of rancid oils. Intestinal absorption of 4-HHE is also associated with formation of 4-HHE-protein adducts that increases the expression of glutathione peroxidise, suggesting that oxidation also increases in plasma. Therefore ingestion of oxidised n-3 fatty acids could be deleterious to health. To avoid this problem only high quality fats and oils that have minimal exposure to heat and light should be consumed in the diet. It is interesting to speculate that some of the benefits of a diet high in antioxidants may occur through inhibition of oxidative stress in the intestine. For example, it might be that some of the benefits of red wine are due to the antioxidants it contains being present in the small intestine at the same time as rancid n-3 fatty acids from food.
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