Alpha linolenic acid (ALA) and linoleic acid (LA) are essential fatty acids in human nutrition. This means that they are required for health, but cannot be synthesised by humans. However, both of these fatty acids are synthesised by plants, and so plant foods are essential to our health. The essential fatty acids ALA and LA are required for health because they are synthesised into eicosanoids, hormones that regulate cell function.
Linoleic acid is converted in humans to gamma linolenic acid (GLA) and then to dihomo gamma linolenic acid (DGLA), which is in turn converted to the series 1 eiscodaoids. Series 1 eicosanoids are anti-inflammatory in nature. Because inflammation is now linked to the initiation and propagation of disease through the generation of free radicals, series 1 eicosanoids are considered to be preventive of disease.
Too Much of A Good Thing
So LA is necessary to form beneficial series 1 eicosanoids. However, too much LA causes DGLA to build up before it can be converted to the series 1 eicosanoids. When this happens, the DGLA is instead converted to arachidonic acid and then to the series 2 eicosanoids. Series 2 eicosanoids are pro-inflammatory and therefore can increase levels of disease through the production of free radicals. Too much LA is therefore detrimental to the health.
Alpha Linolenic Acid
Alpha linolenic acid is converted in humans to eicosapentaenoic acid (EPA). In turn, EPA is converted to the series 3 eicosanoids. The series 3 eicosanoids are neutral in their action possessing neither pro- or anti-inflammatory actions. However, because the synthesis of series 3 eicosanoids inhibits the synthesis of series 2 eicosanoids, the overall action of EPA and the series 3 eicosanoids is anti-inflammatory and therefore disease preventing.
The Western Diet
The Western diet is characterised by processed meat, refined grains and large amounts of vegetable and animals fat. The Western diet causes inflammation because the high concentrations of vegetable fats provide too much LA and this increases synthesis of arachidonic acid and the pro-inflammatory eicosanoids. In addition, the high content of animal fats provides preformed arachidonic acid and this further increases production of the pro-inflammatory eicosanoids.
The best way to reduce the inflammation caused by the Western diet is to stop eating one. Reducing vegetable oil consumption and animal fat intake can significantly reduce arachidonic acid levels and thus reduce inflammation and disease. Switching to a plant based diet such as the mediterranean diet can also provide ALA from green leafy vegetables and preformed EPA from fish to increase production of the beneficial series 3 eicosanoids.
Evening primrose oil and starflower oil are good sources of preformed GLA. Ingesting these oils can significantly reduce inflammation because it provides a source of GLA that can be synthesised to the anti-inflammatory series 1 eicosanoids. It is unclear why the GLA is not converted instead to arachidonic acid to produce pro-inflammatory series 2 eicosanoids, but supplementation with GLA even in the presence of high intakes of LA has a strong anti-inflammatory effect.