Culinary oils and their health effects

Vegetable oils are mainly produced from oilseeds (e.g. rapeseed and sunflower seed) as well as from legumes (e.g. peanut and soybean), nuts (e.g. walnut and almond) and the flesh of some fruits (e.g. olives). Vegetable oils are pressed from the plants and are then processed and refined to produce high-quality oils suitable for use as an ingredient in recipes, for frying, in salad dressings and in the production of margarines and spreads. There has been substantial growth in the oilseeds markets over the past 30 years. This was initially driven by health concerns but, more recently, has been because of the interest in biofuels and the changing market. In 2008, the four main types of vegetable oil that dominated the world market (in terms of the quantities produced) were palm oil, soybean oil, rapeseed oil and sunflower seed oil. In the UK, the only oilseed crops harvested are rapeseed, hempseed and linseed/flaxseed. However, there are substantial imports of groundnuts, soybeans, sunflowers and palm kernels that are processed in the UK resulting in the production of a variety of different oils in the UK. Vegetable oils mainly comprise triacylglycerides. The main nutrient they provide is fat. The only other nutrient that is present in appreciable amounts is vitamin E (tocopherols and tocotrienols). Vegetable oils are also the main source of natural plant sterols in the diet and contain minor components, such as squalene and sphingolipids, all of which may provide a range of health benefits. The fatty acid composition of plant oils varies, and all plant oils are composed of a mixture of different fatty acids, in different proportions. Most culinary oils tend to be high in either monounsaturates or polyunsaturates, with the exception of palm kernel and coconut oils which are high in saturates. Categorising fats as 'saturated fats', 'monounsaturated fats' or 'polyunsaturated fats' (based on the predominant fatty acid) may be helpful for consumer understanding. However, this is an oversimplification of the actual situation. The constituent fatty acids of each culinary oil are primarily responsible for the functionality of the oil, along with the minor components. The properties of the oil can be modified through technical processes including hydrogenation, fractionation and interesterification. The processes used to modify the properties of oils have become a widely debated topic. Hydrogenation was a commonly used process to increase hardness and to stabilise fats without having to increase saturates to any great extent. However, as partial hydrogenation results in the formation of trans fatty acids, its use in the UK for consumer products has now virtually ceased. Some food manufacturers have been using interesterification to ensure that the oil has functional properties and the desired organoleptic qualities, without the formation of trans fatty acids. Alternatively, manufacturers have looked for new oils with more favourable fatty acid profiles. This has been made possible through plant breeding programmes using both conventional techniques and genetic modification and will have an important impact on the fatty acid profile of the UK diet in the future. However, overall there is little evidence to suggest that one vegetable oil should be promoted over any other oil on the basis of additional health effects as there are few good quality trials that have adequately compared the health outcomes of individuals consuming different oils. Indeed, because of the use of a variety of different vegetable-based oils in food manufacturing processes, most people are consuming a wide variety of different oils each day. Instead, the choice of the oil often depends on the functionality of the oil for a particular food application, or on taste, personal preference or cost. The most important message for consumers to understand is that all oils, regardless of whether they contain 'good' or 'bad' fats, are almost 100% fat and that the oils themselves, as well as products manufactured from them, should only ever be included in the diet in moderate quantities.