Of all the topics in nutrition, fats and the nutrition of fats are perhaps the most complex, and as a result the most misunderstood. Fats are chemically complex and this complexity means that the interactions they show with human biochemistry is also highly complex. The chemical complexity of fats also makes their use in cooking problematic as no two fats will react the same way when exposed to heat or light. Generally heating fats results in chemical changes, and we can say that on the whole those changes are detrimental from a nutritional perspective. Heating fats will eventually result in a large heterogeneous group of oxidised and modified products. Once ingested these oxidised and modified products will acts as metabolic inhibitors and oxidisers, which will detrimentally influence normal metabolic regulation. The consumption of the oxidised and modified products of heated fats is associated with cancer, cardiovascular disease, weight gain and type 2 diabetes.
Avoiding the modification and oxidation of fats during cooking is therefore paramounts if good health is to be maintained. Of course, the best way to do this is simply not to cook with fats. However, as most foods contain fats, and eating a diet composed of 100 % raw food is not realistic, it must be accepted that some cooking of fats will occur. Therefore the next best strategy is to adopt behaviours that minimise the amount of fat that is exposed to heat and the temperature that those fats are exposed to. Generally frying in a pan produces the highest cooking temperatures and so it is no surprise that pan frying is particularly problematic with regard the generation of oxidised and modified fat products. Poaching, roasting or grilling may be better alternatives because the temperatures will be lower compared to frying. Adding water to fat is another way to reduce the temperature of the fat, and this can be effective when frying. Exposing fats to heat for the shortest time possible also should be considered.
Chemically dietary fats are glycerol molecules attached to three fatty acids. This forms the basic triglyceride molecule. However, the fatty acid chain can be a straight saturated chain or a kinked unsaturated chain. Unsaturated chains possess double bonds and these are the places that chemical reactions can occur most readily to produce oxidised and modified fats. Therefore those fats with the most double bonds, the polyunsaturated fats, are the most susceptible to modification and oxidation. The most stable fats are the saturated fats for this reasons. In addition, most polyunsaturated oils are already oxidised and contain modified fats because of the way they are manufactured. Cooking with supermarket polyunsaturated oils such as sunflower, rape and corn oil ensures a high content of oxidised and modified fats as these are already at high concentrations in the oil prior to cooking. The use of these oils has been consistently shown to be detrimental to the health, and they should be avoided entirely.
Monounsaturated oils are more stable compared to polyunsaturated oils. Of all the monounsaturated oils available, extra virgin olive oils is perhaps the most useful in cooking. As with all fats, the monounsaturated fats in extra virgin olive oil are susceptible to heat and exposing them with creade modified and oxidised fats. However, extra virgin olive oil fat is protected to some degree because of the phytonutrients from the olives that are present in the oil. These can be seen as green particles in the oils, and they are what gives the oils its dark green colour. The presence of only a single bond on the fatty acid chains is also beneficial compared to the multiple double bonds of polyunsaturated fatty acids as there is only one site for oxidation and modification, rather than multiple. Any olive oil not labelled as extra virgin is already oxidised and modified and is just a detrimental as the majority of polyunsaturated oils. Reheating any fat is detrimental because this will further increase the deterioration of the fatty acids.
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