Vegetable oils are obtained from plants. These are extracted from seeds, nuts and some fruit. Molecules of vegetable oils consist of glycerol and fatty acids. Fatty acids are compounds of carbon, oxygen and hydrogen.
In some vegetable oils, the fatty acids are saturated fatty acids. This means they have single bonds (not double bonds) between their individual carbon atoms. Saturated fatty acids are fats. Saturated fatty acids cannot break down in the body. They get collected in places in the body and stored. The body already contains saturated fats and not need much from outside sources. Such saturated fatty acids are solids at room temperature. Saturated fatty acids are present in coconut, chocolate oil, cocoa, animal fats, fish oils, butter, cheese etc.
The vegetable oils may also contain unsaturated fatty acids. A fatty acid could be classified as saturated acid or unsaturated acid depending on whether or not it contains additional hydrogen atoms. The unsaturated fatty acids have double bonds between some of their carbon atoms. Unsaturated oils are liquid at room temperature. Cooking oils like olive oil, oil obtained from sunflower, nuts or any other vegetable or fruit are rich in unsaturated fatty acids. Unsaturated fatty acids are essential fatty acids as they cannot be produced by the body. They are thought to be healthier option in the diet than the saturated fats like butter and are found useful for frying.
Oils deteriorate during frying leading to a loss of unsaturated fats. Vegetable oils should not be re-heated or reused. Reheating oil that was already used for cooking may convert the unsaturated fatty acids into trans fatty acids. Consumption of trans fatty acids leads to heart diseases.