Oils and fats are all composed of fatty acids, its just that we call the ones that are solid at room temperature fats and those that are liquid, oils.
The difference in properties results from whether it is saturated or unsaturated, where saturation refers to a detail in the fatty acid molecule makeup. Saturated means each fatty acid molecule has the maximum number of hydrogen atoms that it can have, while in unsaturated some positions are missing hydrogen atoms. Mono and poly unsaturated simply means there is either one or more locations where hydrogen atoms are ‘missing’.
Vegetable oils such corn and sunflower are unsaturated whereas butter, lard and coconut oils are saturated.
Unsaturated oils are more reactive than saturated and will undergo chemical reactions producing toxic compounds when heated. According professor Grootveld, of De Montfort University in Leicester, cooking with these can release 100 to 200 times the daily safety limit of aldehydes which have been linked to cancer, heart disease and dementia.
The saturated acid fats by contrast produced much lower levels of aldehydes. Professor Grootveld said “for decades, the authorities have been warning us how bad butter and lard was. But we have found butter is very, very good for frying purposes and so is lard.”
Olive oil is unsaturated but the position of the double bond make it a stronger than other vegetable oils and can also be better for cooking.
A second serious effect of the vegetable oil recommendation has been the imbalance it has created in the amounts of Omega 6 and Omega 3 oils in our diet, according to Professor John Stein, Oxford’s emeritus professor of neuroscience. Vegetable oils are rich in omega 6 oils and these will cause a reduction in omega 3 oils critical to the brain, by replacing them. Deficiencies in omega 3 oils have been shown to be a contributory factor to mental health problems.