Many plant-based oils can be very good for you, such as olive oil, coconut oil, and others. But again like some other foods on this list, the devil is in the details... Many of these same plant-based oils can be incredibly damaging to your health in certain circumstances.
Oils and fats do not contain just one type of fat - they are mixture of several different types, as shown in the picture below.
A definition of the different types of fats and oils is needed here:
- A monounsaturated fat has one (ie, mono) double bond in its chemical structure
- A polyunsaturated fat has two or more (ie, poly) double bonds in its chemical structure
- A saturated fat has no double bonds
- Double bonds make the oil or fat more liquid but less stable, or in other words, more unstable in the presence of oxygen or heat
- By definition and having more double bonds, polyunsaturated fats are the most unstable and tend to oxidise very quickly when exposed to heat, or even at room temperature
- Oxidised fats and oils become rancid very quickly, being very sticky and having a "not right" smell
- Oxidised fats and oils are very inflammatory to your body
- Oxidised fats in your blood is what causes atherosclerotic plaques in the walls of your blood vessels and increases your risk of heart disease or heart attacks
- Saturated fats are stable in the presence of oxygen, tend to be more solid at room temperature (ie, lard, butter and coconut oil), and are much more stable when heated or exposed to oxygen
- The "smoke point" of an oil is irrelevant as to whether it is a healthy oil or not!
One of the worst things you can do with some oils or fats is to HEAT them! This exposes them to more oxygen, breaks the unstable double bonds and turns the oil into an oxidised, rancid and inflammatory oil. As you can see from the figure above, the oils to absolutely avoid for cooking are the polyunsaturated seed oils - those with a higher % of blue or orange shown in the figure above. Amazingly enough, this includes many oils which we are told in their marketing (aka "lies") are "healthy" for you - canola, sunflower and soybean oil and others!
Do NOT use polyunsaturated seed oils for cooking! Especially not canola, safflower, sunflower or olive oils. In fact, with the exception of flaxseed oil, all of your polyunsaturated oils should be thrown away as they are already likely to be rancid and oxidised and causing you adverse health effects.
Monounsaturated oils are not that much better - they will still oxidise and become rancid when heated or exposed to oxygen. So these oils having a high % of yellow in the picture, should NOT be heated either. With the exception of olive oil and perhaps flaxseed oil (which is only kept in the fridge - because it oxidises far too quickly), all the other monounsaturated oils should be thrown away.
Saturated fats on the other hand are stable when heated (as they have no weak double bonds) and do not oxidise as quickly as the other types of oils. Hence ONLY saturated fats should be used for cooking! This means butter and coconut oil. Lard is ok, but has a high polyunsaturated fat % which can oxidise to be rancid.
For making dressings on your salads or vegetables, use olive oil or flaxseed oil. Flaxseed oil can be very healthy as it has a very high % of a plant-based omega-3 oil, which is an anti-inflammatory oil. But this oil is VERY unstable to heat and oxygen, must be kept in the refrigerator and used quickly. Olive oil is very healthy (preferably organic and extra virgin) but for dressings only!
I hope this explains which oils are good for you and why, and which to only use for cooking!