Seed oils increase the risk of cancer

I worked in a fish and chip shop in the early 1990s. I can remember my boss, Ron, chipping out great chunks of beef tallow (solid beef fat) to top up the fryers. Only the deep-fried chicken was cooked in oil. By the time I left the chip shop they had moved away from cooking with beef fat to cooking with vegetable oil. Customers complained about the change in taste, but were pleased about the change to the polyunsaturated oils as these had been found to be healthier than the saturated fats in the tallow.

So it was interesting to read in the paper today, that cooking with these polyunsatured oils may be increasing our risk of cancer and heart disease.   As part of a British TV program, Professor Martin Grootveld, a chemical pathologist, gave volunteers a number of different cooking fats including sunflower oil, vegetable oil, corn oil, cold-pressed rapeseed oil, olive oil, butter, goose fat and lard. These volunteers used these fats and oils in everyday cooking and sent any fat left over in the pan back to for analysis. They found the highest amounts of these damaging compounds in sunflower and corn oils and the least in coconut, butter and goose fats, and in olive and cold-pressed rapeseed oils.

All fats and oils change chemical structure when heated and they can give off compounds called aldehydes. These aldehydes increase the risk of developing a number of metabolic diseases. I can’t quite figure out the mechanism, but oxidative stress increases the amount of reactive aldehydes naturally formed within the body. These all have to be removed by enzymes called aldehyde dehydrogenases (ALDHs). Enzymes can only do so much work, so when we eat foods that have been cooked in hot vegetable oils, we may be overloading these poor enzymes.

One reason why the seed oils came off so poorly is that they are polyunsaturated fats. A fatty acid chain, has a long string of carbon atoms bonded together joined on the end  with a COOH group which makes up the ‘acid’ part of the fatty acids.

COOH group – the ‘acid’ part of fatty acids

The rest of the carbon atoms are bonded together in a long line. Each carbon atom need four bonds to other atoms to keep them stable. Carbon can use two of these bonds to bind to another carbon atom, known as “double-bonds”. These double-bonds are weak and are easily broken by things like heat, or other chemicals and the carbon will bind to another atom and form a new molecule.

Structural bonds of different types of fats.  Arrows indicate the double-bonds.

By contrast, monounsatured oils, have only one of these double-bonds, making it much more stable.

Saturated fats have no double-bonds as all of their carbon atoms are “fully saturated”. This makes them very stable compared to the polyunsaturated oils.

It is important to remember that most foods contain a mixture of fats, and what we refer to as a “saturated” fat, like beef tallow, is only about 50% saturated fats, with monounsaturated fats comprising most of the balance.

We need a healthy amount of fat in the diet, so which fats should we eat and cook with? Everyone has an opinion about fats, but I can’t find anything bad about olive oil. If you normally eat fats, such as with a LCHF diet and are concerned about aldehydes, cook with a minimum of fats and oils, then add more olive oil (or butter in my family) to foods afterwards.