Oils and their effects on health

Oils and fats, which are oils that are solid at room temperature, are important components of the diet. In fact, without any fat or oil in our diet we would quickly die. The reason is that there are two types of fat, omega 6 (w6) and omega 3 (w3) fats that we need for our body to function, but that we cannot produce ourselves. As such we need to eat sources of these fats to survive. Please remmebr w3 and w6 as I'm going to refer to them again in this article.

How oils can damage health

There are two major ways in which oils can damage health. Firstly many oils contain damaged molecules that interfere with vital processes in our body. Secondly most oils that we consume nowadays contain too much w6, and not enough w3 fatty acids.

How oil molecules are damaged.

Oils can be transformed into strange, unhealthy molecules by exposure to air, light and excessive heating. Exposure to air and light causes oxidation while excessive heating causes oil molecules to change shape, stick together and break apart. All these processes damage the original molecule. These damaged molecules are invariably bad for our health as they have not been in our diet for long, and our bodies have not evolved ways of coping with them.

Damage by oxidation (hydroperoxides)

Oxidation of fats and oils produces substances that can increase our risk of heart disease(1), cancer(2) and other chronic inflammatory conditions. This process is one way in which oils become rancid. Typically hydroperoxides are produced which then go on to produce aldehydes. These products damage cell membranes and our liver function.

Damage to triglycerides by heating (acrolein)

When oils are heated, the triglyceride molecules that they are made up of are damaged. Triglycerides are made up of a glycerol backbone and 3 fatty acids. It's a bit like a set of cricket stumps with the 2 bails being joined together. Increased heat causes the fatty acids to become detached, and the more free fatty acids there are the lower the smoke point. The smoke point is a temperature at which the glycerol backbone of the triglyceride may break down into acrolein. Acrolein is a substance that can cause cancer and other chronic diseases, in part by disabling the potent antioxidant glutathione, which is produced mainly in our liver(3).

Damage to fatty acids by heating (trans fats)

The fatty acids that make up triglycerides can be saturated or unsaturated. The unsaturated ones can be transformed into trans fats by heat. The more unsaturated the oil the more damage that is likely to be done. So w3 oils, with typically 3, 4, 5 or 6 unsaturated bonds can be badly damaged as can w6 oils with typically 2,3,4 or 5 unsaturated bonds. The higher the temperature the more trans fats that are created. As a guide, 160C (320F) is considered a point beyond which it is inadvisable to heat an oil. Most refined oils are heated way beyond this during their production. Deep frying also takes oils temperatures beyond this point.

Too much omega 6 oil is bad.

The w6 and w3 molecules are both families of related molecules of different lengths.

When we eat seed oils they contain w6 in the form of linoleic acid which is 18 carbons long. They contain w3 in the form of alpha-linolenic acid, which is also 18 carbons long. The linoleic acid from seed oils is turned into arachidonic acid - AA, while the alpha-linolenic acid is turned into eicosapentanoic acid - EPA and docosahexanoic acid - DHA. These longer chain AA, EPA and DHA are the essential molecules we need.

The problem with w6 in its ultimate form of AA, is that it tends to promote inflammation in the body while w3 in its ultimate forms of EPA nad DHA has the opposite effects. Now some degree of inflammation is required as a defensive mechanism by our immune system. However too much leads to chronic diseases such as heart disease, cancer and arthritis. 

The 18-carbon w6 and w3 molecules require the same enzymes to convert themselves into the longer AA, DHA and EPA that we need to survive. It is thought that if we have too much w6 from seed oils that it uses up all the enzymes, not allowing the 18-carbon w3 to be converted into the anti-inflammatory EPA and DHA. End result - too much inflammation.

How oils can promote health

Polyunsaturated oils are known as EFAs - essential fatty acids, because they are essential for our survival. EFAs in the right quantities can improve our health in a number of ways.


Our body's inflammatory response is mediated by small hormones called prostaglandins. These are made from EFAs. In particular the w6 ones in the form of AA marshall the tropps causing inflammation while the w3 ones obtainable from oily fish such as trout, salmon, mackerel and sardines, help balance this by damping down and controlling the inflammation so that our immune response is proportionate.

Brain function

EFAs are prevalent in brain tissue. In particular the w3 EFA - docosahexanoic acid (DHA) makes up 40% of the EFAs in our brain cells. DHA is found in oily fish and this is what lies behind the oft quoted saying, "fish is good for the brain".

Major oils and their composition

For a breakdown of the amount of different types of fatty acids in a range of popular oils and their smoke points please refer to: Table of oils.

What we can see is that some oils are predominantly saturated while others are mostly monounsaturated or polyunsaturated.

Saturated oils or tropical fats

Coconut oil, palm oil and cocoa butter along with traditional dairy butter are all predominantly saturated. This makes them fairly safe as cooking oils as saturated fats are the most stable kind of fat when heated to high temperatures.

Monounsaturated oils

Almond oil, olive oils, avocado oil and to a lesser extent rape seed oil are predominantly monounsaturated.

Polyunsaturated oils

Sunflower oil, safflower oil, hemp oil and flax oil are not to be used in cooking as they can easily become damaged by heat. They are however good sources of EFAs when they are organic - cold pressed and unrefined.

Which oil should I use

For cooking

The best oils for cooking are those with fewer unsaturated fatty acids, as these are easily destroyed by heat. We are also looking for a higher smoke point. Oils that measure up are:

  • Peanut oil - contains 29% polyunsaturated fats.
  • Avocado oil - contains 10% polyunsaturated fats.
  • Olive oil - contains 9% polyunsaturated fats. Olive oil and oils above contains a lot of monounsaturated fats. It's possible some of these become harmful trans fats. To avoid these altogether, consider the cooking fats below.
  • Butter - contains 3.5% polyunsaturated fats and 30% monounsaturated fats.
  • Coconut oil - contains 3% polyunsaturated fats and 30% monounsaturated fats.
  • Cocoa butter - contains 3% polyunsaturated fats and 6% monounsaturated fats.
  • Palm oil - contains 2% polyunsaturated fats and 13% monounsaturated fats.

For salads and dips

For salads and dips you want an oil with plenty of flavour, that has not been destroyed  in the manufacturing process. This is a great opportunity to use unsaturated oils that are cold pressed and unrefined. Normally these can only be found in health food stores, although olive oil is a common exception. In general organic oils are free of added chemicals and heat treatment, and so should be good candidates for use in salads and dips.

Some examples of companies making premium oils that you may find in health food stores are:

  • Meridian - extra virgin olive oil, safflower oil, sesame oil, sunflower oil.
  • Biona - organic coconut oil, rapeseed oil, sunflower oil, extra virgin olive oil, hemp oil and flax oil.
  • Clearspring - avocado oil, coconut oil, hazelnut oil, extra virgin olive oil, rapeseed oil, safflower oil, sesame oil, soya oil, sunflower oil, walnut oil.

Using oil for cooking

Don't reuse heated oil

The free fatty acids that were liberated the first time round will help the oil degrade at lower temperatures than before, affecting both the taste and the health effects of the oil.

Don't cook above the smoke point

Whichever oil you use, if you go above the smoke point you will cause a lot of damage to the oil. Some oils such as virgin olive oil vary considerably in their smoke point (100-200C). It all depends on how much they have been refined. Some olive oils are processed very little, and these oils have lower smoke points. It is best to use these oils, often the more expensive ones, with salads and as dips, rather than for cooking. 


1. http://www.ncbi.nlm.nih.gov/pubmed/11686005

2. http://www.ncbi.nlm.nih.gov/pubmed/11525870

3. http://en.wikipedia.org/wiki/Acrolein