The different types of fats
Fats are classified by their degree of saturation. A saturated fat is one which contains fatty acids where all the hydrogen bonds (H) are attached to the backbone of carbon (C) atoms in a symmetrical manner. To become unsaturated a double bond is formed between two adjacent carbon atoms resulting in 2 hydrogen atoms on one side of the carbon chain and none on the opposite side. This configuration bends the fat due to an imbalance in electrical charges.
It is possible for a fat to become more and more unsaturated by having more double bonds. When this occurs the fats become more bent. Hence we have mono-unsaturated and poly-unsaturated fats. Polyunsaturated fats can contain 2, 3, 4, 5 or more double bonds. Many in the conventional medical community view saturated fats as harmful and to a lesser extent damaged fats (see below). In contrast an increasing number of nutritional experts, me included, now consider that saturated fats are relatively safe and that damaged fats are the major health concern. Both camps agree that monounsaturated fats are good for health. In my view fats can be classified as good or bad fats in the following way:
If polyunsaturated fats haven't been heated to temperatures approaching their smoke point, then they are good for health. Polyunsaturated fats are sometimes referred to as EFAs (essential fatty acids) as they are needed by the body. They come in two varieties, omega 3 and omega 6.
Omega 3 fats
Omega 3 fats are generally good for health, being mostly anti-inflammatory. They are found in oily fish and in lesser amounts in leafy green vegetables, nuts and seeds.
Omega 6 fats
Omega 6 fats have some inflammatory effects on the body. They are needed, but are generally overconsumed in the standard western diet.
Monounsaturated fats are generally healthy although when damaged by heat they may become damaging.
Saturated fats are vilified by many but evidence they cause harm is not strong. Eaten in moderation they are likely to improve health.
Any fats that have been damaged by heat, light or oxygen exposure are likely to be bad for you. Polyunsaturated fats are more likely to sustain damage than saturated fats. The type of damage that can occur is discussed below.
When oils and fats are heated beyond a certain temperature their nature changes. The double bonds in normal unsaturated fats are termed cis bonds. In Latin cis means on the same side. When these unsaturated fats are heated their cis double bonds can be modified to become trans bonds. In Latin trans means on the other side. The problem with these trans fats is that they are not a shape our bodies are used to. Unsurprisingly there is a lot of good evidence that trans fats are unhealthy and are linked with heart disease1,2, cancers3 and other degenerative conditions such as diabetes. For this reason many countries and regions have banned trans fats, including Denmark, Switzerland, Iceland, Sweden, Austria, New York City, Seattle, and the state of California. Britain unfortunately is not one of them4. In fact in Britain the Food Standards Agency (FSA) has come up with the rather weak guidance that our diet should consist of less than 2% of trans fats. This disgraceful attitude sums up the very cosy relationship between the FSA and the food industry in this country. However, despite this, some supermarkets have tried to eliminate trans fats from their own-brand produce, and these include Waitrose, Marks and Spencer and the Co-op.
There are some naturally occurring trans fats, found in dairy produce. They are naturally hydrogenated by bacteria in the stomachs of cows and other ruminant animals. These types of trans fats do not seem to be particularly bad for our health. This may be due to the fact that they only make up a small amount of our diet, typically making up less than 5% of the fat in an animal. It is also likely that the types of fatty acid present in these natural trans fats are not as damaging as those found in industrially produced trans fats5. The main naturally produced trans fat is trans vaccenic acid (chemical formula 18:1w7). My guess is that this form of trans fat is not particularly unhealthy. In industrially produced trans fats, a number of fatty acids are produced. The most numerous are elaidic acids (chemical formula 18:1w9). These may be unhealthy , but other minor components of industrially produced trans fats such as trans linoleic acid and trans linolenic acid may well cause more damage.
Fats are very often treated to increase their shelf life or change their consistency. This involves heating the fat to a high temperature in the presence of hydrogen and a metal catalyst. This process, hydrogenation can either be partial or full. Trans fat refers to the chemical nature of a fat. Hydrogenation refers to how the fat has been treated.
- Fully hydrogenated fats are theoretically the same as saturated fats. Any difference will relate to the efficiency of the manufacturing process, so it is quite possible that there will be some toxic metals and fatty acid fragments present. You may find hydrogenated fats in chocolate where they have replaced the use of coconut oil.
- Partially hydrogenated fats are the ones which contain industrially produced trans fats. They are found in many packaged foods and will be served up to you in many restaurants. The key is to avoid packaged foods and stick to the healthier oils identified below.
So which oil should I cook with?
Because polyunsaturated oils can easily become damaged when you heat them beyond a certain temperature you should try to avoid using them for cooking. Most of these oils will have been processed already to increase their shelf life and as such will already contain trans fats. The monounsaturated oils and saturated oils are not as easily damaged by heat and are the best ones to use for cooking.
Polyunsaturated oils you shouldn't use**.
- Hemp oil - containing 80% polyunsaturated fats this oil becomes very damaged when heated.
- Safflower oil - contains 75% polyunsaturated fats.
- Flaxseed oil - contains 72% polyunsaturated fats.
- Sunflower oil - contains 65% polyunsaturated fats.
- Soybean oil - contains 57% polyunsaturated fats.
- Corn oil - contains 54% polyunsaturated fats.
Mono and Saturated fats you could use**.
- Sesame oil - containing 45% polyunsaturated fats.
- Rapeseed oil (Canola) - contains 37% polyunsaturated fats.
- 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.
** Exact figures may vary between brands
How can I fry safely?
If an oil reaches its smoke point then it is likely that a lot of chemical changes are taking place and that the oil will be pretty unhealthy. Smoke points vary between different oils6, with increasing smoke point temperatures as the oils get further refined. As such there are a few tips that will reduce any harm to health from frying. They are:
- Add water and/or food into a pan before frying. This automatically lowers the cooking temperature down towards 100C, reducing the damage from oxidation and production of free radicals in the oil.
- Fry with onion and garlic, which contain sulphur compounds which can reduce the amount of oxidation of the oils.
- Keep your eye on the pan to ensure it doesn't overheat and create smoke, an indication of oils being destroyed.
There are some who avoid cooking with virgin olive oil as they claim it can get damaged easily. In theory as extra virgin olive oil contains more healthy impurities, these could lower the smoke point of the olive oil7. I'm not convinced that this is necessarily true as smoke points can very between different oils and even batches of oil. In the case of olive oil there is evidence that it is relatively stable when heated to 180C8. My take is that it is fine to use most extra virgin olive oil as long as you avoid overheating it. If you insist on a whizz-bang hot fry then I'd urge you to use palm oil as the healthiest bet.
If you feel frying is unhealthy then you should also consider that various healthy plant chemicals such as lycopene, carotenoids and lutein are better absorbed when cooked in oil. These compounds are associated with significant health benefits, so on balance I'd say that frying at reasonable temperature using any oil on the list below sesame oil is going to be beneficial to health.