What are the differences between fats and oils and what do the terms 'saturated', 'monunsaturated' and 'polyunsaturated' mean?

This answer is brought to you by many of the Australian nutrition professionals who regularly contribute to a nutrition email discussion group.

This is a complex question and a full understanding requires some knowledge of fat chemistry. The answer provided here attempts to avoid the more difficult aspects of the chemistry involved. The Appendix below provides a more detailed answer and may be worth reading if you are reasonably familiar with organic chemistry.

Fat and oils are similar chemically, but the subtle differences that do exist have major effects on health. Fats and oils both consist of molecules that have 'backbones' of long chains of carbon and hydrogen atoms chemically bonded together. These molecules are called 'fatty acids'. Fats and oils in the diet (and most fat in the body) contain three fatty acids chemically bonded to form one fat molecule.

A fatty acid may be 'saturated' with hydrogen (ie, all the carbon atoms along the fatty acid backbone have the maximum possible number of hydrogen atoms attached), 'monunsaturated' (two adjacent carbon atoms have less hydrogen atoms) or 'polyunsaturated' (more than two carbon atoms along the fatty acid backbone have less hydrogen atoms) - see diagram.

The most obvious difference between a 'fat' and an 'oil' is that a fat is solid at room temperature while an oil is liquid. A fat is usually solid because it consists mostly of saturated fatty acids. Oils usually have mostly polyunsaturated fatty acids. All edible oils and fats also contain monunsaturated fatty acids. Margarines and other semi-solid fats will have a mix of mostly monounsaturated and polyunsaturated fatty acids. Although there are no naturally-occurring fats that contain only saturated, monounsaturated or polyunsaturated fatty acids, for simplicity, a fat that has mostly saturated fatty acids will commonly be referred to as a 'saturated fat'. Similar reasoning leads to use of the terms 'monounsaturated fat' (most of the fatty acids are monounsaturated) and 'polyunsaturated fat'. Also for simplicity, in the answers below, reference is made to 'fat' as a collective term for 'fats and oils' except where the discussion refers specifically to oils.

What are examples of saturated, monounsaturated and polyunsaturated fats? (See the answer to Question 4 in the Appendix for more detail on this).

Virtually all edible fats contain significant quantities of monounsaturated fatty acids, but the quantities of saturated and polyunsaturated fatty acids vary depending on the origin of the fat. Animal fats derived from ruminants (sheep and cattle) are largely saturated. Fats from other animals (eg pork, chicken) are less saturated than ruminant fat and contain some polyunsaturated fatty acids. Fats from game meat (eg, kangaroo, deer) tend to be lower still in saturated fatty acids and often contain significant quantities of a range of polyunsaturated fatty acids. Fats from fish are generally highly unsaturated and contain a form of polyunsaturated fatty acid known as 'omega-3' - see Question 3 in the Appendix for details).

How do edible fats and oils differ in the amount of energy (kilojoules) they provide and how does this affect weight gain?

Any pure, edible fat will provide about 37 kilojoules (kJ) per gram. This is more than twice the energy level of one gram of protein or carbohydrate. Because butter and margarine both contain some water (about 20%), they provide ~30 kJ per gram. Cream contains even more water, so the energy content of 1mL (~ 1gram) of cream is ~14 kJ. Cooking oils and fats generally contain little or no water, so they provide the full 37 kJ per gram. Although all pure fats provide about the same amount of energy, there is some evidence that the effects of fats on weight gain differ according to their chemical composition. It appears that the degree of saturation influences the amount of weight gained or lost on an otherwise similar diet. That is, consumption of a diet that is high in saturated fat seems to associated with increased risk of putting on weight, while polyunsaturated fats may be less 'fattening' (and may even be protective against obesity). The evidence for this is strong in animals and preliminary studies suggest that the same effect may occur in people too.

What effect do 'saturated' fats have on blood cholesterol?

Because they are derived from a ruminant animal, lard, dripping, butter and cream contain far more saturated fat than do the polyunsaturated or monounsaturated margarines and cooking oils. In the body, cholesterol production can be increased in response to the consumption of foods rich in particular saturated fatty acids. High blood cholesterol is associated strongly with increased risk of heart disease. Coconut and palm-kernel oil also contain the saturated fatty acids found in butter, so they can promote higher blood cholesterol too. These two plant-derived oils are similar to butter in their ability to raise blood cholesterol. Most other plant oils have little saturated fat - eg, avocados and peanut (and other nut) oils are largely monounsaturated and/or polyunsaturated and are therefore considered to be relatively heart-friendly.

What effects do polyunsaturated fats have on blood cholesterol?

Many of the polyunsaturated fats are believed to have cholesterol-lowering properties, particularly those rich in omega-6 fatty acids (see Question 3 in the Appendix for details), eg safflower, sunflower, soybean and cottonseed oils. The other major polyunsaturated fats - the omega-3 series (found in significant quantities in fish, but also in canola, linseed and walnut oils) - are also believed to be helpful in reducing the risk of heart disease, but by means other than lowering cholesterol. Currently, omega-6 fatty acids predominate in the Australian diet. The National Health and Medical Research Council recommends a 'moderate increase (especially in infant formulae)' in intake of omega-3 fatty acids.

What about the monounsaturates - how do they affect blood cholesterol?

Oils and margarines that are rich in monounsaturated fat (eg canola and olive oil) are also believed by many nutritionists to be 'heart-friendly'. In fact, high consumption of olive oil has been suggested as a possible major component of the 'Mediterranean Diet' that is believed to be protective against heart disease. That is, people who live near the Mediterranean Sea, where olive oil is widely consumed, have much lower rates of heart disease than people in the UK, USA, Australia and other countries where olive oil is not eaten in large quantities. Of course this could be coincidence.

Some nutritionists believe that olive oil and other monunsaturated fats are simply neutral with respect to cholesterol. That is, it may be that any cholesterol-lowering that follows replacement of saturated with monounsaturated fat is the result of reduced saturated fat consumption rather than due to a positive effect of the monounsaturated fat. The relationship of olive oil (and other monounsaturates) to health, especially heart health, deserves (and is receiving) more investigation.

I have heard that there are new margarines that are supposed to help lower blood cholesterol - what do these contain and how effective are they?

A new spread containing compounds called phytosterols was released onto the Australian market recently. It is claimed by the manufacturers that this spread is more effective than the existing monounsaturated and polyunsaturated margarines in combating high blood cholesterol. Studies are being conducted to test the validity of this claim and to ensure that these products have no undesirable long-term effects. All the scientific studies so far conducted demonstrate a cholesterol-lowering effect and animal studies indicate that toxicity is very unlikely in people. The product has been accepted by the United States Food and Drug Administration as being safe for human use.

Should we reduce intake of fat, and if so, of which fats?

The National Health and Medical Research Council's dietary guideline number 2 recommends that we should 'eat a diet low in fat and, in particular, low in saturated fat.' Switching to low- or reduced-fat milk is one simple way of reducing total fat (and saturated fat) intake. Replacing spreads and cooking fats derived from ruminant animals with polyunsaturated and/or monounsaturated spreads and cooking oils will also assist in reducing intake of saturated fat.

APPENDIX 

ADDITIONAL MATERIAL FOR PEOPLE WITH A BACKGROUND IN CHEMISTRY

1. What do fats and oils consist of and how do they differ chemically?

Fats and oils in our diet (and in our body fat deposits) commonly consist of three fatty acids chemically combined with a molecule of glycerol to form a 'triglyceride'. Each fatty acid consists of a methyl group (CH3) at one end, a carboxyl group (COOH) at the other, and a chain of carbon atoms connecting these ends. The 'degree of saturation' of each fatty acid depends on how many atoms of hydrogen the backbone of the fatty acid molecule contains. If every carbon atom has nothing but hydrogen bound to it, that fatty acid molecule is 'saturated' (ie, saturated with hydrogen) so there are no double bonds along the molecule. A 'monounsaturated' fatty acid molecule has one double bond. 'Polyunsaturated' means that there is more than one double bond along the fatty acid molecule. The greater the degree of saturation, the 'harder' the fat is likely to be - that is, predominantly saturated fats are usually solid at room temperature, while polyunsaturated fats are liquid ('oils'). Fats with mostly monounsaturated fatty acids will generally be semi-solid. (See diagram above).

2. What forms do fats take in the body and in food?

All fats contain a variety of fatty acids - that is, there are no naturally-occurring fats containing exclusively saturated, monounsaturated or polyunsaturated fatty acids. Some fats will have mostly saturated fatty acids, in others monounsaturated fatty acids will predominate, while oils have mostly polyunsaturated or a combination of monounsaturated and polyunsaturated fatty acids. For convenience, fats that contain mostly saturated fatty acids are commonly called 'saturated fats', those with mostly monounsaturated fatty acids are referred to as 'monounsaturated fats' and oils in which polyunsaturated fatty acids predominate are called 'polyunsaturated oils'.

3. Are any fats/oils essential in our diets and, if so, what foods are they found in?

Only two types of fatty acid are essential in the human diet - the polyunsaturated omega-6 and omega-3 fatty acids. An omega-6 polyunsaturated fatty acid has the first of its double bonds on the sixth carbon atom from the methyl (CH3) end. An omega-3 polyunsaturated fatty acid has its first double bond on the third carbon atom from the methyl end. Polyunsaturated margarines and seed-based cooking oils are the major sources of omega-6 fatty acids in the Australian diet. In addition, sunflower seeds, sesame and pumpkin seeds, walnuts and oats are moderate sources. Fish and other seafoods are the main sources of omega-3 fatty acids. Canola oil is the second most important source. Linseed and walnut oils are moderate sources of omega-3 fatty acids.

4. What percentages of polyunsaturated, monounsaturated and saturated fats occur in edible fats/oils?

The percentages of polyunsaturated (P), monounsaturated (M) and saturated (S) fatty acids in common margarines, cooking oils and fats are (in the order P:M:S):

• Canola oil 30:63:7
• Safflower oil 77:14:9
• Sunflower oil 66:23:11
• Olive oil 10:76:14
• Soybean oil 62:23:15
• Peanut oil 36:45:19
• Cottonseed oil 58:16:26
• Palm oil 30:39:51
• Monounsaturated margarine* 22:63:15
• Polyunsaturated margarine* 41:40:19
• Table margarine* 33:33:34
• Animal frying fats 5:45:50
• Butter 7:36:57

*Percentage content varies depending on the raw materials.

Although 'saturated fat' has the reputation for increasing blood cholesterol, this applies only to a few saturated fatty acids, specifically the C12 (lauric), C14 (myristic) and C16 (palmitic) fatty acids (the C-number refers to the number of carbon atoms in the fatty acid molecule). Butter, palm oil and coconut oil contain appreciable quantities of these fatty acids, so these fats/oils can increase blood cholesterol.

5. Trans fatty acids - polyunsaturated or saturated fat?

A complicating factor is the existence of 'trans' fatty acids. These are technically unsaturated fatty acids, but their structure has been altered so that they now behave in the body more like saturated fats. Small quantities of trans fatty acids will be found in many margarines that have undergone partial hydrogenation (to make them spreadable). The National Health and Medical Research Council recommends that trans fatty acids be considered as equivalent to saturated fats. Australian margarines generally contain only small amounts of trans fatty acids. Current intakes of trans fatty acids are relatively small in this country and they are not considered to represent a nutritional hazard.

The word 'trans' refers to the orientation of the carbon chains around a double bond - in this case the carbon chain ends are oriented on opposite sides of the double bond. It is thought that this actually makes the molecule behave like a 'saturated' fat because it is relatively straight in character. The other form of the 'isomer' is in the 'cis' orientation where the carbon chains occur on the same side of the double bond giving them a V-shaped character.

For more information Nutrition Australia recommends:

• Eat Less Fat

Back to Frequently Asked Questions Index

[Date issued: June 1999]

Disclaimer: This material is provided on the basis that it constitutes advice of a general nature only. It is not intended to replace the advice of a physician or a dietitian.