Fats and Oils: Introduction

Dietary fat can be classified into four groups: saturated, mono-unsaturated polyunsaturated and trans-fats. Each group behaves differently inside the body. All four types of fat consist of 'fatty acids', which are made up of the molecules carbon and hydrogen in various combinations. Saturated fats are literally saturated with hydrogen molecules, while mono-unsaturated and polyunsaturated fats have substantially fewer molecules of hydrogen attached.

Saturated, mono-unsaturated and polyunsaturated fats
Saturated fats contribute to the risks of heart disease by boosting blood cholesterol levels. Mono-unsaturated and polyunsaturated fats both tend to lower blood cholesterol when they replace saturated fats in the diet, but the polyunsaturated fatty acids have a larger impact than mono-unsaturated fatty acids.Most sources of dietary fat contain mono-unsaturated fats. It is the ratio of saturated to polyunsaturated fat that tends to differ from one food to the next, depending on its origin. On the whole, saturated fats are found in greater amounts in ruminant (sheep/cattle) animal products.

Trans-fatty acids
Trans-fatty acids (and saturated fats), such as elaidic acid, are formed when mono-unsaturated or polyunsaturated vegetable oils are hydrogenated (hydrogen is added) and hardened to form margarines, especially table margarine and the even harder vegetable fat or shortening used by the food industry. They also occur naturally in small quantities in some meats, butter and dairy products. The amount and type of fat in the diet may result in the narrowing or widening of the arteries, which has the effect of altering the flow of blood. This may increase the tendency for blood to clot. These are important factors which influence the risk of developing cardiovascular disease.

Common fat-containing foods
Different foods contain different ratios of fatty acids:
Saturated fats - sources include beef, lamb, milk, cheese, other dairy products and some processed foods containing hydrogenated vegetable shortening, such as pastries and fried fast foods.
Mono-unsaturated fats - sources include avocado, olive oil, canola oil and peanuts.
Polyunsaturated fats - sources include fish oils, seafood and vegetable oils - especially safflower, sunflower, corn or soy oils.

Sources of omega-6 and omega-3 fats
Polyunsaturated fats can be divided into two categories:
Omega-3 fats which are found in fish, seafood, lean meat, plant foods (such as cereal grains, seeds, nuts, legumes, dark green leafy vegetables, figs), plant oils including soybean, canola, linseed and walnut. Oily fish (such as mackerel, salmon, trout, tuna, sardines, herring and gemfish) have more omega-3 fatty acids than white fish.
Omega-6 fats which are found primarily in nuts, seeds and plant oils, such as corn, soy and safflower.

Benefits of omega fats
Research is ongoing, but the benefits of omega fats in the diet seem to include:
Lowering blood cholesterol levels - which reduces one important risk factor in coronary heart disease (omega 6 and 3).
Producing anti-inflammatory compounds called leukotrienes, which can help ease a variety of inflammatory disorders, such as psoriasis, rheumatoid arthritis, ulcerative colitis and inflammation of blood vessel walls associated with atherosclerosis (omega 3).
Contributing to the normal development of the foetal brain.

Omega-3
Studies have shown that high intakes of omega-3 fatty acids are associated with a lower risk of heart disease and strokes. They may also reduce blood pressure and reduce the risk of cancer. Omega-3 fatty acids:
Lower the levels of both LDL cholesterol and triglycerides (fats) in the blood
Improve blood vessel elasticity
Thin the blood, which makes it less sticky and less likely to clot.

When blood is thinner, it is less likely to stick to the blood vessel walls and less likely to cause damage. Studies have found that Eskimos, who consume a lot of fish in their diet, bleed for longer periods of time.

Omega-6 and 3 may compete
Omega-6 fatty acids are polyunsaturated fatty acids which decrease LDL cholesterol - that is, they have a good effect. However, recent studies have found that the ratio of omega-6 to omega-3 may be important to reduce the risk of cardiovascular disease. A competitive interaction exists between the metabolism of omega-6 and omega-3 fatty acids. If your intake of omega-6 fat is too high, it can compete with the omega-3 fats and stop them doing their good work, which may lead to an omega-3 fatty acid deficiency. For a healthy balance, it is recommended that the ratio of omega-6 to omega-3 should be less than 5 to 1. This can be achieved by consuming a variety of foods, especially a variety of plant oils and spreads.
The right balance of omega-6 to omega-3 fatty acids enables the body to:
Reduce inflammation
Lower blood pressure
Prevent irregular heart beats
Promote healthy blood flow.

Energy density
Dietary fat has more than double the amount of kilojoules per gram (37) than carbohydrate or protein (17), making it energy dense. A diet high in fat has been associated with overweight and obesity. Carrying too much body fat is a risk factor in many diseases, including coronary heart disease and diabetes. Some research suggests that saturated fats are more likely to contribute to weight gain (especially around the middle) than omega-3 polyunsaturated fat and possibly mono-unsaturated fats, even though they have the same kilojoule content.

Dietary fats and blood cholesterol
There are basically two types of blood cholesterol: low density lipoprotein (LDL), which contributes to the narrowing of arteries and high density lipoprotein (HDL), which does the opposite. High LDL levels in the blood have been linked to coronary heart disease. Saturated fats tend to raise LDL levels. The saturated fats which contain mainly myristic and palmitic fatty acids are the most powerful in raising blood cholesterol. Palmitic acid is the major fatty acid in palm oil. It is also found in cottonseed oil, lard, cocoa butter and beef tallow. These fats are commonly found in many fast foods and commercial products, such as biscuits and pastries. Mystiric acid is found in dairy foods, in particular cheese. Not all saturated fats raise LDL cholesterol levels. Stearic acid, which is found predominantly in meat, dairy products and chocolate, does not appear to affect blood cholesterol levels.

Trans fatty acids appear to act just like saturated fatty acids in the way that they raise blood cholesterol levels. Unlike saturated fats, they also tend to lower HDL cholesterol, so are potentially more damaging than saturated fats. However, they are found in much smaller amounts in the diet than saturated fats and do not pose as great a risk of heart disease as saturated fats. Polyunsaturated fats lower LDL levels, but some studies have shown they can also cause a decrease in the levels of HDL, the 'good' blood cholesterol. Mono-unsaturated fats are considered the most beneficial to health because they lower LDL levels and maintain, or even raise, HDL cholesterol levels.

The Mediterranean diet
Researchers are investigating the possibility that a diet rich in mono-unsaturated fats, such as olive oil, may be protective against the development of coronary heart disease. People who have a high consumption of mono-unsaturated fats from olive oil (for example, in Greece and Italy) tend to have low rates of coronary heart disease, regardless of their body weight. We must remember, though, that the Mediterranean diet contains much more than olive oil. It's possible that the low rate of coronary heart disease in these countries relates to a high intake of vegetables, legumes, fruits and cereals, which are rich in antioxidants. The evidence so far is inconclusive. Adding olive oil to vegetables may increase the amount eaten because they taste better. Also, olive oil may aid in the absorption of fat- soluble antioxidant vitamins and phytochemicals, resulting in higher levels in the blood - 'it's not what you eat but how you eat it'!

Current recommendations
Nutritionists recommend that we limit the amount of fats in the daily diet, particularly saturated and trans fats. Simple suggestions include:
Trim visible fat from all cuts of meat.
Include one to two fish or seafood meals a week. Oily fish may be preferable.
Don't batter or fry fish in animal or hydrogenated vegetable fats; pan-frying and deep-frying may decrease the omega-3 content of the fish.
Switch to reduced fat or non-fat varieties of milk and other dairy products.
Instead of frying foods, try steaming, stir-frying or baking.
Limit takeaway foods, butter, potato chips, biscuits and cake, and other processed food containing vegetable shortening.
Choose fat from unrefined sources - for example, oily fish, nuts, soy, avocado, seeds (in bread) and virgin/cold pressed oils
Use a variety of different oils - for example, virgin olive oil, canola oil or peanut oil
Use non-stick pans to cut back on the amount of fats that are used solely to stop the food from sticking.

Things to remember
Dietary fat contains more than double the amount of kilojoules per gram than carbohydrate or protein.
Animal products and some processed foods, especially fried fast food, are generally high in saturated fats, which have been linked to increased blood cholesterol levels.
Mono-unsaturated and polyunsaturated fats tend to improve blood cholesterol levels (decreasing the ratio of 'bad' LDL cholesterol to 'good' HDL cholesterol).

For more information on Fats & Oils click here

Article co-authored by Better Health Channel
(Australian -Victorian Government website)

Last Updated: June 2003.