Fat is important for many body processes. You need to eat some fat. Fat protects your organs, keeps you warm and it helps your body absorb and move nutrients around. It also helps hormone production. However some fats are better than others and having too much of any type is not a good idea.

Dietary fats are classified by their structure. Different types of fats react differently inside the body. Saturated fats (found mostly in animal products) increase blood cholesterol, which is a risk factor in coronary heart disease. Monounsaturated and polyunsaturated fats tend to lower blood cholesterol.Dietary fats and blood cholesterol

There are two types of blood cholesterol. Low density lipoprotein (LDL) cholesterol and high density lipoprotein (HDL) cholesterol. LDL is considered the ‘bad’ cholesterol because it contributes to the narrowing and silting up of the arteries, which can lead to heart disease and stroke. The HDL cholesterol is considered to be the ‘good’ cholesterol because it actually cleans out the arteries, reducing the risk of cardiovascular disease.Groups of fats

Each group of fats behaves differently inside the body. Dietary fat can be classified into four groups:

• Saturated
• Monounsaturated
• Polyunsaturated
• Trans fats.

Saturated, monounsaturated and polyunsaturated fats

Saturated fats contribute to the risk of heart disease by raising blood cholesterol levels. Monounsaturated and polyunsaturated fats both tend to lower blood cholesterol when they replace saturated fats in the diet, but the polyunsaturated fatty acids have a slightly greater impact than monounsaturated fatty acids.Trans fatty acids

Trans fatty acids are rare in nature. They are only created in the rumen of cows and sheep, and are only found in small amounts in milk, cheese, beef and lamb. Trans fatty acids are also created during the manufacture of some table margarines and solid spreads used in the food industry. Trans fatty acids are considered to behave like saturated fats in the body; they raise LDL levels and increase the risk of heart disease. Look for margarines that have less than one per cent fatty acids on the label.Common fat-containing foods

Different foods contain different ratios of fatty acids:

• Saturated fats - sources include fatty cuts of meat, full fat milk and cheese, butter, cream, most commercially baked products such as biscuits and pastries, most deep fried fast foods, coconut and palm oil.
• Monounsaturated fats - sources include margarine spreads such as canola or olive oil based choices, oils such as olive, canola and peanut oils, avocado, and nuts such as peanuts, hazelnuts, cashews and almonds.
• Polyunsaturated fats - sources include fish oils, seafood, polyunsaturated margarines, vegetable oils such as safflower, sunflower, corn or soy oils, nuts such as walnuts and brazil nuts, and seeds.

Sources of omega-6 and omega-3 fats

Polyunsaturated fats can be divided into two categories:

• Omega-3 fats are found in both plant and marine foods and have been found to reduce the risk of heart disease. Food sources include canola and soy oils and canola based margarines. Marine sources include fish especially oily fish such as Atlantic salmon, mackerel, Southern blue fin tuna, trevally and sardines.
• Omega-6 fats 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 that they:

• Lower blood cholesterol levels, which reduces an important risk factor in coronary heart disease
• Improve blood vessel elasticity
• Thin the blood, which makes it less sticky and less likely to clot
• Reduce inflammation and boost the immune system
• Contribute to the normal development of the foetal brain.

Plant sterols

Plant sterols are present in all plants. Intakes of 2-3g plant sterols per day have been shown to reduce blood cholesterol levels by an average of 10 per cent. This is because they block the body’s ability to absorb cholesterol, which leads to a reduced level of cholesterol in the blood.

It is hard to eat this much from natural sources so there are now plant sterol enriched margarines on the market. Eating 1 to 1½ tablespoons of sterol enriched margarine each day can help to lower blood cholesterol levels.

Energy density

Dietary fat has more than double the amount of kilojoules per gram (37) than carbohydrate or protein (17), making it very energy dense. Some research suggests that saturated fats are more likely to contribute to weight gain (especially around the middle) than polyunsaturated fat and monounsaturated fats, even though they have the same kilojoule content.

Carrying too much body fat is a risk factor in many diseases, including coronary heart disease and type 2 diabetes. Saturated fats should be kept to a minimum.

Saturated fats

Saturated fats tend to raise LDL levels. Most saturated fats raise blood cholesterol. Butter, coconut and palm oil, cottonseed oil, lard, cocoa butter and beef tallow are all high in saturated fats. These fats are commonly found in many fast foods and in commercial products such as biscuits and pastries.

Trans fatty acids

Trans fatty acids from hydrogenated plant fats 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. However, they are found in much smaller amounts in the diet than saturated fats and so do not pose as great a risk of heart disease.Other fats

Other dietary fats that influence blood cholesterol are polyunsaturated fats and monounsaturated fats. They both reduce blood cholesterol levels. You should try to replace saturated fats in your diet with either monounsaturated or polyunsaturated fats. For example, replace butter in some cooked dishes with olive or canola oil.Cholesterol in food

People with high blood cholesterol or who are at risk of heart disease should try to limit their intake of cholesterol-rich foods. Dietary cholesterol is only found in animal products such as:

• Full fat dairy products
• Fatty meats
• Egg yolks
• Offal - for example liver, kidney and brains.

The Mediterranean diet

Researchers are investigating the possibility that a diet rich in monounsaturated fats, such as olive oil, may be protective against the development of coronary heart disease. People who have a high consumption of monounsaturated 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.

Current recommendations

Nutritionists recommend that we limit the amount of fats in the daily diet, particularly saturated and trans fats. Simple suggestions include:

• Use margarine spreads instead of butter or dairy blends.
• Use salad dressings and mayonnaise made from oils such as canola, sunflower, soy and olive oils.
• Use low or reduced fat milk and yoghurt or ‘added calcium’ soy beverages.
• Try to limit cheese and icecream to twice a week.
• Have fish (any type of fresh or canned) at least twice a week.
• Select lean meat (meat trimmed of fat and chicken without skin). Try to limit fatty meats including sausages and delicatessen meats such as salami.
• Snack on plain, unsalted nuts and fresh fruit.
• Incorporate dried peas (for example split peas), dried beans (for example haricot beans, kidney beans, three bean mix) or lentils into two meals a week.
• Make vegetables and grain based foods such as breakfast cereals, bread, pasta, noodles and rice the major part of each meal.
• Try to limit take-away to once a week or less.
• Try to limit snack foods such as potato crisps and corn crisps to once a week or less.
• Try to limit cakes, pastries and chocolate or creamy biscuits to once a week or less.
• Try to limit cholesterol-rich foods such as egg yolks and offal like liver, kidney and brains..

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.
• Replacing saturated fats with monounsaturated and polyunsaturated fats tends to improve blood cholesterol levels.