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Fats and oils |
Fat in the body
Energy is stored in the human body as fat. All animals, including humans, make fats in their bodies while plants make oils. Scientists call fats and oils of biological origin lipids. Plants are exposed to the climatic elements. To continue functioning at extremely low temperatures they need their lipids to be semi fluid. In animals a higher melting compound is preferable because the lipids also have a structural part to play and must not be too fluid.
Types of fats and oils
There are three main types of fatty acid: saturated, mono-unsaturated and polyunsaturated. Animal fats are mainly saturated. Plant oils are mainly unsaturated.
Most fats are a mixture of saturated, mono-unsaturated and polyunsaturated fatty acids. They are classified according to which predominates.
When we eat fats and oils, we consume fatty acids. Essential fatty acids play a vital role in the functioning of the human body. All fatty acids are molecules composed mostly of carbon and hydrogen.
Saturated fats
Saturated fats are found largely in meat and dairy products as well as coconut and palm oil.
Saturated fatty acids have all the hydrogen the carbon atoms can hold. Saturated fats are usually solid at room temperature, and they're more stable, that is, they don't combine readily with oxygen and turn rancid. Saturated fatty acids raise blood cholesterol, which raises the risk of coronary heart disease and stroke.
Monounsaturated fats
Mono-unsaturated fatty acids are found in olive and canola oils. Monounsaturated fatty acids have only one unsaturated bond. Monounsaturated oils are liquid at room temperature but start to solidify at refrigerator temperatures. For example, salad dressing containing olive oil turns cloudy when refrigerated but is clear at room temperature. Monounsaturated fatty acids seem to lower blood cholesterol when substituted for saturated fats.
Polyunsaturated fats
Polyunsaturated fatty acids are derived mainly from seed oils. Polyunsaturated fatty acids are missing more than one pair of hydrogen atoms. Polyunsaturated fatty acids are classed as omega 3 or omega 6 depending on where in the molecule the missing hydrogen atoms occur.
Polyunsaturated fatty acids have more than one unsaturated bond. Polyunsaturated oils, which contain mostly polyunsaturated fatty acids, are liquid at room temperature and in the refrigerator. They easily combine with oxygen in the air to become rancid. Polyunsaturated fatty acids help lower total blood cholesterol when substituted for saturated fats.
Cis and trans fatty acids
Essential fatty acids are important in the structure of cell membranes and are involved in the production of hormone-like chemicals called prostaglandins, which control many bodily processes. All the essential fatty acids are polyunsaturated and the major essential fatty acid is linoleic acid.
A fatty acid molecule consists of a chain of carbon atoms in carbon-carbon double bonds with hydrogen atoms "attached." In nature most unsaturated fatty acids are cis fatty acids. This means that the hydrogen atoms are on the same side of the double carbon bond. In trans fatty acids the two hydrogen atoms are on opposite sides of the double bond.
Trans double bonds can occur in nature as the result of fermentation in grazing animals. People eat them in the form of meat and dairy products.
Trans double bonds are also formed during the hydrogenation of either vegetable or fish oils. French fries, donuts, biscuits, chips and other snack foods are high in trans fatty acids. In fact, nearly all fried or baked goods have some trans fats.
Margarine is made by putting some of the missing hydrogen atoms back into polyunsaturated oils. This converts the fatty acids into a more solid form. These hardened oil products are called trans fatty acids. A similar process occurs in the rumen of sheep and cattle. In fact, much of our trans fatty acid intake is from dairy, beef and sheep fat. Trans fatty acids have the same effect as saturated fatty acids in raising blood cholesterol. However, the level of trans fatty acids in margarines in Australia is lower and overall polyunsaturated and mono-unsaturated margarines have been shown to lower LDL cholesterol.
Saturated fats such as palm oil and tallow are used extensively in the commercial preparation of food because of their stability. In deep fat frying these fats contribute desirable flavour and texture to fried foods. Commercial situations require fats to be stable at high temperatures and when exposed to moisture.
In commercial kitchens, oils are used repeatedly and frying may occur over several hours. The stability of fats also affects shelf life of stored snack items. The more saturated the oil, the less likely it is to break down and become rancid.
Polyunsaturated oils are unsuitable for commercial purposes because the fatty acid molecules polymerise or link up to form long chains when heated. This results in a greasy product. Canola and soybean oils are particularly susceptible to undesirable changes. Hardening of these oils renders them more stable but results in trans fatty acid levels which negate their cholesterol lowering potential. Recently a variant of sunflower oil has been developed which has the stability and nutritionally desirable characteristics required in frying fat.
How are trans fatty acids harmful?
In clinical studies, trans fatty acids or hydrogenated fats tend to raise total blood cholesterol levels and LDL ("bad") cholesterol and lower HDL ("good") cholesterol when used instead of cis fatty acids or natural oils. These changes may increase the risk of heart disease. It's not clear if trans fats that occur naturally have the same effect on cholesterol and heart disease as those produced by hydrogenating vegetable oils.
Cholesterol
Cholesterol is carried in the bloodstream in particles that scientists call lipoproteins. These particles are named according to how big they are. The very large particles are called Very Low-Density Lipoprotein (VLDL). These carry triglycerides in the blood. Triglycerides are the form in which fat is transported and stored in the body. The intermediate-sized particles are called Low-Density Lipoprotein (LDL) and are an agent in causing heart disease. The smallest particles are High-Density Lipoproteins (HDL) and actually protect against heart disease.
Cholesterol levels are altered by eating different types of fat. Eating saturated fatty acids increases LDL levels. Mono-unsaturated fatty acids lower LDL cholesterol but to a lesser extent than do omega 6 or polyunsaturated fatty acids, such as sunflower oil. Omega 3 polyunsaturated fatty acids which are found in oily fish, soybean and canola oil, lower triglyceride levels in the blood.
The characteristics of heart disease
Coronary heart disease is characterised by a narrowing of the arteries (atherosclerosis). Fatty deposits, or plaques, cling to the artery walls and can clog the arteries, making it more likely that a blood clot will form.
A heart attack occurs when a blood clot blocks one of the arteries of the heart. This prevents the flow of blood, cuts off the oxygen supply to the heart and damages or kills the heart cells.
Risk factors for heart disease
A number of factors are associated with the build-up of fatty deposits in the coronary arteries, including cigarette smoking, lack of physical activity and a family history of the disease. Other risk factors include:
Type of fat eaten - saturated and trans fats increase blood cholesterol and heart attack rates. Polyunsaturated and monounsaturated fats lower the risk of heart attacks.
Obesity - overweight and obese people usually have diets high in fat, particularly saturated fat. A person who carries the bulk of their body fat around their stomach (an 'apple') is at greater risk of heart disease than someone whose body fat tends to settle around their bottom, hips and thighs (a 'pear').
High blood pressure (hypertension) - blood pressure is the amount of pressure within the arteries (blood vessels that carry blood around the body). High blood pressure, or hypertension, means that the pressure in the arteries is higher than normal. This may be because the arteries are less elastic, there is more blood volume or more blood is being pumped out of the heart.
Uncontrolled diabetes and impaired glucose tolerance
In healthy people, insulin keeps the blood sugar level relatively constant. However, for the 25 per cent of Australians with vulnerability to type 2 diabetes, the body gradually loses its sensitivity to insulin, leading to chronically elevated blood sugar levels, also known as impaired glucose tolerance.
Uncontrolled diabetes can damage the artery walls and contribute to coronary heart disease. People who are obese are more likely to develop type 2 diabetes than those of normal weight. Australian Aborigines and Torres Strait Islanders have much higher rates of diabetes than other Australians, even at lower body weights.
Foods that help prevent heart disease
There is no 'magic' food to decrease the risk of developing heart disease. You need to eat a healthy diet and have plenty of exercise. There is, however, evidence that plant food - especially wholegrain cereals, legumes, nuts, fruits and vegetables - may decrease the risk of heart disease. The foods that best protect against heart disease include:
Oily fish - such as mackerel, sardines, tuna and salmon, which contain omega-3 fatty acids. This type of fat lowers cholesterol, improves blood vessel elasticity and thins the blood, making it less likely to clot and block blood flow.
Some vegetables oils - such as corn, soy and safflower, which contain omega-6 fatty acids, and those containing omega-3 fatty acids such as canola and olive oil. All of these can help to lower LDL cholesterol when used instead of saturated fats such as butter.
Fruit and vegetables - antioxidants in fruit and vegetables offer protection against heart disease. Fruit and vegetables are also important sources of folate, which helps lower the blood levels of the amino acid homocysteine, which appears to be linked to an increased risk of heart disease.
Fibre from wholegrain cereals - offers greater protection against the risk of heart attack than fibre from fruits and vegetables.
Unrefined carbohydrate sources with a low glycaemic index - foods such as wholegrain breads and breakfast cereals, legumes, certain types of rice and pasta are important for people prone to diabetes because they help keep blood sugar levels in check.
Legumes and soy - soy protein has been shown to lower LDL cholesterol levels, especially if blood cholesterol levels are high.
Nuts and seeds - nuts can protect against heart disease, but they need to be eaten in moderation as they are high in kilojoules.
Tea - some evidence suggests that the antioxidants in tea can help prevent the build-up of fatty deposits in the arteries, and that it may act as an anti-blood clotting agent and improve blood vessel dilation to allow increased blood flow.
Alcohol - in moderation is associated with reduced risk of heart disease (no more than two drinks per day). Some types of alcohol, such as red wine, may contain other protective factors like antioxidants, which may increase good cholesterol levels. However, a high intake of alcohol tends to increase triglycerides (a type of fat) in the blood and raise blood pressure, increasing the risk of heart disease.
Foods containing vitamin E - some studies indicate that vitamin E acts as an antioxidant, helping to protect against 'bad' cholesterol. Good sources of vitamin E include avocados, dark green vegetables, vegetable oils and wholegrain products. It is better to eat foods containing vitamin E rather than take supplements, which do not have the same protective effects.
Garlic - a compound in fresh garlic called allicin has been found in some studies to lower blood cholesterol.
Foods enriched with plant sterols - may be useful for people who have trouble lowering their blood cholesterol - around 2g plant sterols daily (found in 1 tablespoon of sterol-enriched margarine) may help.
General dietary recommendations
To substantially reduce your risk of developing coronary heart disease:
Avoid fried fast food and processed foods containing vegetable shortening.
Choose a variety of oils (extra virgin olive oil, canola, peanut) and foods containing natural fats (nuts, seeds, avocado, olives, soy, fish).
Switch to low fat or non-fat dairy products.
Increase the amount and variety of plant foods consumed - eat more unrefined vegetables, fruits and wholegrain cereals. Reduce intake of refined sources of carbohydrates with higher glycaemic indices.
Include legumes (like baked beans, soybeans, lentils and tofu) in your diet.
Have a handful (9-15) of a variety of nuts on most days of the week, especially walnuts and almonds (instead of biscuits, cakes, chips).
Eat oily fish at least once per week.
If you drink alcohol, have no more than two drinks per day.
Trim all visible fat from meat.
Remove poultry skin and eat only the meat.
Don't forget exercise
Diet alone is not enough
Exercise is vital to reduce the risk of heart disease. Walking at least 30 minutes each day at a vigorous pace (at least 3km per hour) reduces heart disease risk by 30 per cent. How do you measure how briskly you are walking? Rule of thumb - walk as fast as you can, so you can still talk but not sing.
If you are over 40, have a heart condition or haven't exercised for a long time, see your doctor before you start any exercise program.
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