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Fats are also known as lipids. A lipid is a substance that is poorly soluble or insoluble in water The term ‘dietary fat’ encompasses many different types of fat. Over 90% of dietary fats are called triacylglycerols or triglycerides Other dietary fats include cholesterol.
Triacylglycerols contain three fatty acids attached to a glycerol molecule. Fatty acids vary according to their length, which is composed of carbon and hydrogen atoms joined together to form a hydrocarbon chain. The number of double bonds that occur between the carbon molecules also varies. The chemical structure of each type of fatty acid determines its physical characteristics and its nutritional and physiological function. Regardless of the type of fatty acid present, all triacylglycerols provide 9 kcal (37 KJ) per gram; this makes fat the most concentrated source of energy in the diet. Fatty acids should provide no more than 30–35% of dietary energy or approximately no more than 70 g aday for women and no more than 90 g a day for men.
Typical high sources of fat in the diet include cooking fats and oils, fried food, fatty and processed meats. These should form a very small part of the diet. Care should be taken to reduce fried foods; avoid adding fats and oils during cooking; to grill food, which allows fat to drip out; and to choose lean meats and low fat dairy products. A product is thought to be low in fat if it contains less than 3 g fat per 100 g and high in fat if it contains more than 20 g fat per 100 g or 21 g fat per serving.
Some types of fatty acids are essential nutrients. They must be consumed in the diet for the body to function properly. Fats form the structure of cell membranes, they are involved in the transport, breakdown and excretion of cholesterol and they are the building blocks for many important compounds such as hormones, blood clotting agents, and compounds involved in immune and inflammatory responses. Fats also transport fat soluble vitamins and antioxidants; provide the body with insulation and form a protective layer around organs; are a structural component of the brain and nervous system; and provide a reserve supply of energy in the form of adipose tissue (body fat). Excess amounts of adipose tissue defines obesity and may lead to health problems such as diabetes, cancer and heart disease.
Saturated fatty acids
Saturated fatty acids have a hydrocarbon chain where each carbon atom carries its maximum number of hydrogen atoms except for the end carboxyl group and they do not have any double bonds. The molecules are straight, allowing them to pack closely together. For this reason, they are solid at room temperature with a high melting point. Saturated fatty acids are chemically stable both within the body and in food.
Saturated fatty acids are named according to the number of carbon atoms they contain. Each one has a common name (e.g., stearic acid), a systematic name (e.g., octadecanoic acid because stearic acid has 18 carbon atoms), and a notational name (e.g., 18:0 as stearic acid has 18 carbon atoms but no double bonds).
Animal products such as meat fat, dripping, lard, milk, butter, cheese and cream are the primary sources of saturated fatty acids. Most plant products have a lower amount of saturated fat with the exception of coconut and palm oil.
SATURATED FATTY ACIDS AND HEALTH Saturated fatty acids increase the body’ levels of cholesterol, including low density lipoprotein (LDL) cholesterol. LDL cholesterol is commonly known as ‘bad’ cholesterol. High levels of LDL cholesterol in the blood increase the risk of cardiovascular disease. LDL cholesterol transports excess cholesterol through the bloodstream where it can become deposited in the walls of the arteries and form a hardened plaque. This is called atherosclerosis. This thickening of the artery walls reduces the flow of blood supplying the heart, brain, and other organs. A heart attack or stroke is caused by a blood clot blocking these narrowed arteries. Saturated fatty acids also contribute to production of these blood clots as they are converted into substances that can increase the stickiness of the blood and increase its tendency to clot. For this reason dietary guidelines recommend that no more than 10% of dietary energy should come from saturated fatty acids. This means that on a daily basis approximately no more than 22 g saturated fat should be consumed.
(Illustration by GGS Information Services/Thomson Gale).
by a woman consuming 2,000 calories a day and no more than 28 g saturated fat should be consumed by a man consuming 2,500 calories a day. A product is considered low in saturated fat if it contains less than 1.5 g per 100 g and high in saturated fat if it contains more than 5 g of fat per 100 g.
Monounsaturated fatty acids
Monounsaturated fatty acids have a hydrocarbon chain that contains one unsaturated carbon bond that is not fully saturated with hydrogen atoms. Instead, it has a double bond to the adjoining carbon atom. Double bonds are either in a cis or trans formation. In the cis formation the hydrogen atoms bonded to the carbon atoms in the double bond are positioned on the same side of the double bond. This creates a kink in the hydrocarbon chain. There is also a free electron or slightly negative charge surrounding the double bond causing them to repel each other. The molecules are not packed closely together and become liquid (oil) at room temperature. In the trans formation the hydrogen atoms are on opposite sides of the carbon-carbon double bond resembling the characteristics of a saturated fatty acid. There is less kinking of the hydrocarbon chain and the fat is more solid at room temperature. trans bonds are rarely seen in nature.
Monounsaturated fatty acids are named according to the number of carbons they contain and the position of their double bond. Like saturated fatty acids, they each have a common name, a systematic name, and a notational name. Fatty acids with double bonds in the ninth position are sometimes called n-9s or omega-9s.
The most concentrated sources of monounsaturated fatty acids in the diet are olive oil and rapeseed oil. They are present in many other foods including nuts and seeds, avocados, eggs, fish and meat fat.
MONOUNSATURATED FATTY ACIDS AND HEALTH Monounsaturated fatty acids reduce the level of total and LDL cholesterol. It also has a significant effect on increasing and maintaining the body’ level of high density lipoprotein (HDL) cholesterol. HDL cholesterol is commonly known as ‘good’ cholesterol because it removes cholesterol from the blood transferring it to body tissues where it is used to make hormones and other substances the body needs. Therefore, higher levels of HDL cholesterol are associated with a reduction in the risk of cardiovascular disease. Between 10–20% of dietary energy should come from monounsaturated fat.
Polyunsaturated fatty acids
Polyunsaturated fatty acids have a hydrocarbon chain containing two or more double bonds not fully saturated with hydrogen atoms. The double bonds may either be in the cis or trans formation. The majority of naturally occurring polyunsaturated fats are in the cis form. In this form the hydrogen atoms bonded to the carbon atoms in the double bond are positioned on the same side of the double bond. This creates a kink in the hydrocarbon chain. There is also a free electron or slightly negative charge surrounding the double bond causing them to repel each other. The molecules are not packed closely together and become liquid (oil) at room temperature. The presence of one or more double bonds with free electrons and a negative charge makes them unstable molecules ready to.
react with other chemicals. Polyunsaturated fatty acids are susceptible to chemical changes or oxidation within food leading to cell damage in the body.
Polyunsaturated fatty acids are named similar to other fatty acids. They have a common name, a systematic name, and a notational name. Fatty acids with double bonds starting in the sixth position are commonly known as n-6s or omega-6s.
POLYUNSATURATED FATTY ACIDS AND HEALTH Polyunsaturated fatty acids are divided into two groups, omega-6s and omega-3s. There is one essential fatty acid in each of these groups from which all other fatty acids can be made in the human body. These essential fatty acids cannot be made by the body and must be obtained from the diet. They are a necessary component of the diet; without them deficiency symptoms and poor health would result. Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are the essential fatty acids. Linoleic acid should provide at least 1% of dietary energy and alpha-linoleic acid should provide 0.2% dietary energy. These essential fatty acids are converted into longer chain fatty acids that form important substances in the body such as hormones, blood clotting agents, and compounds involved in immune and inflammatory responses.
These long chain fatty acids are not technically essential, but they have an important role in the body. Examples of long chain fatty acids include aracha-donic acid (AA), eicosapentaenoic acid (EPA), and docosahexanoic acid (DHA). Long chain omega-3
fatty acids become essential if there is insufficient linoleic and aplha-linolenic acid available in the diet. These fats play a significant role in development of the brain, nervous system, and retina in fetal development and early life.
OMEGA-6 The most concentrated sources of omega-6 in the diet is vegetable oils, such as sunflower, safflower, corn, cottonseed, canola, and soya oils. They are also present in plant seeds, nuts, vegetables, fruit and cereals. In addition to being a source of linoleic acid, omega-6s have been shown to have a lowering effect on both LDL and HDL cholesterol. However, there are health concerns with excessive omega-6 intakes. Omega-6 fats are susceptible to oxidation within the body and may contribute to tissue damage that leads to atherosclerosis and cancer. Omega-6 fats should contribute no more than 10% of dietary energy. Antioxidant nutrients such as vitamin E are required to reduce this oxidation with higher intakes of omega-6 fats. Omega-6s compete with the more beneficial omega-3 fatty acids, so it is recommended that the omega-6:omega-3 ratio is reduced to 4:1.
OMEGA-3 Short chain omega-3 fats are found in flaxseed or linseed oil, walnut oil, canola oil, and rapeseed oil. The best sources of long chain omega-3s are fish and fish oil.
Evidence suggests that consuming long chain omega-3 fats has cadiovascular health benefits. This believed to be the result of their anti-clotting effect. Growing evidence also suggests that consuming long chain omega-3s has benefits beyond those achieved when consuming shorter chain fatty acids. The United Kingdom’ government Food Standards Agency recommends that oily fish be consumed at least once a week.
There has been much interest in the effect of EPA and DHA deficiency and supplementation on behavior in children, particularly those with learning difficulties. Although there is some evidence of benefit with EPA, in 2006 the U.K. Food Standards Agency concluded that there was insufficient evidence to reach a firm conclusion and additional clinical trials were needed.
There is also interest in the anti-inflammatory properties of long chain omega-3s in inflammatory conditions such as Crohn’ disease and rheumatoid arthritis. The role of omega-3s has been evaluated in treatment of depression and prevention of cognitive decline but more research is needed to confirm these benefits.
Omega-3 fats have been shown to reduce blood pressure and triglyceride levels (another fat in the blood that contributes to raising the risk of cardiovas cular disease). To achieve these benefits, omega-3s must be taken in pharmacological doses and there are small risks associated with these high doses such as raised LDL cholesterol, poor control of diabetes, and increased risk of bleeding. Large doses of omega-3s should only be taken under the supervision of a qualified medical doctor.
Trans fatty acids
Trans fatty acids are monounsaturated or polyun-saturated fatty acids where the double bond is in the trans rather than cis formation. They occur naturally in small amounts in lamb, beef, milk, and cheese as they are created in the rumen of cows and sheep. The majority of trans fat in the diet comes from the partial hydrogenation of vegetable oils. This is a process in food manufacture that adds hydrogen atoms to unsaturated fatty acids so that oils become more hardened at room temperature. The process results in some of the double bonds of the fatty acid molecules becoming saturated and some of the remaining double bonds changing from a cis to a trans formation. For example, when partially hydrogenated oleic acid becomes elaidic acid or 9 trans-octadecenoic acid. Trans fats are semi-solid at room temperature and more stable within food. Partial hydrogenation of oils has traditionally been used to develop spreading fats and margarines, for fast food, and in cakes and biscuits. Manufacturers are using it less because of the health problems associated with it. In 2006, New York City adopted the United States’ first major ban on all but trace amounts of artificial trans fats in restaurant cooking. As of July 2008, a serving of food must not contain more than half a gram of trans fat. Food legislation in the United States and the European Union states that hydrogen-ated or partially hydrogenated fats must be labeled in the ingredients of food and in some cases the amounts of trans fat must also be labeled.
TRANS FATTY ACIDS AND HEALTH Trans fat raises LDL cholesterol in a similar way to saturated fat and it reduces HDL cholesterol. It may also raise blood triglyceride levels. The combination of both these effects means that it is most likely to increase cardiovascular risk. The World Health Organization recommends phasing out trans fat in food manufacture and reducing trans fat consumption to no more than 1% of dietary energy or 2.5 g per day.
Cholesterol is essential to the structure of cell membranes and production of bile acids for digestion, steroid hormones, and vitamin D. Dietary cholesterol
has little effect on blood cholesterol levels because an increased dietary intake reduces the amount the body produces. Only extreme dietary levels of cholesterol need to be restricted. For most individuals, dietary measures that reduce saturated fat also avoid excessive cholesterol consumption. However, individuals with familial hypercholesterolemia may need to consume less than 300 mg/day, which requires avoidance of most animal products.
The most concentrated dietary sources of cholesterol include liver, offal, and products made from egg yolk, mayonnaise, fish roes, and shellfish.
The guidelines for the recommended levels of dietary fat is not appropriate for those under two years of age, for those who are ill or malnourished, or for those diagnosed with anorexia nervosa
A study conducted by the International Study of Asthma and Allergies in Childhood (ISAAC) found a strong correlation between trans fatty acids and increased occurrence of allergies in adolescents. Parents should provide healthy alternatives to foods containing fatty acids and monitor the amount and type of fats consumed in their diet. These preventative measures help avoid serious health problems including heart disease and stroke that can result from high levels of fatty acids in the diet.
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American Dietetic Association (ADA). 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. Telephone: (800): 877-1600. Website: <http://www.eatright.org>.
American Heart Association. 7272 Greenville Avenue, Dallas, TX 75231. Telephone: (800) 242-8721. Website: <http://americanheart.org>.
British Dietetic Association. 5th Floor, Charles House, 148/9 Great Charles Street, Queensway, Birmingham, B3 3HT. Telephone: 0121 200 8080. Website: <www.bda.uk.com>.
British Heart Foundation. 14 Fitzgerald Street, London W1H 6DH. Telephone: 020 7935 0185. Website: <www.bhf.org.uk>.
British Nutrition Foundation. High Holborn House, 52-54 High Holborn, London WC1V 6RQ. Telephone: 020 7404 6504. Website: <www.nutrition.org.uk>. Food Standards Agency. Aviation House, 125 Kingsway, London WC2B 6NH. Telephone: 020 7276 8000. Web sites: <www.food.gov.uk>
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Deborah Lycett, BSc(Hons) RD MBDA.