Fats are also called lipids. A lipid is a substance that is poorly soluble or insoluble (does not dissolve) in water. The term “dietary fat” encompasses many different types of fat. More than 90% of dietary fats are called triacylglycerols or triglycerides. Cholesterol is another type of dietary fat.
Triacylglycerols contain three fatty acids attached to a glycerol molecule. Fatty acids are composed of carbon and hydrogen atoms joined together to form a hydrocarbon chain and vary in length. 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 functions. Regardless of the type of fatty acids present, all triacylglycerols provide 9 kcal (37 kJ) per gram of energy. This makes fat the most concentrated source of energy in the diet. Dietary guidelines recommend that fats should provide no more than 30%–35% of dietary energy, which is approximately no more than 70 g a day for women (with an average intake of 2,000 kcal) and no more than 90 g a day for men (with an average intake of 2,500 kcal).
Typical sources of concentrated fat in the diet include cooking fats and oils, fried food, and fatty and processed meats (e.g., hot dogs). These should form only a small part of the diet. Care should be taken to reduce the amount of fried and fatty foods eaten. Avoiding adding fats and oils during cooking, grilling food so as to allow fat to drip out, and choosing lean meats and low-fat dairy products can help promote better health. 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. Care should be taken when selecting low-fat food products however, as they may still be high in calories and salt.
There are two fatty acids that are considered essential and must be obtained in the diet as they cannot be made in the body. They must be consumed in the diet for the body to function properly. Fats form the structure of cell membranes; are involved in the transport, breakdown, and excretion of cholesterol; and 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, 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 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 (e.g., butter) 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).
SATURATED FATTY ACIDS AND HEALTH. Saturated fatty acids increase the body's 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 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 have a hydrocarbon chain that contains one unsaturated carbon bond that is not fully filled (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.
MONOUNSATURATED FATTY ACIDS AND HEALTH. Monounsaturated fatty acids are able to reduce the level of total and LDL cholesterol while increasing or maintaining the body's 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. Approximately 10%–20% of dietary energy should come from monounsaturated fat.
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 will 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 arachadonic acid (AA), eicosapentaenoic acid (EPA), and docosahexanoic acid (DHA). Long-chain omega-3 fatty acids become essential if there is insufficient linoleic and alpha-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 cardiovascular health benefits. This is believed to be the result of their anti-clotting effect. Growing evidence also suggests that consuming longchain omega-3s has benefits beyond those achieved when consuming shorter-chain fatty acids. Research trials are ongoing to determine the effects of including omega-3s in the diet on a regular basis.
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.
Trans fatty acids are monounsaturated or polyunsaturated 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 solid 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. A serving of commercially prepared food must not contain more than half a gram of trans fat. The city of Philadelphia and the state of California soon followed suit. As of July 2011, similar legislation was underway in many other municipalities. Food legislation in the United States and European Union states that hydrogenated 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.
The most concentrated dietary sources of cholesterol include liver, offal, and products made from egg yolk, mayonnaise, fish roes, and shellfish.
If a person's blood lipid levels are high, dietary treatment is often initiated to lower the low-density lipoprotein fraction and total blood cholesterol levels. Current American Heart Association recommendations include reducing saturated fat to less than 7% of calories, limiting cholesterol to a blood reading of less than 200, losing weight if appropriate, and increasing soluble fiber to 0.35–0.88 oz. (10–25 g) per day. High-density lipoprotein (HDL) levels, also referred to as the good cholesterol, is related more to genetic and other factors than to diet, although moderate alcohol consumption raises HDL.
Some people also have high plasma triglyceride (TG) levels, which accumulating evidence is showing may be an important heart disease risk factor. Dietary guidelines involve low simple sugars, low-saturated fat diets, restricted alcohol and weight loss if indicated. Drug therapy is often initiated.
The recommended dietary intake of fats is generally considered sufficient fat intake for most athletes, even those engaging in endurance or other events. Fat intake should be made up primarily of monounsaturated fats such as those found in seeds, nuts, and olive oil.
Many athletes who engage in professional or very rigorous sports and fitness activities try to limit the amount of fat in their diets. This is admirable; however, reducing fat intake too much can also have serious negative consequences. Fat is required for the body to operate in peak condition. Muscles primarily burn energy from carbohydrates during exercise, but they also burn fat. A diet that severely restricts fat can hurt muscle performance. One research study compared athletes who were on very low-, medium-, and high-fat diets. The researchers found that athletes on the very-low-fat diet did not perform as well or have the endurance of the athletes on the medium-fat diet. The athletes on the high-fat diet performed about as well as those on the medium-fat diet. These results show that moderate amount of fats, especially monounsaturated fats, are likely to be necessary for peak athletic performance.
See also Calories ; Cholestrol .
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Deborah Lycett, BSc (Hons) RD MBDA
Revised by Tish Davidson, AM