Fats are a subset of the class of nutrients 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. More than 90% of dietary fats are called triacylglycerols or triglycerides. Other dietary fats include cholesterol.
Triglycerides contain three fatty acids attached to a glycerol molecule. Fatty acids vary according to their length and are 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. Saturated fatty acids have no double bonds. Monounsaturated fatty acids have one double bond. Polyunsaturated fatty acids have more than one double bond. 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 (g); this makes fat the most concentrated source of energy in the diet. Fats come in a variety of different forms, from liquid (unsaturated) to solid (saturated) at room temperature.
Saturated fatty acids have a hydrocarbon chain in which each carbon atom carries its maximum number of hydrogen atoms except for the end carboxyl group. 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 because stearic acid has 18 carbon atoms but no double bonds).
Animal products such as meat fat, meat drippings, beef tallow, 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, palm kernel, and palm oil.
SATURATED FATTY ACIDS AND HEALTH. Saturated fatty acids increase the body's levels of cholesterol, including low density lipoprotein (LDL) cholesterol. The prevailing belief for the past decades has been that high levels of LDL cholesterol in the blood increase the risk of cardiovascular disease. For this reason, dietary guidelines recommend that no more than 10% of dietary energy should come from saturated fatty acids. Women should consume no more than 22 g of saturated fat if consuming 2,000 calories a day, and men should have no more than 28 g of saturated fat if 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 it is high in saturated fat if it contains more than 5 g of fat per 100 g.
In the 2010s, researchers debated about the danger of saturated fats in the diet. Several studies published in that decade failed to find an association between saturated fat, high cholesterol, and heart disease. In 2017, the British Journal of Sports Medicine published an article expressly contradicting the claim that saturated fat clogs arteries and causes heart disease, and called for a shift in guidelines involving saturated fat consumption. Some experts now recommend that people actually add fats to their diets and reduce carbohydrates and sugars in an effort to reduce insulin levels and prevent weight gain, type 2 diabetes, and other conditions associated with high insulin. Conversely, the American Heart Association and most medical bodies continue to recommend that people eat a diet low in saturated fats to protect their hearts. The DASH diet, for example, is a low-fat diet specifically recommended to patients with hypertension and a risk of heart disease.
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. They also have 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. With less kinking of the hydrocarbon chain, 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 canola (rapeseed) oil. These fatty acids are present in many other foods including nuts and seeds, avocados, eggs, fish, and meat fats, especially chicken.
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 which the hydrogen atoms bond to the carbon atoms on the same side of the double bond. This creates a kink in the hydrocarbon chain. A free electron or slightly negative charge surrounding the double bond causes 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.
Like other fatty acids, polyunsaturated fatty acids 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 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-linolenic acid should provide 0.2% of 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 arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosa-hexaenoic 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 are vegetable oils, such as sunflower, safflower, corn, cottonseed, canola, and soybean oils. They are also present in plant seeds, nuts, vegetables, fruits, 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 intake. 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 be reduced to 4:1.
OMEGA-3. Short chain omega-3 fats are found in flaxseed or linseed oil, walnut oil, and canola oil. They are also present in nuts, seeds, and wheat germ, but 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, which is 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's Food Standards Agency recommends that oily fish be consumed at least once a week.
Researchers have shown much interest in the effect of EPA and DHA deficiency and supplementation on behavior in children, particularly those with learning difficulties. As of 2018, however, no good evidence supported this hypothesis.
Studies are also being done on the anti-inflammatory properties of long chain omega-3s in inflammatory conditions such as Crohn's 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 in which the double bond is in the trans rather than cis formation. They are no longer a significant factor in the food supply in many places, having been banned due to their deleterious effects on health. Trans fats occur naturally in small amounts in lamb, beef, milk, and cheese as they are created in the rumen of cows and sheep. During the twentieth century, the majority of trans fat in diets came from the partial hydrogenation of vegetable oils, 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. Trans fats are semi-solid at room temperature and more stable within food. Partial hydrogenation of oils was widely used in the previous century to develop spreading fats and margarines and was also present in fast food, cakes, and biscuits.
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.
The most concentrated dietary sources of cholesterol include liver (foie gras), egg yolks and products made from egg yolk, caviar (fish roes), butter, and shrimp.
After years of recommending a low-fat diet, health organizations are changing slightly in light of research suggesting that dietary fat is not as harmful as previously believed. The USDA's 2015–2020 dietary guidelines do not encourage a low-fat diet, and state that a healthy diet can contain up to 35% of calories from fat. The World Health Organization/Food and Agriculture Organization (WHO/FAO) recommendations for fat intake are similar. Fatty acids should provide no more than 30%–35% of total calories, which equates to approximately no more than 70 g/day for adult women and no more than 90 g/day for adult men. Amounts for children are slightly higher, starting at 40%–60% for infants up to six months and then gradually tapering off to 25%–35% by age two. Saturated fats should comprise no more than 10% of total energy intake in adults and 8% in children. A person should not drop his or her fat intake to less than 15% of calories unless prescribed by a medical professional, with guidance from a registered dietitian. People should also not increase fat intake to more than 35% unless being monitored by a doctor.
A number of experts do not believe that these recommendations are correct, and favor instead a diet that is very high in fat (up to 85%) and low in both protein and carbohydrates (as low as 5% carbohydrates in ketogenic diets). Proponents of the low-carb high-fat (LCHF) diet point out that fat does not raise insulin levels, making it less likely for the body to store excess calories as fat. Fat also keeps people satisfied longer than carbohydrates, allowing them to wait much longer between meals. They advocate the consumption of saturated fats such as butter and cream, and recommend seeking out fatty cuts of meat instead of lean ones. Benefits of a LCHF diet can include weight loss, lower insulin levels, and the reversal of type 2 diabetes, all effects that have been verified in scientific studies.
Research has begun refuting the connection between dietary consumption of fat and heart disease. For example, a 2016 Norwegian diet intervention study called FATFUNC found that a high intake of total and saturated fat did not increase the calculated risk of heart disease. In 2017, the European Society of Cardiology called for a reconsideration of dietary recommendations in light of research indicating that high carbohydrate diets are linked with worse mortality and high fat intake is associated with lower risk of cardiovascular mortality. They observed that limiting fat consumption does not improve health. They suggested 35% as a dietary fat goal.
Typical high sources of fat in the diet include cooking fats and oils, fried food, fatty and processed meats, nuts, coconut, cheeses and whole-fat dairy products. A product is considered 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.
The guidelines for the recommended levels of dietary fat are not appropriate for people under two years of age, those who are ill or malnourished, or those who have been diagnosed with a medical condition that requires higher or lower intake of dietary fat.
See also Coronary heart disease; Diet drugs; Dietary cholesterol; Dietary guidelines; Fat replacers; Hyperlipidemia; Hypertension; Hypertriglyceridemia; Low-fat diet; Omega-3 and omega-6 fatty acids; TLC diet; Trans fats; Triglycerides.
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Deborah Lycett, BSc(Hons) RD MBDA
Revised by Amy Hackney Blackwell, PhD