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What are Fats?

The primary function of fats within the human body is storage of excess energy. Fats are also necessary for the absorption of carotenoids (naturally occurring pigments from plant sources) and fat-soluble vitamins (vitamins A, D, E, and K are fat soluble), and they play a structural role in cell membranes.

Fats, along with proteins and carbohydrates, are macronutrients. That means that fats are nutrients that are required in relatively large amounts within the body, and that they are capable of providing caloric energy when consumed (as opposed to vitamins and minerals, which are micronutrients and do not provide any caloric energy when consumed).

There are three basic types of fats: unsaturated fats, saturated fats, and trans fats. Each one of these fats is discussed in more detail below.

Unsaturated Fats are Good Fats

olive oil dripping off a spoon

Unsaturated fats, which can be further subcategorized into monounsaturated fats and polyunsaturated fats, are generally considered to be good fats because they are known to have a number of health benefits (as is discussed further below). These types of fats are commonly found in plant sources such as nuts, seeds, and vegetable oils, and some types of unsaturated fats are also found in fish. Unsaturated fats exist in liquid form at room temperature.

Monounsaturated Fats

Monounsaturated fats are considered to be good fats, as they are known to have a number of health benefits, including reduction of LDL (or "bad") cholesterol levels and reduced risk of cancer (specifically breast cancer). The Mediterranean Diet, which is high in monounsaturated fats derived primarily from olive oil, is good for the brain insofar as it has been linked to reduced cognitive decline with aging, and it is also good for the heart as it is linked to lower risk of cardiovascular disease. A decrease in saturated fatty acids and an increase in monounsaturated fatty acids has been shown to improve insulin sensitivity (increased insulin sensitivity is good, as it enables your body tissues to better absorb glucose from your bloodstream and thereby regulate your blood sugar levels more efficiently).

Most dietary sources of monounsaturated fats are plant based. Some good dietary sources of monounsaturated fats are olive oil, canola oil, peanut oil, safflower oil, sunflower oil, peanut butter, hazelnuts, almonds, brazil nuts, cashews, avocado, pecans, sesame seeds, and pumpkin seeds.

Spiller GA, Jenkins DJ, Cragen LN, Gates JE, Bosello O, Berra K, Rudd C,Stevenson M, Superko R. Effect of a diet high in monounsaturated fat from almonds on plasma cholesterol and lipoproteins. J Am Coll Nutr. 1992 Apr;11(2):126-30.

Wolk A, Bergström R, Hunter D, Willett W, Ljung H, Holmberg L, Bergkvist L, Bruce A, Adami HO. A prospective study of association of monounsaturated fat and other types of fat with risk of breast cancer. Arch Intern Med. 1998 Jan 12;158(1):41-5.

Feart C, Samieri C, Rondeau V, et al. Adherence to a Mediterranean diet, cognitive decline, and risk of dementia. JAMA 2009; 302:638-48.

Vessby B, Uusitupa M, Hermansen K, Riccardi G, Rivellese AA, Tapsell LC, Nälsén C, Berglund L, Louheranta A, Calvert GD, Maffetone A, Pedersen E, Gustafsson IB, Storlien LH, Rasmussen BM; KANWU Study. Substituting dietary saturated for monounsaturated fat impairs insulin sensitivity in healthy men and women: The KANWU Study. Diabetologia. 2001 Mar;44(3):312-9.

Gaskins AJ, Rovner AJ, Mumford SL, Yeung E, Browne RW, Trevisan M, Perkins NJ, Wactawski-Wende J, Schisterman EF; for BioCycle Study Group. Adherence to a Mediterranean diet and plasma concentrations of lipid peroxidation in premenopausal women. Am J Clin Nutr. 2010 Oct 13.

Polyunsaturated Fats

Polyunsaturated fats, like monounsaturated fats, are considered to be good fats because of their many health benefits. Polyunsaturated fats include the well known omega 3 and omega 6 varieties of fats that have become popular supplementary additions to a large range of supermarket foods. Omega 3 and omega 6 polyunsaturated fats are considered to be essential fats, because the human body cannot create them from other consumed nutrients. It is therefore essential that omega 3 and omega 6 polyunsaturated fatty acids are consumed in the diet. A diet that is deficient in omega 3 polyunsaturated fats can result in growth problems and neurological problems. A diet that is deficient in omega 6 polyunsaturated fats can result in growth problems and skin rash problems.

Omega 3 Polyunsaturated Fats

Some possible health benefits associated with consumption of omega 3 polyunsaturated fats include prevention of certain types of cancer, heart disease, strokes, blood clotting, and heart arrhythmias (i.e. irregular heartbeat), and treatment of inflammatory diseases, asthma, and depression.

Omega 3 polyunsaturated fats can be classified into two main types. The first type, alpha linolenic acid, or ALA, is a form of omega 3 polyunsaturated fat that is derived primarily from plant sources like flaxseed oil, perilla seed oil, canola oil, soybean oil, pumpkin seed oil, walnut oil, tofu, walnuts, pumpkin seeds, and soybeans. The other type of omega 3 polyunsaturated fat includes both eicosapentaenoic acid, or EPA, and docosahexaenoic acid, or DHA. EPAand DHA can be made within the bodies of humans and other animals from ALA precursors consumed in the diet. EPAand DHA can be found in fatty fish, like salmon, mackerel, halibut, and herring.

Omega 6 Polyunsaturated Fats

Some possible health benefits associated with consumption of omega 6 polyunsaturated fats include reduction of LDL (or "bad") cholesterol levels and inflammation, and prevention of heart disease.

Omega 6 polyunsaturated fats can be found in plant food sources in the form of linoleic acid (LA). Food sources that contain LA include safflower oil, sunflower oil, corn oil, soybean oil, sesame oil, sunflower seeds, pine nuts, pecans, and brazil nuts. Humans and other animals can convert LA into arachidonic acid (AA), another form of omega 6 polyunsaturated fat. AA, therefore, can be found in animal sources like poultry and eggs, but the quantity is generally smaller per serving than the amount of LA to be found in a serving of the plant sources listed above.

Sacks FM, Katan M. Randomized clinical trials on the effects of dietary fat and carbohydrate on plasma lipoproteins and cardiovascular disease. Am J Med. 2002 Dec 30;113 Suppl 9B:13S-24S.

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Leitzmann MF, Stampfer MJ, Michaud DS, Augustsson K, Colditz GC, Willett WC, Giovannucci EL. Dietary intake of n-3 and n-6 fatty acids and the risk of prostate cancer. Am J Clin Nutr. 2004 Jul;80(1):204-16.

Bruinsma KA, Taren DL. Dieting, essential fatty acid intake, and depression.Nutr Rev. 2000 Apr;58(4):98-108.

Okamoto M, Mitsunobu F, Ashida K, Mifune T, Hosaki Y, Tsugeno H, Harada S, Tanizaki Y. Effects of dietary supplementation with n-3 fatty acids compared with n-6 fatty acids on bronchial asthma. Intern Med. 2000 Feb;39(2):107-11.

Saturated Fats are Bad Fats

Saturated fats are generally considered to be bad fats, as they are known to increase risk of cardiovascular disease. Saturated fats do actually increase HDL (or "good") cholesterol levels, but they also increase LDL (or "bad") cholesterol levels. Since saturated fats are not essential fats (i.e. your body is capable of making them on its own), and since unsaturated fats are known to increase HDL (or "good") cholesterol levels and decrease LDL (or "bad") cholesterol levels, you should aim to minimize the amount of saturated fats that are consumed in your diet and replace them with unsaturated fats.

Saturated fats are mostly found in animal sources like beef, pork, lamb, seafood, poultry, and higher fat dairy products like cream, whole milk, cheese, and ice cream. Some plant sources, like coconut and palm oil, also contain saturated fats.

Siri-Tarino PW, Sun Q, Hu FB, Krauss RM. Saturated fat, carbohydrate, and cardiovascular disease. Am J Clin Nutr. 2010 Mar;91(3):502-9. Epub 2010 Jan 20. Review.

Siri-Tarino PW, Sun Q, Hu FB, Krauss RM. Saturated fatty acids and risk of coronary heart disease: modulation by replacement nutrients. Curr Atheroscler Rep. 2010 Nov;12(6):384-90.

Trans Fats are Very Bad Fats

Like saturated fats, trans fats are bad fats. In fact, trans fats are considered to be even worse than saturated fats. Whereas saturated fats increase both HDL (or "good") and LDL (or "bad") cholesterol levels, trans fats reduce HDL (or "good") cholesterol levels and increase LDL (or "bad") cholesterol levels. Consumption of trans fats increases risk of cardiovascular disease and systemic inflammation (which can possibly lead to stroke, diabetes, and cardiovascular disease). Studies have shown that for each 2% increase in calories derived from trans fats rather than carbohydrates, saturated fats, monounsaturated fats, or polyunsaturated fats, risk of death due to heart disease or heart attack will increase by 24%, 20%, 27%, or 32%, respectively (depending on which calorie source is replaced with trans fats).

Trans fats are found in dietary sources that contain partially hydrogenated vegetable oils. In the food industry, vegetable oils are sometimes heated in the presence of hydrogen so that they will become solid at room temperature. This process is called partial hydrogenation, and it reduces the likelihood that the oils will spoil, it makes them easier to transport, it allows for continuous reheating without degradation, and it provides some desirable characteristics like, for example, prevention of oil separation from other ingredients. Partial hydrogenation is, however, also responsible for the creation of trans fats, which is very undesirable. Dietary sources of trans fats include some margarines, vegetable shortening, crackers, cookies, and any foods that have been fried in partially hydrogenated oils.

It's worth noting that fully, rather than partially, hydrogenated oils have almost no trans fats. Fully hydrogenated oils are high in saturated fats, which makes fully hydrogenated oils a better (although still not ideal) choice than the high trans fat partially hydrogenated oils. Like saturated fats, trans fats are not essential fats (i.e. your body is capable of making them on its own), so they don't need to be consumed in the diet. Consumption of trans fats should be reduced as much as is possible, or even eliminated entirely.

Castro-Martínez MG, Bolado-García VE, Landa-Anell MV, Liceaga-Cravioto MG, Soto-González J, López-Alvarenga JC. [Dietary trans fatty acids and its metabolic implications]. Gac Med Mex. 2010 Jul-Aug;146(4):281-8.

Mozaffarian D, Pischon T, Hankinson SE, Rifai N, Joshipura K, Willett WC, Rimm EB. Dietary intake of trans fatty acids and systemic inflammation in women. Am J Clin Nutr. 2004 Apr;79(4):606-12.

Mozaffarian D, Aro A, Willett WC. Health effects of trans-fatty acids: experimental and observational evidence. Eur J Clin Nutr. 2009 May;63 Suppl 2:S5-21.

Dietary Guidelines for Consumption of Fats

Unlike the articles about proteins and carbohydrates, this article does not include a specific grams per day recommendation for consumption of fats because, according to the reference listed below, "there are insufficient data to determine a defined level of fat intake at which risk of inadequacy or prevention of chronic disease occurs."

However, when broken down into a percentage of total energy intake, your consumption of fats should follow the general guidelines in the table below:

1 to 3 years30% to 40% of energy requirements
4 to 18 years25% to 35% of energy requirements
19 years and older20% to 35% of energy requirements

For example, if you are an adult that consumes 2000 calories per day it is recommended that 20% to 35%, or from 400 calories to 700 calories, are derived from fats. As discussed above, those fat calories should ideally come from monounsaturated and polyunsaturated fats, rather than saturated and trans fats.

Trumbo P, Schlicker S, Yates AA, Poos M; Food and Nutrition Board of the Institute of Medicine, The National Academies. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. J Am Diet Assoc. 2002 Nov;102(11):1621-30. Erratum in: J Am Diet Assoc. 2003 May;103(5):563.

Additional Resources

Harvard School of Public Health - Fats and Cholesterol: Out with the Bad, In with the Good

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