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Am Fam Physician. 2009;80(4):345-350

See related editorial on page 330.

Patient information: See related handout on trans fats, written by the authors of this article.

Author disclosure: Nothing to disclose.

Fatty acids can be divided into four general categories: saturated, monounsaturated, polyunsaturated, and trans fats. Saturated fatty acids and trans fats are associated with an increased risk of coronary heart disease. Monounsaturated fatty acids and polyunsaturated fatty acids are associated with a decreased risk of coronary heart disease, although these associations are not uniformly supported in the literature. Omega-3 fatty acids, which are a type of polyunsaturated fatty acid, have been studied as potential therapy for a variety of medical conditions because of their suspected anti-inflammatory properties. Omega-3 fatty acids have also been shown to provide some benefit to patients with cystic fibrosis, and may have a protective effect against dementia. Physicians should counsel patients about the importance of avoiding hydrogenated oils and foods containing trans fats because of their association with coronary heart disease in observational studies.

Fatty acids are long-chain hydrocarbons that can be separated into four categories: saturated, mono-unsaturated, polyunsaturated, and trans fats. More than 20 types of fatty acids are found in foods; some of these are listed in Table 1. Sources of fatty acids include fruits, vegetable oils, seeds, nuts, animal fats, and fish oils. Essential fatty acids, such as omega-3 fatty acids, serve important cellular functions. They are a necessary part of the human diet because the body has no biochemical pathway to produce these molecules on its own.

Clinical recommendationEvidence ratingReferences
Increased intake of saturated fats and trans fats may increase the risk of coronary heart disease.B47, 9, 40, 41
Increased intake of monounsaturated and polyunsaturated fats may decrease the risk of coronary heart disease.B4, 713
Regular omega-3 fatty acid supplementation may provide some benefits for persons with cystic fibrosis and dementia.B20, 21
There is no clear effect of omega-3 fatty acid consumption on cardiovascular events in persons with, or at high risk of, cardiovascular disease.B18
NameNumber of carbon atomsType of fatty acidEssential fatty acidCommon sources
Palmitic acid16SaturatedNoPalm oil
Stearic acid18SaturatedNoAnimal fat
Oleic acid18MonounsaturatedNoOlive oil
Linoleic acid18PolyunsaturatedYesSafflower oil
Linolenic acid18PolyunsaturatedYesSoybean oil
Arachidonic acid20PolyunsaturatedYesMeat, dairy
Eicosapentaenoic acid20PolyunsaturatedYesFish oil
Docosahexaenoic acid22PolyunsaturatedYesFish oil

Biochemical Structure

In saturated fatty acids, the carbon chain has the maximum number of hydrogen atoms attached to every carbon atom. If a pair of hydrogen atoms is missing because of a double bond between two carbon atoms, it is called an unsaturated fatty acid. A fatty acid with a single double bond is monounsaturated, whereas a fatty acid with more than one double bond is polyunsaturated (Figure 1). The carbon-carbon double bond found in monounsaturated or polyunsaturated fatty acids can exist in the cis or trans configuration. When the two hydrogen atoms are on opposite sides of the double bond, the configuration is called trans. When the hydrogen atoms are on the same side of the double bond, the configuration is called cis (Figure 2).

Saturated Fats

Studies have shown that consuming saturated fatty acids has a detrimental effect on serum lipids by increasing low-density lipoprotein (LDL) cholesterol levels.1 There is some evidence that short-chain fatty acids (fewer than 10 carbon atoms) are less likely to affect serum cholesterol levels, whereas longer-chain fatty acids (12, 14, or 16 carbon atoms) are more likely to raise LDL levels.2 One exception to this is stearic acid (18 carbon atoms), which does not appear to raise serum cholesterol levels.3 Increased consumption of saturated fatty acids has also been associated with an increased risk of coronary heart disease (CHD; Table 2).413

Monounsaturated Fats

Several large observational studies have found an association between an increased intake of monounsaturated fatty acids and a decreased risk of CHD (Table 2).413 One large study failed to find a similar association, although it was limited to a trial of beta-carotene and alpha-tocopherol in persons who smoke.9 Evidence from controlled clinical studies has shown that monounsaturated fatty acids favorably affect a number of risk factors for CHD, including lowering total and LDL cholesterol levels, protecting against thrombogenesis, reducing LDL susceptibility to oxidation, and producing a more favorable glycemic profile.14

StudyNumber of patientsStudy typeYears of follow-upFindings
Saturated fatty acids
Seven Countries Study4 12,770 menCross-population5, 10, 15Strong correlation between total cholesterol and the percent of energy intake from saturated fatty acids
Japan-Honolulu-San Francisco Study5 11,900 menCross-populationNACorrelation between increased consumption of saturated fatty acids, and increased serum cholesterol levels and increased rate of CHD mortality
Ireland-Boston Diet-Heart Study6 1,001 menProspective cohort20Patients who died of CHD had a higher intake of saturated fatty acids and cholesterol
Nurses' Health Study7 80,082 womenProspective cohort14Positive association between percent of energy intake from saturated fatty acids and increased risk of CHD
Monounsaturated fatty acids
Seven Countries Study4 12,770 menCross-population5, 10, 15Low death rate from heart disease in men consuming large quantities of olive oils (primarily monounsaturated fatty acids)
Nurses' Health Study7 80,082 womenProspective cohort14Association between increased energy intake from monounsaturated fatty acids and a relative risk reduction in coronary disease
Coronary Mortality in France and Finland8 NAEpidemiologicNAAssociation between increased consumption of monounsaturated fatty acids and low rates of CHD mortality
Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study9 21,930 men who smokeProspective cohort6.1No association between monounsaturated fatty acids and the risk of CHD
Polyunsaturated fatty acids
Finnish Mental Hospital Study10 676 menRCT6Increased consumption of polyunsaturated fatty acids associated with reduction of CHD
Los Angeles Veteran Hospital11 424 menRCT8Increased consumption of polyunsaturated fatty acids associated with reduction of CHD
Oslo Diet-Heart Study12 206 men with history of myocardial infarctionRCT5Increased energy from polyunsaturated fatty acids associated with significant reduction in cholesterol and coronary events
Minnesota Coronary Survey13 4,393 men, 4,664 womenRCT4.5Increased consumption of polyunsaturated fatty acids associated with reduction in serum cholesterol but no change in CHD

Polyunsaturated Fats

Polyunsaturated fatty acids, which include omega-3 fatty acids, have been studied extensively for their effect on several diseases (Table 3).1525 Omega-3 fatty acids have been shown to provide some benefit to patients with cystic fibrosis, and may have a protective effect against dementia.20,21 Omega-3 fatty acids are thought to be beneficial in some inflammatory-related diseases because they displace omega-6 fatty acids, including arachidonic acid, in the cell membrane. This reduces the creation of metabolic end products, including prostaglandins, thromboxanes, and leukotrienes.26

Disease processCochrane review conclusions
AsthmaLittle evidence to recommend persons with asthma modify intake of omega-3 fatty acids15
Cancer cachexiaInsufficient data16
ClaudicationOmega-3 has limited benefit in persons with intermittent claudication, but no evidence of consistent improved clinical outcomes17
Coronary heart diseaseNo clear effect of omega-3 fatty acid consumption on cardiovascular events in persons with, or at high risk of, cardiovascular disease18
Crohn diseaseInsufficient data to recommend routine use of omega-3 supplements for maintenance of remission19
Cystic fibrosisRegular omega-3 supplements may provide some benefits for persons with cystic fibrosis20
DementiaMay be a protective effect of omega-3 fatty acids against dementia21
Diabetes (type 2)Omega-3 supplementation lowers triglyceride and very-low-density lipoprotein levels in patients with diabetes, but may raise low-density lipoprotein levels; no effect on glycemic control or fasting insulin22
Kidney transplant recipientsInsufficient evidence to recommend fish oil therapy23
SchizophreniaInconclusive results; use of omega-3 remains experimental24
Ulcerative colitisNo evidence to support omega-3 fatty acids in maintenance of remission25

Additionally, maternal omega-3 fatty acid supplementation during pregnancy and lactation may provide a beneficial effect on the cognitive development of infants and children, but evidence is inconclusive about the benefits of omega-3 supplementation in pre-term and full-term infants.27 Several review articles have examined the relationship between omega-3 fatty acid intake and cancer incidence, including prostate cancer, colon cancer, and skin cancer.2830 A recent systematic review concluded that the literature does not support an association between increased omega-3 fatty acid intake and reduced cancer incidence.31

Various studies413 and a report from the Institute of Medicine32 support the increased consumption of polyunsaturated fatty acid for the prevention of CHD (Table 2413). However, a Cochrane review of 48 randomized controlled trials (RCTs) and 41 cohort analyses suggests that further high-quality trials are needed to confirm the protective effect of omega-3 fatty acids in persons at increased risk of cardiovascular disease.18 The authors concluded that increased consumption of omega-3 fatty acids did not significantly alter total mortality or combined cardiovascular events in persons with cardiovascular disease, in persons at high risk of cardiovascular disease, or in the general population.18 Excessive consumption of polyunsaturated fats is not recommended because of the increased risk of excessive weight gain and the increased risk of gallstone formation in some persons.33

Trans Fats

Most trans fats are manufactured through a process called hydrogenation, which is the artificial addition of hydrogen atoms to unsaturated oils. Hydrogenation converts liquid vegetable oils to solid or semi-solid fats that remain stable at room temperature. These fats can then be incorporated into certain food products (e.g., cookies, chips) to increase shelf life (Table 434). Until recently, trans fats have been the predominant fat used in most types of commercial baked goods.

ProductCommon serving sizeTotal fat (g)Saturated fat (g)Percent daily value for saturated fatTrans fat (g)Combined saturated and trans fat (g)Cholesterol (mg)Percent daily value for cholesterol
Butter1 tablespoon11735073010
Cake (pound cake)1 slice (80 g)163.5184.5800
Candy bar1 (40 g)1042037<51
Cookies (cream filled)3 (30 g)6152300
French fried potatoes (fast food)Medium size (147 g)2773581500
Margarine, stick§1 tablespoon112103500
Margarine, tub§1 tablespoon7150.51.500
Mayonnaise (soybean oil)1 tablespoon111.5801.552
Milk, skim§1 cup0000052
Milk, whole1 cup74.52304.53512
Potato chipsSmall bag (42.5 g)112103500
Shortening1 tablespoon133.51847.500

The consumption of trans fats has been directly linked to an increase in CHD. Trans fats appear to have a detrimental effect on serum lipids by increasing LDL cholesterol and triglyceride levels, and reducing high-density lipoprotein cholesterol levels.3539 Studies have shown that substituting 2 percent of total energy intake with trans-fatty acids is associated with a 14 to 36 percent increase in the incidence of CHD (Table 5).7,9,40,41 Observational data from the Nurses' Health Study suggests that replacing 5 percent of energy from saturated fat with energy from unsaturated fats could reduce the risk of CHD by 42 percent; however, replacing only 2 percent of energy from trans fats with energy from nonhydrogenated, unsaturated fats could reduce the risk of CHD by 53 percent.7 There are no prospective RCTs that have demonstrated a reduction in morbidity and mortality through selective reduction of trans-fat consumption.

StudyNumber of patientsYears of follow-upRelative risk of CHD with trans-fat consumption*
Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study9 21,930 men who smoke6.11.14 (95% CI, 0.96 to 1.35)
Zutphen Elderly Study39 667 men101.28 (95% CI, 1.01 to 1.61)
Health Professionals Follow-up Study40 43,757 men61.36 (95% CI, 1.03 to 1.81)
Nurses' Health Study7 80,082 women141.33 (95% CI, 1.07 to 1.66)

Dietary Recommendations

The Nutrition Committee of the American Heart Association has recommended that no more than 30 percent of a person's daily calories come from fat.42 Of that, less than 7 percent of total calories should be from saturated fatty acids, and less than 1 percent should be from trans-fatty acids.42 Certain fatty acids, such omega-3 fatty acids, are preferable to saturated fats and should be substituted for saturated fats when possible. Physicians should emphasize to patients that consuming too much of any fat contributes to caloric intake and weight gain. Physicians should also stress the importance of minimizing or avoiding trans fats from hydrogenated oils because of their harmful effects on cholesterol levels and their link to heart disease.

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