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.

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).^4–13

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).^4–13 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

Polyunsaturated Fats

Polyunsaturated fatty acids, which include omega-3 fatty acids, have been studied extensively for their effect on several diseases (Table 3).^15–25 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

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.^28–30 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 studies^4–13 and a report from the Institute of Medicine³² support the increased consumption of polyunsaturated fatty acid for the prevention of CHD (Table 2^4–13). 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.

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.^35–39 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.

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.