Although not considered vitamins, certain fatty acids which cannot be synthesized by the human body, are necessary for normal metabolism. These fatty acids must therefore be obtained from dietary sources. Deficiency of these essential fatty acids, though uncommon, results in a variety of disorders of body growth and function. In order to understand why certain fats are essential, while others are unhealthy, a general overview of fat metabolism and function is necessary.

Fatty acids are long chains of carbon atoms with an acid group at one end and methyl group at the other end. Saturated fatty acids are carbon atoms are linked together by single bonds and contain the maximum number of hydrogen atoms. The fat molecule is saturated with hydrogen. Saturated fats are primarily derived from animal and dairy products. Unsaturated fatty acids are chains of carbon atoms some of which are linked by an ‘extra’ or double bonds. The ‘extra’ carbon to carbon bond replaces a hydrogen bond and therefore the fat molecule is not saturated with hydrogen. Unsaturated fats are derived primarily from vegetable and fish oils.

Unsaturated fatty acids are classified by the location of the first double bond along the carbon chain. An omega 3 fatty acid has the first double bond between the third and fourth carbon atoms (counting from the methyl end). With omega 6 fats, the first double bond occurs between the sixth and seventh carbon atoms. Fats with one double bond are monounsaturated, while fats with more than one double bond are polyunsaturated. Humans are able to synthesize fatty acids omega 9 and higher. Therefore omega 3 and 6 fats are essential because they must be obtained from dietary sources.

Unsaturated fats may be artificially saturated by a chemical process called hydrogenation. Polyunsaturated fats from vegetable oils are partially hydrogenated to monounsaturated fats to solidify them at room temperature. Margarine and shortening are examples of polyunsaturated fats which have been hydrogenated. The hydrogenation process however creates a trans monounsaturated molecule in which the hydrogen atoms at the double bond are situated on opposite sides of the carbon chain. Most naturally occurring unsaturated fats are cis configuration in which hydrogen atoms at the double bond are on the same side of the carbon chain. This slight difference in configuration results in significant differences in the biological activity of the stereoisomers.

Increased consumption of saturated fats raises not only the total cholesterol, but also the LDL or ‘bad’ cholesterol. Hydrogenated trans monounsaturated fats not only increase LDL cholesterol, but also lower HDL or ‘good’ cholesterol. Saturated and hydrogenated fats are not surprisingly linked to atherosclerosis and coronary heart disease.

Essential omega 6 and 3 polyunsaturated fats on the other hand are healthful. Studies have shown that replacing dietary saturated fats with omega 6 polyunsaturated fats lowers cholesterol levels. Omega 3 fats lower triglyceride levels and have been linked to a decreased incidence of coronary heart disease.

Essential fatty acids are also used in the synthesis of cell membranes. They become incorporated into membrane structures called phospholipids that serve cell structural and maintenance functions. Omega 6 and 3 polyunsaturated fats are also required for synthesis eicosaniods, which are metabolites that are converted into a variety of hormone like molecules (e.g. prostaglandins, thromboxane, leukotrienes). These regulatory molecules play vital roles in the modulation of reproductive, immune, secretory cardiovasuclar and pulmonary functions. Biosynthesis of eicosanoids is dependent upon a adequate dietary supply of omega 3 and 6 fatty acids. (1)

Dietary factors are known to affect the composition of phospholipids in cell membranes. A diet relatively deficient in essential fatty acids may result in the incorporation of trans monounsaturated fatty acids instead of the essential fatty acid. This substitution results in sub-optimal cell function such as decreased membrane fluidity and impaired cell regulatory and maintenance functions. (2)

Essential fatty acid deficiency experimentally produced in animals results in decreased growth, skin abnormalities, infertility, impaired immunity, as well as blood cell, liver and kidney abnormalities. Frank deficiency states in humans is rare but have been seen in conditions requiring prolonged intravenous nutrition lacking in fatty acids. The symptoms of deficiency in humans are similar to those seen in animals models.

The human requirements for essential fatty acids are not known. The amounts needed to prevent obvious deficiency symptoms are probably very low. However, a diet relatively insufficient in essential fatty acids may result in vague and nonspecific complaints such as fatigue, generalized weakness, dry skin, rash, frequent infections, and consequently be very difficult to diagnose.

In summary, it is important to understand and distinguish between dietary fats which are harmful and those which are necessary for good health. Excessive saturated fats primarily from animal and dairy products or from hydrogenation of vegetable oils raises cholesterol and is associated with accelerated atherosclerosis and heart disease. Omega 3 and 6 polyunsaturated fats found primarily in vegetable and fish oils lower the cholesterol level and are associated with decreased risk of heart disease. A recommended healthy diet contains less than 30% of calories from total fat, less than 10% from saturated fat, and less than 300 mg per day of cholesterol.

1. Jones PJH, Kubow S. Lipids, Sterols, and Their Metabolites, In: Shills ME, et. al., eds. Modern Nutrition in Health and Disease, 9^th ed. Baltimore: Williams and Wilkens, 1999, 84-85

2. Grundy SM. Nutrition and Diet in the Management of Hyperlipidemia and Atherosclerosis. In: Shills ME, et. al., eds. Modern Nutrition in Health and Disease, 9^th ed. Baltimore: Williams and Wilkens, 1999, 1203

Howard Chew, MD