Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Carbohydrate and Lipid Metabolism
Metabolism of Unsaturated Fatty Acids and Eicosanoids
Essential Fatty Acids (EFAs)

In 1928, Evans and Burr discovered that rats fed a fat-free diet containing Vitamins A and D exhibited growth retardation and reduced fertility. Subsequent studies demonstrated that this deficiency syndrome could be alleviated by supplementing the diet with linoleic, α-linolenic, and arachidonic acids. Other symptoms of the syndrome include scaly dermatitis, tail necrosis, and Urinary Tract lesions; however, it is rarely fatal. Essential Fatty acids are abundant in vegetable oils (see p. 153) and present in smaller amounts in animal Tissues.

In addition to serving as precursors in The Biosynthesis of Prostaglandins and Leukotrienes (see below), essential fatty acids appear to fulfill several other Functions that are not yet fully understood. They are components of cellular structural Lipids, play a crucial role in maintaining mitochondrial membrane architecture, and are found in high concentrations in reproductive Organs. In their structural capacity, essential fatty acids are integrated into Phospholipids (predominantly at the C-2 position). When essential fatty acids are deficient, they are replaced in phospholipids, other Complex Lipids, and membranes by other polyenoic acids of the ω-9 series, typically ∆^5,8,11-eicosatrienoic acid. The ratio of triene to tetraene (arachidonic) acids in plasma lipids can be used to assess the severity of essential fatty acid deficiency.

Humans consuming diets devoid of essential fatty acids developed scaly dermatitis and impaired lipid transport. Under normal dietary conditions, adults do not exhibit symptoms of essential fatty acid deficiency. However, infants fed artificial formulas with very low fat content developed scaly dermatitis, which was easily treated with linoleic acid preparations. Disorders associated with a deficiency of essential fatty acids, including α-linolenic acid, are also observed in patients maintained for extended periods on total parenteral Nutrition virtually lacking fats. To prevent these complications, essential fatty acids should account for at least 1–2% of the total caloric intake.

Trans-fatty acids

Trans-fatty acids are present in trace amounts in the fats of ruminants, where they are produced in the rumen through microbial action. The presence of large amounts of trans-fatty acids in partially hydrogenated vegetable oils (such as margarine) raises concerns about their safety as a dietary component. The long-term effects of these acids on The Human Body remain unclear, though autopsy data show that up to 15% of fatty acids are in the trans configuration. To date, no conclusive Evidence of the Adverse effects of trans-fatty acids has been obtained. Their metabolic pathway resembles that of saturated fatty acids rather than cis-Unsaturated fatty acids; this is likely because saturated and trans-fatty acids share a similar linear carbon chain conformation (see Chapter 15). Trans-polyunsaturated fatty acids lack essential fatty acid activity and may act as antagonists to the latter, thereby exacerbating the symptoms of essential fatty acid deficiency in the body.

Clinical Aspects

Along with essential fatty acid deficiency and alterations in the ratio of individual unsaturated fatty acids—which may occur during chronic starvation—impaired essential fatty acid METABOLISM is also observed in cystic fibrosis, acrodermatitis enteropathica, hepatorenal syndrome, Sjögren–Larsson syndrome, multisystem neuronal degeneration, Crohn's disease, Liver cirrhosis, chronic alcoholism, and Reye's syndrome. When consuming a diet with a high ratio of polyunsaturated to saturated fatty acids, plasma Cholesterol levels, particularly in low-density Lipoproteins, decrease. This is considered beneficial in light of the relationship between serum cholesterol levels and CORONARY Heart DISEASE.