Introduction

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Fats are unique among the macronutrients in that they function as both energy stores and signaling molecules. They are also unique in that the cell membranes in tissues and organs largely reflect the fatty acid composition of the diet.1 Polyunsaturated fatty acids (PuFA) can alter the formation of eicosanoids,2-4 cytokines,56 and enzymes of cell activation, such as protein kinase C7; change membrane receptor expression;8,9 and also influence the expression of numerous genes.10-12 Therefore, no other nutrient class has the capability of causing such a profound functional change in cells throughout the body. Because there is a significant storage capacity for this macronutrient, a consideration of the role of fat in the healing process needs to take into account both dietary and stored fat. The fatty acids (Figure 3.1) incorporated into cellular phospholipids and adipose tissue could be considered to be a type of dietary memory. Physiological and healing processes may be influenced when fatty acids are released from membranes via phospholipase or if they are mobilized from adipocytes for energy. In addition to the storage function, white adipose tissue (WAT) is a source of proinflammatory cytokines, such as tumor necrosis factor-a (TNF-a) and interleukin-6 (IL-6)13 and other bioactive molecules, such as leptin and adiponectin, currently referred to as adipokines.14-16 Taken collectively, the products of fat metabolism and homeostasis may be viewed as being one of the key regulators of inflammation and the healing process. This property takes on further significance if one considers that individuals suffering from poor wound healing often have additional complicating factors characterized by dysfunctions in lipid metabolism (e.g., diabetes in combination with obesity).

Stearic acid cooh

18:0, saturated

Oleic acid

Oleic acid

'cooh

18:1, m9, monounsaturated cis-Linoleic aci

'cooh cis-Linoleic aci cooh cooh

18:2, 0)6, polyunsaturated a-Linolenic acid a-Linolenic acid cooh

FIGURE 3.1 Examples of the structure for saturated, monounsaturated, and polyunsaturated fatty acids. No double bonds are present in saturated fatty acids. Monounsaturated fatty acids have a single double bond, and polyunsaturated fatty acids have two or more double bonds. Humans can insert a double bond at carbon atoms 4, 5, 6, and 9 numbered from the carboxyl terminal. The ro numbers of the carbon atoms in the fatty acid side chain are numbered from the methyl end of the molecule (ro-1, ro-2, ro-3, etc.).

cooh

18:3, 0)3, polyunsaturated

FIGURE 3.1 Examples of the structure for saturated, monounsaturated, and polyunsaturated fatty acids. No double bonds are present in saturated fatty acids. Monounsaturated fatty acids have a single double bond, and polyunsaturated fatty acids have two or more double bonds. Humans can insert a double bond at carbon atoms 4, 5, 6, and 9 numbered from the carboxyl terminal. The ro numbers of the carbon atoms in the fatty acid side chain are numbered from the methyl end of the molecule (ro-1, ro-2, ro-3, etc.).

Fat in the diet is almost completely absorbed in the intestine. The process of digestion begins with the introduction of salivary and then gastric lipases.17,18 The majority of lipolysis then occurs in the small intestine with the addition of pancreatic lipase in combination with a colipase.19 Bile salts and phospholipids secreted by the liver then emulsify the hydrolyzed fat for absorption by the enterocytes. Approximately 90 to 95% of dietary fats are in the form of triacylglycerols (TAGs) (Figure 3.2). TAGs are composed of three fatty acid side chains attached to a glycerol skeleton, and they are the primary concentrated store of metabolic energy found in adipose cells and tissues. The fatty acids on the glycerol backbone are usually different and often consist of two saturated fatty acids and one unsaturated fatty acid side chain. Pancreatic lipase cleaves fatty acids from the TAGs at the sn-1 and sn-3 positions, with the result being free fatty acids and 2-monoacylglycerol. These components are then absorbed by the enterocyte. The TAGs are then reassembled and packaged into chylomicrons along with cholesterol, phospholipids, and apoproteins. Shorter-chain fatty acids (< 12 carbon) may be bound to albumin and directly transported to the liver as nonesterifed fatty acids (NEFAs).20,21

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