Macronutrients provide the energy for all body functions as well as the major building blocks for all tissues and reparative processes (Table 1.2). As will be discussed in Chapter 2 and Chapter 3, carbohydrate and fat may both be used to support the energy needs of the organism, although certain cells may be obligate carbohydrate consumers. This is especially true in the healing wound, with a density of inflammatory cells and fibroblasts that use carbohydrate as an energy source.35 36 Studies suggest that, even when carbohydrates are readily available, a substantial percentage of the total energy needs are met by fat.37 These findings are discussed in greater detail in Chapter 3 and Chapter 11. In addition, fats provide the building blocks for the cell membrane and certain inflammatory mediators. Carbohydrates may minimize the burden for fat intake by providing substrate for fatty acid synthesis, except for essential fatty acids that must be obtained from dietary sources.
Protein is the third component of macronutrients. Amino acids provide the major structural building blocks of all proteins in the body, including collagen, the major protein of the healing wound. Amino acids are necessary for the cell membrane and enzymes and cytokine production in the healing wound. In the inflammatory and proliferative phases, amino acid requirements in the wound will be maximal due to the high level of enzymatic activity and the high rate of cell turnover.
Protein metabolism is closely interrelated to carbohydrate and fat metabolism. Glucose and fatty acid metabolites may be used as substrate for endogenous amino acid synthesis (Figure 1.4). Conversely, metabolites of amino acid breakdown may be deaminated to provide gluconeogenic precursors and substrate for fatty acid production. However, much like the fatty acids that cannot be synthesized by the human body, certain amino acids are essential or conditionally essential and must, therefore, be obtained through the diet. This concept is discussed in greater detail in Chapter 4.
Micronutrients function primarily as cofactors in biochemical reactions and, as such, are critical to all of the activities of macronutrients. Protein synthesis cannot continue without adequate quantities of vitamin B6, zinc, and copper.26,37,38 Collagen synthesis will be impaired without vitamin C, iron, and copper. Carbohydrate utilization is impaired without chromium and manganese. Vitamin B12, folate, and zinc are essential for nucleic acid metabolism and, thus, are essential in the healing wound with rapid cellular proliferation.
Function of Some Key Nutrients Involved in Wound Healing
Glucose Fatty acids Cholesterol
Vitamin B complex
Vitamin E Vitamin K
Needed for platelet function, neovascularization, lymphocyte formation, fibroblast proliferation, collagen synthesis, and wound remodeling Required for certain cell-mediated responses, including phagocytosis and intracellular killing of bacteria Gluconeogenic precursors Energy substrate of leukocytes and fibroblasts Serve as building blocks for prostaglandins, isoprostanes that are inflammatory mediators; energy source for some cell types Are constituents of triglycerides and fatty acids contained in cellular and subcellular membranes Hydroxylates proline and lysine in collagen synthesis Free radical scavenger
Is a necessary component of complement that functions in immune reactions and increases defenses to infection Serves as cofactor of enzyme systems
Required for antibody formation and white blood cell function, essential for nucleic acid metabolism Enhances epithelialization of cell membranes Enhances rate of collagen systhesis and cross-linking of newly formed collagen Antagonizes the inhibitory effects of glucocorticoids on cell membranes Necessary for absorption, transport, and metabolism of calcium
Indirectly affects phosphorus metabolism Free radical scavenger
Needed for synthesis of prothrombin and clotting factors VII, IX, and X Required for synthesis of calcium-binding protein Stabilizes cell membranes; enzyme cofactor Needed for cell mitosis and cell proliferation in wound repair
Needed for hydroxylation of proline and lysine in collagen synthesis Enhances bactericidal activity of leukocytes Hemoglobin oxygen transport to wound Integral part of the enzyme lysyloxidase, which catalyzes formation of stable collagen cross-links
Source: Modified from Schumann, D., Nurs. Clin. North Am., 14, 683, 1979. With permission.
Micronutrients are ubiquitous in a normal diet, and therefore, severe deficiencies are uncommon without pathologic stress. In many cases, they are only cofactors in chemical reactions, not altered or consumed, so they may ultimately be reutilized. However, with certain micronutrients, we have come to recognize the value of intake orders of magnitude greater than needed for cofactors of biochemical reactions. Vitamin C, for instance, in large quantities may be a useful free-radical scavenger, as discussed in greater detail in Chapter 8.3940 Vitamin A in large quantities may be useful to offset the adverse effects of corticosteroids on wound healing.3841 42 In Chapter 9, this is discussed in greater detail.
Malnutrition may be defined as a state of nutrient deficiency or excess. This fact is often overlooked when providing nutrition, despite the fact that some of the adverse effects of nutrient excess are evident to everyone. Carbohydrate excess, for example, clearly may lead to a state of obesity, with the well-recognized risks of cardiovascular disease, diabetes, and shorter life expectancy. What is less obvious is that carbohydrate excess in the critically ill patient may also result in liver damage and the inability to wean from ventilators.26 In Chapter 2 and Chapter 11, this will be discussed in greater detail. Other nutrients, such as vitamin A, have the potential for toxicity that is even less obvious but equally dangerous.3842 This is discussed in greater detail in Chapter 9. It is apparent that in providing nutrition to stimulate healing, the caregiver cannot assume that "more is better."
In the chapters that follow, not only will the need for nutritional supplementation to optimize healing be evaluated, but also how to optimize that supplementation to avoid the consequences of nutrient excess will be addressed.
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