Nutrition, Anabolism, and the Wound Healing Process: An Overview

Robert H. Demling, MD

Disclosures

ePlasty. 2009;9:65-94. 

In This Article

Body Composition and LBM

Components of Body Composition

To better understand the impact or erosion of LBM and the normal or abnormal utilization of protein and fat for fuel, a general understanding of normal body composition is required[19–21] (Table 1).

Body composition can be divided into a fat and a fat-free component or LBM. LBM contains all of the body's protein content and water content, making up 75% of the normal body weight. Every protein molecule has a role in maintaining body homeostasis. Loss of any body protein is deleterious. The majority of the protein in the LBM is in the skeletal muscle mass. LBM is 50% to 60% muscle mass by weight.

It is the loss of body protein, not fat loss, that produces the complications caused by involuntary weight loss. Protein makes up the critical cell structure in muscle, viscera, red cells, and connective tissue. Enzymes that direct metabolism and antibodies that maintain immune functions are also proteins. Skin is composed primarily of the protein collagen. Protein synthesis is essential for any tissue repair. Therefore, LBM is highly metabolically active and necessary for survival.

There are only 40,000 calories in the LBM compartment in a 70-kg individual; each gram of protein generates 4 calories (Fig 2). It is not possible to burn more than 50% of LBM.[22] Fat mass comprises about 25% of body composition. For all intents, the fat compartment is a calorie reservoir where day-to-day excess calories are stored and fat is removed when demands need to be met. There are, however, some necessary essential fats, which make up a small fraction of this compartment.

Figure 2.

Body composition is divided into lean mass containing all the protein in the body plus water and fat mass composed mainly of a fat store, for a deposition of excess energy.

For the most part, fat is not responsible for any essential metabolic activity. This energy reservoir contains about 110,000 calories stored, as 1 g of fat generates 10 calories (Fig 2). There are a number of body adaptations that attempt to maintain normal LBM or body protein (Table 2).[23]

There is an ongoing homeostatic drive to preserve LBM as a self-protective process since lost protein is deleterious. However, activation of the stress response, caused by a wound, will block these adaptive responses and body protein will be burned for fuel.[6–9]

Measuring Body Composition (Common Approaches)

Involuntary weight loss is a marker of potential problems, and weight restoration is a potential solution. However, the real key diagnostic information is the status of body composition (Table 3). Since normal body composition for the individual of concern is not known prior to the insult, a host of normalized tables and equations, with an assumed normal value, are used. Therefore, the actual alteration of body composition caused by an insult or poor nutrition (or usually both) is not known. The complications, for example, the weakness seen in the patient, as well as the presence of a catabolic state that will lead to LBM loss, are often the best clinical markers. Of the available methods (Table 3), skin-fold thickness and bioelective impendence are valuable if taken sequentially over time, but some form of baseline is needed; on the other hand, nitrogen balance provides direct information as to whether the patient was catabolic or anabolic on the measurement day, and how catabolic.[22–28]

Loss of LBM

Loss of any LBM is deleterious as there are no spare proteins. The loss of LBM, relative to normal, corresponds with major complications. A loss of more than 15% of total will impair wound healing, the greater the loss, the more the healing deficit. A loss of 30% or more leads to the development of spontaneous wounds such as pressure ulcers, and wound dehiscence at a late stage. Death occurs with 40% LBM loss, usually from pneumonia (Table 4).[22]

This table assumes no preexisting involuntary weight loss.[17,18] Someone with PEM will always have a preexisting loss, which needs to be added as part of total. One can assume that with any stress-induced PEM, LBM loss is about half of the involuntary weight loss. The relationship between LBM and wound healing is based on the manner of utilization of available protein for either the wound or maintaining the overall LBM compartment (Fig 3).

Figure 3.

With a loss of lean mass less than 10%, the wound takes priority over the available protein substrate. As lean mass decreases, more consumed protein is used to restore LBM, with less being available to the wound. Wound healing rate decreases until lean mass is restored. With a loss of lean mass exceeding 30% of total, spontaneous wounds can develop due to the thinning of skin from lost collagen.

Wound closure is an important genetically determined drive for survival. With a loss of less than 20% of LBM, the wound takes priority for the protein for healing. With a loss of 20%, there is equal competition for the protein between the wound and the restoration of LBM, so the healing rate will slow down. With a loss of 30% or more, where risk to survival is high, the LBM takes complete priority for protein intake. The wound essentially stops healing till LBM is restored at least partially.[6–8]

Body compositional changes before and after a wound, therefore, have a major impact on healing irrespective of the local wound care. In addition, nutritional support needs to be increased in both calories and protein (1.5 g/kg body weight) if there is a preexisting deficit, as would be present with any previous PEM. The rate of healing is directly related to the rate of restoration of body composition (Fig 3). Wound healing is directly related to the degree of LBM loss (Figs 4–7).[29]

Figure 4.

Lean mass loss 20% of the total: Clean but poorly healing acute wound responding to LBM loss.

Figure 5.

Lean mass loss 25% the total: thinning of skin with loss of collagen as LBM decreases.

Figure 6.

Lean mass loss 25% to 30% of the total: dehiscence stump closure now with open nonhealing wound.

Figure 7.

Lean mass loss 30% of the total: spontaneous pressure ulcer on the sacrum.

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