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

Robert H. Demling, MD

Disclosures

ePlasty. 2009;9:65-94. 

In This Article

Assessing the Nutritional Needs

To optimize substrate flow to the healing wound, an assessment of required intake is made. There are many present values, which have been scientifically defined over the past 3 decades ( Table 7 ).

There are a number of specific processes that need to be completed before the calories and protein intake can be determined. Assessment of nutritional needs can be divided into the following 3 components[46–48] ( Table 6 and Table 8 ):

  • Energy or calorie requirements

  • Protein requirements

  • Micronutrient requirements

Calculation of Energy Needs

Daily energy expenditures (calories used) can be calculated or directly measured.[49–52] Calculation is usually the preferred approach for the outpatient as the requirement for direct measurement is often available only in an acute care setting. Direct measurement using the method of indirect calorimetry is the most precise approach.[51,52]

The first step in calculating energy expenditure is to determine the basal metabolic rate (BMR) using predictive equations.[49–52] This value reflects the energy to maintain homeostasis at rest shortly after awakening and in a fasting state for 12 to 18 hours[49–54] (Table 6).

Usually, the basal or resting energy expenditure is about 25 kcal/kg ideal body weight for the young adult and about 20 kcal/kg for the elderly. Requirements for the injured or ill patient are usually 30% to 50% higher.[49–54]

Malnourished patients, who already have a deficit and have lost weight, require a 50% increase over calculated maintenance calories (energy).[47,55–57]

The second step is to adjust the BMR for the added energy caused by the "stress" from injury and wounds.[47,52–57] This value, expressed as a present increase over the BMR, is an estimate of the value found for a number of bodily insults. The metabolic rate (energy demands) increases 20% after elective surgery and 100% after a severe burn.[47,48,52–57] A wound, an infection, or a traumatic injury will fall between these 2 extremes. One simple formula for defining the stress factor is described below (Table 9). The stress factor is the multiplier of the BMR.[44,45,48] The relative increase in the BMR has been defined for a number of disease processes. The data have been converted into a stress factor increase in the BMR (Table 9).

The third step is to determine the physical activity level of the patient. Physical activity is added by multiplying by an activity factor: for patients out of bed, 1.2 and for active exercise, 1.5 or more. Thus, the energy requirements can be calculated as follows:

Indirect Calorimetry The reference standard for measuring energy expenditure in the clinical setting is indirect calorimetry. Indirect calorimetry is a technique that measures oxygen consumption and carbon dioxide production to calculate resting energy expenditure since 99% of oxygen is used for energy production. Oxygen used can be converted into calories required.[51,52]

Protein Requirements

After determining caloric (energy) requirements, protein requirements are assessed. A healthy adult requires about 0.8 g of protein per kilogram of body weight per day or about 60 to 70 g of protein to maintain homeostasis, that is, tissue synthesis equals tissue breakdown. Stressed patients need more protein, in the range of 1.5 g of protein per kilogram of body weight per day.[47,48,58–63] The increased needs stem from both increased demands for protein synthesis and increased losses of amino acids from the abnormal protein synthesis channeling where protein substrate is also used for fuel. Urinary nitrogen losses increase after injury and illness, with an increase in the degree of stress. Nitrogen content is used as a marker for protein (6.25 g of protein is equal to 1 g of nitrogen). Nitrogen balance studies, such as a 24-hour urinary urea nitrogen measurement, that compare nitrogen intake with nitrogen excretion can be helpful in determining needs by at least matching losses with intake. Nutritionally depleted but nonstressed patients, especially the elderly, also require 1.5 g/kg/day to restore the lost body protein.[59–63] Stressed, depleted patients usually cannot metabolize more than 1.5 g/kg/day of protein unless an anabolic agent is added, which can override the catabolic stimulus. The required protein intake for a number of clinical states has been defined and can be used as estimates (Table 10). Simply, aging increases protein requirements to avoid sarcopenia.

Micronutrient Support

Micronutrients are compounds found in small quantities in all tissues. They are essential for cellular function and, therefore, for survival. It is becoming increasingly clear that marked deficiencies in key micronutrients occur during the severe stress response or with any superimposed PEM as a result of increased losses, increased consumption during metabolism, and inadequate replacement.[64–68] Because micronutrients are essential for cellular function, a deficiency further amplifies stress, metabolic derangements, and ongoing catabolism.

The micronutrients include organic compounds (vitamins) and inorganic compounds (trace minerals). These compounds are both utilized and excreted at a more rapid rate after injury, leading to well-documented deficiencies. However, because measurement of levels is difficult, if not impossible, prevention of a deficiency is accomplished only by providing increased intake. Deficiency states can lead to severe morbidity. Specific properties of these important molecules will be described later. Although the doses of the various micronutrients required to manage wound stress are not well defined, a dose of 5 to 10 times the recommended daily allowance is recommended until wound stress is resolved and the wound has healed.[47,64–68] There are specific micronutrients required for wound healing. Replacement in sufficient amounts is essential (Table 11).

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