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Prior to initiation of nutritional support, the patient’s estimated nutritional requirements should be determined. In most situations, solutions of equal nutrient value can be designed for delivery via enteral and parenteral routes, but differences in absorption should be considered. A complete nutritional support solution must contain water, carbohydrates, amino acids, fat, electrolytes, vitamins, and minerals.
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One method for calculating fluid requirements in adults is to allot 1500 mL for the first 20 kg of body weight plus 20 mL for every kilogram above 20 kg. Another way to calculate fluid requirement is 25–35 mL/kg or approximately 1 mL/kcal energy required. It is important to note that patients with certain medical conditions will benefit from fluid restrictions.
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Indirect calorimetry is considered the gold standard for estimating energy requirements, but it may not be available in every clinical setting. In its absence, predictive energy expenditure equations and simple weight-based calculations can be utilized.
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The most widely used and validated predictive energy expenditure equations for healthy individuals include the Harris-Benedict Equation and Mifflin-St. Jeor Equation, which calculate RMR with the addition of stress and/or activity factors. The Penn State Equation is the recommended predictive equation for critically ill patients due to its superior accuracy. This formula calculates RMR combined with dynamic physiological variables such as maximum temperature in a 24-hour period and minute ventilation. Different versions of the equation exist for patients above 60 years in age or with BMI greater than 30. Actual body weight should be used for all predictive equations.
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Energy requirements can also be estimated by multiplying actual body weight in kilograms by 25–35 kcal/kg/day. This method is simple and easy to use, but validation studies have shown an accuracy rate of less than 50%.
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The above methods provide imprecise estimates of actual energy expenditure, especially for markedly underweight, overweight, and critically ill patients who are ideal candidates for indirect calorimetry. Studies using indirect calorimetry have demonstrated that as many as 30–40% of patients will have measured expenditures 10% above or below estimated values.
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In standard diets and balanced nutritional support solutions, carbohydrates provide the greatest amount of energy compared to the other macronutrients; however, protein and fat are required in optimal amounts to provide the body energy and assist with other functions.
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Protein and energy requirements are closely related. If adequate calories are provided, stable and non-stressed patients should receive 0.8–1.2 g/kg/day of protein. Patients under moderate to severe stress require at least 1.5 g/kg/day, although patients with trauma and burns may need closer to 2.5 g/kg/day. Actual weight should be used for normal and underweight patients while adjusted body weight is used for patients with obesity.
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Patients who are receiving protein without adequate calories will catabolize protein for energy rather than utilizing ...