Each patient’s nutritional requirements should be determined independently of the method of nutritional support. In most situations, solutions of equal nutrient value can be designed for delivery via enteral and parenteral routes, but differences in absorption must be considered. A complete nutritional support solution must contain water, energy, amino acids, electrolytes, vitamins, minerals, and essential fatty acids.
For most patients, water requirements can be calculated by allowing 1500 mL for the first 20 kg of body weight plus 20 mL for every kilogram over 20. Additional losses should be replaced as they occur. For patients with a normal BMI, fluid needs are about 30–35 mL/kg, or approximately 1 mL/kcal of energy required.
Energy requirements can be estimated by one of three methods: (1) by using standard equations to calculate BEE plus additional calories for activity and illness, (2) by applying a simple calculation based on calories per kilogram of body weight, or (3) by measuring energy expenditure with indirect calorimetry.
BEE can be estimated by the Harris–Benedict equation: for men, BEE = 666 + (13.7 × weight in kg) + (5 × height in cm) – (6.8 × age in years). For women, BEE = 655 + (9.5 × weight in kg) + (1.8 × height in cm) – (4.7 × age in years). For undernourished patients, actual body weight should be used. For patients with obesity, a weight in between ideal body weight and actual body weight can be used: ideal body weight + 0.4(actual body weight – ideal body weight). For most patients, an additional 20–50% of BEE is administered as nonprotein calories to accommodate energy expenditures during activity or relating to illness. Occasional patients are noted to have energy expenditures greater than 150% of BEE.
Energy requirements can also be estimated by multiplying actual body weight in kilograms by 25–30 kcal/kg/day.
Both of these methods provide imprecise estimates of actual energy expenditures, especially for markedly underweight, overweight, and critically ill patients. Studies using indirect calorimetry have demonstrated that as many as 30–40% of patients will have measured expenditures 10% above or below estimated values.
Protein and energy requirements are closely related. If adequate calories are provided, most patients can be given 0.8–1.2 g of protein per kilogram per day. Patients under moderate to severe stress should receive up to 1.5 g/kg/day. As in the case of energy requirements, actual weights should be used for normal and underweight patients and ideal body weight + 0.4(actual body weight – ideal body weight) can be used for patients with obesity.
Patients who are receiving protein without adequate calories will catabolize protein for energy rather than utilizing it for protein synthesis. Thus, when energy intake is low, excess protein is needed for nitrogen balance.