Nutritional Requirements

The fundamental goal of nutritional support is to provide the daily nutrient and energy needs of each patient. This chapter will describe how to determine those needs in critically ill patients (1).

I. Calorie Requirements

A. Oxidation of Nutrient Fuels

Oxidative metabolism captures the energy stored in nutrient fuels (carbohydrates, lipids, and proteins), and uses this energy to sustain life. This process consumes O₂, and generates CO₂, H₂O, and heat. The quantities involved in the oxidation of each nutrient fuel are shown in Table 36.1.

The heat generated by the complete oxidation of a nutrient fuel is the energy yield (in kcal/g) of that fuel. Lipids have the highest energy yield (9.1 kcal/g), while glucose has the lowest energy yield (3.7 kcal/g).
The summed oxidation of all three nutrient fuels determines the whole-body O₂ consumption (VO₂), CO₂ production (VCO₂), and heat production for any given time period. The 24-hour heat production (daily energy expenditure) in kcal, determines how many calories to provide each day in nutritional support. The daily energy expenditure can be measured (indirectly) or estimated, as described next.

Table 36.1 Oxidation of Nutrient Fuels

Fuel	O₂ Consumption	CO₂ Production	Heat Production^†
Glucose	0.74 L/g	0.74 L/g	3.7 kcal/g
Lipid	2.00 L/g	1.40 L/g	9.1 kcal/g
Protein	0.96 L/g	0.78 L/g	4.0 kcal/g
^† The energy yield from each nutrient fuel.

B. Indirect Calorimetry

1. The Principle

It is not possible to measure metabolic heat production in hospitalized patients, so the daily energy expenditure is determined indirectly using the whole-body O₂ consumption (VO₂) and CO₂ production (VCO₂), and the relationships in Table 36.1. This is the principle of indirect calorimetry, which measures the resting energy expenditure (REE) in kcal/min, using the following relationships (2):

2. The Method

Indirect calorimetry is performed with “metabolic carts” that measure whole-body VO₂ and VCO₂ at the bedside by measuring the concentrations of O₂ and CO₂ in inhaled and exhaled gas (usually in intubated patients). Steady-state measurements are obtained for 15–30 minutes to determine the REE (kcal/min), which is then multiplied by 1,440 (the number of minutes in 24 hours) to derive the daily energy expenditure (kcal/24 hr) (3).

3. Indirect calorimetry is not readily available in many ICUs, and daily energy requirements are usually estimated, as described next.

C. The Simple Way

More than 200 cumbersome equations are available for estimating daily energy requirements (1), but none is more accurate than the following simple relationship (1,4):
Body weight adjustments have been proposed for obese patients (5), but such adjustments are not recommended in the current guidelines on nutrition support (1).

Table 36.2 Calorie-Restricted, High-Protein Feeding Regimen for Obese ICU Patients

If indirect calorimetry is available, measure the REE, and provide 70% of the daily calorie requirement.
If indirect calorimetry is not available, use the patient’s body mass index (BMI), in kg/m², to determine the daily calorie and protein intake.
The daily calorie intake should be 11–14 kcal/kg (actual body weight) for a BMI of 30–50, and 22–25 kcal/kg (ideal body weight) for a BMI >50.
The daily protein intake should be 2 g/kg (ideal body weight) for a BMI of 30–40, and 2.5 g/kg (ideal body weight) for a BMI >40.

From the Clinical Practice Guidelines in Reference 1.

D. Calorie Restriction

Calorie restriction has several potential advantages, including a decrease in O₂ consumption (which creates less demand on the cardiac output), a decrease in CO₂
production (which is advantageous in ventilator-dependent patients.), and improved glycemic control.
At least six clinical trials have shown no apparent harm when the daily caloric intake is reduced by about 50% (while protein intake is maintained) (6).
The current guidelines for nutrition support in the ICU in-cludes a recommendation for calorie restriction in obese patients (1). This recommendation is summarized in Table 36.2

II. Substrate Requirements

The daily energy requirement is provided by nonprotein calories (from carbohydrates and lipids), while protein intake is used to maintain lean body mass (among other things).

Table 36.3 Endogenous Fuel Stores in Healthy Adults

Fuel Source	Amount (kg)	Energy Yield (kcal)
Adipose Tissue Fat	15.0	141,000
Muscle Protein	6.0	24,000
Total Glycogen	0.09	900
		Total: 165,900
Data from Cahill GF, Jr. N Eng J Med 1970; 282:668–675.

A. Carbohydrates

Standard nutrition regimens use carbohydrates (dextrose) to provide about 70% of the nonprotein calories. The human body has limited carbohydrate stores (see glycogen stores in Table 36.3), and daily intake of carbohydrates is essential for proper functioning of the brain, which relies heavily on glucose as a nutritive fuel.

B. Lipids

Lipids are used to provide about 30% of the nonprotein calories. As mentioned, lipids have the highest energy yield of the three nutrient fuels (see Table 36.1), and lipid stores in adipose tissues represent the major endogenous fuel source in healthy adults (see Table 36.3).

1. Linoleic Acid

Dietary lipids are triglycerides, which are composed of a glycerol molecule linked to three fatty acids. The only dietary fatty acid that is essential (i.e., must be provided in the diet) is linoleic acid, a long chain, polyunsaturated fatty acid.