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The body requires the intake of water, energy, and nutrients in order to function properly and maintain body mass. Energy substrates are derived from ingested carbohydrates, proteins, and fats or mobilized from stored sources. A final source of nutrients may be the catabolism of muscle. A large amount of nutrients required for bodily function can be synthesized, but essential nutrients must be ingested, which include various amino acids, fats, vitamins, and minerals.

Metabolic rates can be calculated via indirect calorimetry, in which oxygen consumption and carbon dioxide production are measured using equipment known as a metabolic cart, and then used to determine the respiratory quotient (RQ) and resting energy expenditure (REE):

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The Harris–Benedict equation was derived from indirect calorimetry studies. The Harris–Benedict equation described in 1919 was revised by Roza and Shizgal in 1984, as shown below.

First, the basal metabolic rate is calculated based on gender, body weight, height, and age:

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Once the basal metabolic rate is calculated, a multiplier based on activity level is applied to estimate the subject’s daily kilocalorie requirement:

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Little to no exercise BMR × 1.2
Light exercise (one to three times per week) BMR × 1.375
Moderate exercise (3–5 d per week) BMR × 1.55
Heavy exercise (6–7 d per week) BMR × 1.725
Very heavy exercise (two times per day) BMR × 1.9

A simplified formula for energy requirements of a healthy adult recommended by The American College of Chest Physicians is 25 kcal/kg/d, with 15%–20% composed of protein, based on the patient’s ideal body weight.

Under the stressful conditions of critical illness, metabolic requirements increase. Septic patients may have an increased nutritional requirement of 30% and severe burn patients may increase requirements by 100% of their basal needs. Severe malnutrition can lead to morbidity and mortality in the critically ill, whereas nutritional repletion may improve healing, and immune function.

Studies have shown that enteral nutrition (EN) has benefits over parenteral nutrition (PN), including reduced infection, reduced organ failure, and reduced hospital stay. Peripheral nutrition should be reserved only for situations where enteral feeding is not an option such as bowel obstruction and short gut syndrome, or when nutritional requirement are not met by enteral means.

Optimal start time for EN is unknown, but some evidence points towards reduced infectious complications of critically-ill patients when initiation is within 24–48 hours of injury or ICU admission.

Diarrhea is a complication associated with enteral feeding which may be related to hyperosmolarity of the solution or to lactose intolerance. Gastric distention due to decreased gut motility can lead to increased aspiration risk, and may be decreased with prokinetic agents, elevation of the head of the bed to 30°–45°, and postpyloric feeding ...

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