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Overview of Nutritional Support

By David R. Thomas, MD, Professor Emeritus, St. Louis University School of Medicine

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Many undernourished patients need nutritional support, which aims to increase lean body mass. Oral feeding can be difficult for some patients with anorexia or with eating or absorption problems. Nutritional support is often needed for critically ill patients (1).

Behavioral measures that sometimes enhance oral intake include the following:

  • Encouraging patients to eat

  • Heating or seasoning foods

  • Providing favorite or strongly flavored foods

  • Encouraging patients to eat small portions

  • Scheduling around meals

  • Assisting patients with feeding

If behavioral measures are ineffective, nutritional support—oral nutrition, enteral tube nutrition, or parenteral nutrition—is indicated, except sometimes for dying or severely demented patients.

General reference

  • 1. McClave SA, Taylor BE, Martindale RG, et al: Guidelines for the provision and assessment of nutrition support therapy in the adult critically ill patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.). JPEN J Parenter Enteral Nutr 40 (2): 159–211, 2016. doi: 10.1177/0148607115621863.

Predicting Nutritional Requirements

Nutritional requirements are predicted so that interventions can be planned. Requirements can be estimated by formulas or measured by indirect calorimetry. Indirect calorimetry requires use of a metabolic cart (a closed rebreathing system that determines energy expenditure based on total CO2 production), which requires special expertise and is not always available. Thus, total energy expenditure (TEE) and protein requirements usually are estimated.

Energy expenditure

TEE varies based on the patient’s weight, activity level, and degree of metabolic stress (metabolic demands); TEE ranges from 25 kcal/kg/day for people who are sedentary and not under stress to about 40 kcal/kg/day for people who are critically ill. TEE equals the sum of

  • Resting metabolic rate (RMR, or resting energy expenditure rate), which is normally about 70% of TEE

  • Energy dissipated by metabolism of food (10% of TEE)

  • Energy expended during physical activity (20% of TEE)

Undernutrition can decrease RMR up to 20%. Conditions that increase metabolic stress (eg, critical illness, infection, inflammation, trauma, surgery) can increase RMR but rarely by >50%.

The Mifflin–St. Jeor equation estimates RMR more precisely and with fewer errors than the commonly used Harris-Benedict equation, usually providing results that are within 20% of those measured by indirect calorimetry. The Mifflin–St. Jeor equation estimates RMR as follows:


TEE can be estimated by adding about 10% (for sedentary people) to about 40% (for people who are critically ill) to RMR.

Protein requirements

For healthy people, protein requirements are estimated at 0.8 g/kg/day. However, requirements may be higher (see Table: Estimated Adult Daily Protein Requirement) for the following:

  • Patients with metabolic stress

  • Patients with kidney failure requiring dialysis

  • Patients > 70 yr

Estimated Adult Daily Protein Requirement


Requirement (g/kg of ideal body wt/day)



Age > 70 yr


Kidney failure without dialysis (GFR < 25 mL/min/1.73 m2)


Kidney failure with dialysis


Metabolic stress (eg, critical illness, trauma, burns, surgery)


Assessing Response to Nutritional Support

There is no gold standard to assess response. Clinicians commonly use indicators of lean body mass such as the following:

Nitrogen balance, response to skin antigens, muscle strength measurement, and indirect calorimetry can also be used.

Nitrogen balance, which reflects the balance between protein needs and supplies, is the difference between amount of nitrogen ingested and amount lost. A positive balance (ie, more ingested than lost) implies adequate intake. Precise measurement is impractical, but estimates help assess response to nutritional support:

  • Nitrogen intake is estimated from protein intake: nitrogen (g) equals protein (g)/6.25.

  • Estimated nitrogen losses consist of urinary nitrogen losses (estimated by measuring urea nitrogen content of an accurately obtained 24-h urine collection) plus stool losses (estimated at 1 g/day if stool is produced; negligible if stool is not produced) plus insensible and other unmeasured losses (estimated at 3 g).

Response to skin antigens, a measure of delayed hypersensitivity, often increases to normal as undernourished patients respond to nutritional support. However, other factors can affect response to skin antigens.

Muscle strength indirectly reflects increases in lean body mass. It can be measured quantitatively, by hand-grip dynamometry, or electrophysiologically (typically by stimulating the ulnar nerve with an electrode).

Levels of acute-phase reactant serum proteins (particularly short-lived proteins such as prealbumin [transthyretin], retinol-binding protein, and transferrin) sometimes correlate with improved nutritional status, but these levels correlate better with inflammatory conditions.

Key Points

  • Behavioral measures may avert the need for nutritional support.

  • Predict the patient's energy requirements based on weight, sex, activity level, and degree of metabolic stress (eg, due to critical illness, trauma, burns, or recent surgery).

  • Normal protein requirement is 0.8 mg/kg/day, but this amount is adjusted if age is > 70 or if the patient has kidney failure or metabolic stress.

  • Assess the response to nutritional support by indicators of lean body mass and/or other indicators (eg, nitrogen balance, response to skin antigens, muscle strength measurement, indirect calorimetry).

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