• Clinical science

Specialized nutrition support


Specialized nutrition support (SNS) is required when oral intake is either limited or not possible for a prolonged period of time. Common indications for SNS include patients in critical care, those with dysphagia, unconscious patients who cannot be fed, severely malnourished patients, and those with intestinal malabsorption. The two forms of SNS are enteral nutrition and parenteral nutrition. Enteral feeding is always preferred whenever possible, but parenteral feeding may be instituted if the patient has a nonfunctional GI tract (e.g., gastroschisis, short bowel syndromes), and/or if enteral feeding is contraindicated. Patients who are on SNS may develop several complications related to feeding tubes or intravenous catheters, as well as additional metabolic complications such as electrolyte imbalances, hyperglycemia, refeeding syndrome, gallstone disease, and nonalcoholic fatty liver disease.



  • First-line: enteral feeding
    • Advantages
      • Easier to perform
      • Metabolic complications occur less often
      • Intestinal motility is stimulated, preventing mucosal atrophy
      • Lower risk of blood stream infection
  • Second-line: parenteral feeding

The following principle applies in most situations: oral before enteral, enteral before parenteral!

Enteral feeding

Parenteral nutrition

  • Definition: intravenous administration of nutrients that bypasses the gastrointestinal tract (from Greek “ para” = besides; “ enteron” = the inside)
  • Contraindications
  • Routes
    • Parenteral nutrition is required for < 2 weeks: peripheral venous line, or peripherally inserted central catheter
    • Parenteral nutrition is required for > 2 weeks: tunneled central venous catheter or a port
  • Regimens
    • Continuous parenteral nutrition
      • Most commonly used, especially in hospital settings
      • Slower infusion rate
      • Higher risk of fatty liver
    • Cyclical parenteral nutrition
      • TPN is given mostly at night
      • Faster infusion rates
      • Advantage: allows TPN in an outpatient setting
      • Disadvantage: higher risk of fluid overload, electrolyte imbalances, and unstable glucose levels
  • Composition of parenteral feeding solutions
  • Complications
  • Discontinuation of parenteral nutrition
    • Indication: > 75% of the recommended calorie and protein intake can be achieved by oral or enteral feeding.
    • PN is tapered off by halving the infusion rate hourly.

Parenteral feeding solutions

  • Modular preparations: Modular preparations are more expensive but they allow individualized TPN solutions to be compounded.
    • Amino acids: Commonly available protein modules contain 50–125 g/L of amino acids (8–20 g/L of nitrogen).
    • Glucose: Glucose is available as 5%, 10%, 25%, 50% and 70% dextrose solutions.
    • Fats: Preparations are available as 10, 20 or 30% of soybean oil emulsions.
  • Standard preparations: These preparations are available as prepacked solutions and have fixed macronutrient compositions.
Type of standard solution Energy content % of energy from proteins % of energy from fats % of energy from carbohydrates
Three-in-one preparations High glucose solutions 0.6–1.2 kcal/mL 10–15% 30–35% 45–55%
Low glucose solutions 35–45% 35–45%
Two-in-one preparations 15–25% 0% 75–85%

Nutritional requirements during specialized nutrition support

Energy content

  • 20–25 kcal/kg/day
  • Critically ill patients
    • Initial 10–14 days: a hypocaloric intake of approximately 10–15 kcal/kg/day
    • After 2 weeks: The energy intake may be increased to meet the recommended daily intake of 25 kcal/kg/day.

Macronutrient requirements


  • Initial 10–14 days: 0.8 g/kg/day.
  • After 2 weeks: the dose may be increased to 1.5 g/kg/day
    • Patients with burns may require up to 2 g/kg/day until the wound heals.
  • Protein preparations: Both modular and standard formulations contain a mixture of all 9 essential amino acids and some non-essential amino acids in varying proportions.
  • Nitrogen balance
    • Nitrogen balance measures the effectiveness of protein supplementation.
    • Nitrogen balance = Nitrogenintake - Nitrogenloss = (Protein intake in grams/6.25) - (24 hour urine nitrogen + 4)
    • 1 g of lost nitrogen → loss of 30 g of lean mody mass
    • A negative nitrogen balance of -2 to -4 g/d may be allowed.


  • The recommended dose of carbohydrates for TPN is 4 g/kg/day.
  • Carbohydrate preparations: Both modular and standard formulations use D-glucose (dextrose).


  • The recommended dose of fats for TPN is 1 g/kg/day.
  • Fat preparations: In the US, lipid formulations consist of soybean oil stabilized in an emulsion of egg yolk lecithin.

Fluid, electrolytes, and micronutrient requirements

Fluid and electrolytes
  • Water: daily maintenance fluid requirements (30 ml/kg/day or 1 ml/kcal infused) + replacement of ongoing fluid loss if necessary
  • Sodium: 1–3 mEq/kg/day (= daily maintenance requirements) + replacement of any lost sodium
  • Potassium: 0.5–1 mEq/day (= daily maintenance requirements) + replacement of any lost potassium
  • Chloride: daily parenteral dosage is determined based on acid-base balance and is administered mostly in a 2:1 or 1:1 ratio with acetate
  • Acetate: daily parenteral dosage is determined based on acid-base balance
  • Calcium: 10 mEq/day
  • Magnesium: 10 mEq/day
  • Phosphate: 30 mmol/day
Vitamins Trace elements
  • Zinc: 2.5–4 mg (+ 10–15 mg for every liter of stool output or stomal drainage)
  • Copper: 0.5–1.5 mg
  • Manganese: 0.1–0.3 mg
  • Chromium: 10–15 μg
  • Selenium: 20–100 μg (not necessary for short-term PN)
  • Molybdenum: 20–120 μg (not necessary for short-term PN)
  • Iodine: 70–150 μg (not necessary for short-term PN)

A patient's nutritional requirements must be considered when choosing a PN solution. It is important to note that patients with life-threatening illnesses have higher energy requirements.

Metabolic complications

Metabolic complications are more common with parenteral nutrition than enteral nutrition!

last updated 11/19/2018
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