Nutrition use and indications

Malnutrition leads to an increased risk of infection due to immune compromise (related to decreased intake of trace elements, amino acids and vitamins, plus decreased production of leptin), increased fatigability and inability to wean/mobilise due to loss of muscle bulk, and poor wound healing. Gut mucosal atrophy occurs within days of non-feeding and may compromise the ability to feed enterally.

Adequate nutritional support should, in general, be provided early during critical illness. Improved outcomes from early nutritional support exist for patients with trauma and burns. Enteral feed is also a gastric protectant. However, the patient should be resuscitated and stabilised before enteral feeding is contemplated as gut hypoperfusion will compromise the ability to absorb, and feeding may render the gut more ischaemic. Increasing abdominal distension, pain/discomfort, large gastric aspirates, and diarrhoea suggest the need for a period of bowel rest rather than persisting with feeding and addition of prokinetics.

Enteral nutrition is indicated when swallowing is inadequate or impossible but GI function is otherwise intact. Parenteral nutrition is indicated when the GI tract cannot be used to provide adequate nutritional support, e.g. obstruction, ileus, high small bowel fi stula, or malabsorption. Parenteral nutrition may be used to supplement enteral nutrition.

Calorie requirements

As with nitrogen requirements below, this is inexact and optimal day-today intake is not known for individual patients. Various formulae can calculate basal metabolic rate but are misleading in critical illness. Metabolic rate can be measured by indirect calorimetry, but most patients are assumed to require 2000–2700Cal/d or less if starved or underweight. Burn-injured patients generally receive more feed.

Nitrogen requirements

Nitrogen excretion can be calculated in the absence of renal failure according to the 24h urea excretion.

Nitrogen (g/24h) = 2 + urinary urea (mmol/24h) x 0.028

However, as with most formulae, this method lacks accuracy. Most patients require 7-14g/d.

Other requirements

The normal requirements of substrates, vitamins, and trace elements are tabled opposite. Most long-term, critically ill patients require folic acid and vitamin supplementation during nutritional support. Trace elements are usually supplemented in parenteral formulae but should not be required during enteral nutrition.

Underfeeding	Overfeeding
Loss of muscle mass	Increased VO2
Reduced respiratory function	Increased VCO2
Reduced immune function	Hyperglycaemia
Poor wound healing	Fatty infiltration of liver
Gut mucosal atrophy
Reduced protein synthesis

Normal daily requirements (for a 70kg adult)

Water	2100mL
Energy	2000–2700Cal
Nitrogen	7–14g
Glucose	210g
Lipid	140g
Sodium	70–140mmol
Potassium	50–120mmol
Calcium	5–10mmol
Magnesium	5–10mmol
Phosphate	10–20mmol

Vitamins

Thiamine	16–19mg
Ribofl avin	3–8mg
Niacin	33–34mg
Pyridoxine	5–10mg
Folate	0.3–0.5mg
Vitamin C	250–450mg
Vitamin A	2800–3300IU
Vitamin D	280–330IU
Vitamin E	1.4–1.7IU
Vitamin K	0.7mg

Trace elements

Iron	1–2mg
Copper	0.5–1.0mg
Manganese	1–2mcg
Zinc	2–4mg
Iodide	70–140mcg
Fluoride	1–2mg